Phosphate Rocks for Direct Application to Soils

PHOSPHATE ROCKSFOR DIRECT APPLICATION TO SOILS S. S. S. Rajan', J. H. Watkinsonl, and A. G. Sinclair2 IAgResearch, Ruakura Agricultural Research Center, 3 123, Hamilton, New Zealand LInvermay Agricultural Center, 50034, Mosgiel, New Zealand

I. Introduction 11. Reactivity of Phosphate Rocks

A. Definition of Reactiviry B. Measurement of Reactivity C. Mineralogy and Reactivity 111. Measurement of Phosphate Rock Dissolution in Soil A. Measurement in Acid Soils B. Measurement in Calcareous Soils IV. Factors Affecting Phosphate Rock Dissolution in Soil and Availability to Plants A. Factors Affecting Rate of P Release from Phosphate Rock Applied to Soil B. Factors Affecting Plant Availability of P from Dissolved Phosphate Rock V. Modeling the Rate of Phosphate Rock Dissolution in Field Soil A. Kirk and Nye Model B. Watkinson Model VI. Agronomic Effectiveness of Phosphate Rock A. Determining Agronomic Effectiveness B. Quantifying Comparative Performance of Phosphate Rocks C. Residual Effectiveness of Phosphate Rocks VII. Economics of Using Phosphate Rock Fertilizers VIII. Soil Testing Where Phosphate Rocks Are Used A. Current Research B. Future Research Needs IX. Amendments to Phosphate Rocks A. Composting with Organic Manures B. Phosphate Rock-Sulfur Assemblages C. Partially Acidulated Phosphate Rocks X. Concluding Remarks References

77 Adilnces in Agrnnmny, Vobine Y7

Copyright 0 1996 by Academic Press, Inc. All rights of reproduction in any form reserved

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

I. INTRODUCTION Interest in phosphate rocks (PRs) as direct application fertilizer stems from the facts that (i) per kilogram of P, PR is usually the cheapest fertilizer; (ii) direct application, with or without amendments, enables utilization of PRs which are unsuitable for manufacturing phosphoric acid and other soluble fertilizers such as triple (TSP) or single superphosphate (SSP); (iii) because PRs are natural minerals requiring minimum processing they are environmentally benign (Schultz, 1992); and (iv) PRs could be more efficient than soluble fertilizers in terms of recovery of phosphate by plants, even for short term crops in soils where soluble P is readily leached, as in sandy soils (Yeates, 1993) and possibly for long-term crops also in other soils (Rajan et al., 1994).

In spite of this PRs are not widely used as direct application fertilizers. The reasons are: (i) not all soils and cropping situations are suitable for use of PRs from different sources; (ii) the large number of factors controlling their dissolution in soil and availability to plants coupled with the inability to predict their agronomic effectiveness in a given soil climatic and crop situation; and (iii) their lower P content compared with high-analysis fertilizers which make PRs more expensive at the point of application if long-distance transportation is required. It has been more than 15 years since the last comprehensive review on PR for direct application was published by Khasawneh and Doll (1978). More recently Hammond er al. (1986b) reviewed the use of PRs and amended PRs in tropical soils. Since then considerable progress has been made in several areas of PR research. This includes a better understanding of the factors that affect PR dissolution, critical evaluations of the methods used to measure PR dissolution in soils, and its availability to plants and development of mechanistic models to predict the dissolution and availability to plants of PR-P. We found the literature on PR research rather overwhelming. In this chapter, instead of reviewing the numerous published reports, we will concentrate on the advances made on the fundamentals of PRs dissolution and their agronomic use, with specific examples. We have mostly quoted references published since 1978, although for the sake of continuity and comprehensiveness we have also cited some earlier publications. The philosophy behind this review will be that, paraphrasing Nye (1992), if we really understand the fate of PRs applied to soil and

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

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develop mechanistic models describing their reactions we should be able to predict their effectiveness for any combination of soil, climate, crop, and management. This should enable decisions regarding PR use to be made without time-consuming and expensive field trials in every location.

11. REACTIVITY OF PHOSPHATE ROCKS

A. DEFINITION OF REACTIVITY There are two occasions when some measure of the agronomic performance of PRs would be desirable. One is the performance of a given PR when added to a given soil/plant system, and the other is the relative performance of a number of PRs when added to a soil suitable for using PRs. It seems preferable to restrict the usage of reactivity to the second occasion because it will then depend on only PR properties. The first would also require a knowledge of whether the soil/plant system would be suitable for even the most reactive PR. Although Khasawneh and Doll (1978) indicated that the reactivity of a PR is related to agronomic effectiveness, they did not define the term explicitly. Rather it was discussed in relation to measured PR properties, such as the relative amount dissolved in a particular organic acid solution. More recently Sinclair et al. (1992) described reactivity as the ability of a PR to release P, or the rate of release of P to soil and plant; Rajan et al. (1 992) defined it as the magnitude and rate of dissolution of a PR. Building on these ideas, it is proposed that Reactivity is the combination of PR properties that determines the rate of dissolution of the PR in a given soil under given field conditions. Reactivity is defined as a property of the PR, and deliberately excludes properties of soils and plants. It is a direct measure of the amount of PR that dissolves in a given time, and hence is related to agronomic effectiveness. Sometimes there is apparently no relation because it is masked by other factors (see Section 1V.B). Also, the relation is discontinuous since the relative agronomic effectiveness (RAE) is not increased by a decrease in size below about 0.15 mm (Khasawneh and Doll, 1978), even though the amount dissolved will still continue to increase with decreasing size (Kanabo and Gilkes, 1988a,b,c,d). Reactivity is also defined in terms of the dissolution in a soil in the field, not in the laboratory or glasshouse, i.e., under the most appropriate conditions for providing information on the use of PR as a fertilizer. Reactivity as defined is a kinetic property, which is also consistent with the use of PR as a fertilizer in the open system of a field soil. Any method of measurement must therefore avoid the establishment of concentrations approaching a

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

saturated solution, or conditions approaching a quasi-equilibrium. Such equilibrium values would exclude purely kinetic properties, particularly surface area, which is an important factor in the dissolution rate modeling of RPRs (Kirk and Nye, 1986a,b; Watkinson, 1994a). The definition of reactivity should not be based on methods, as at present (Khasawneh and Doll, 1978), such as the amount extracted by a solution of an organic acid. The emphasis should be on dissolution in field soil, and any method can only give an estimate of this. Relative reactivities could be measured from the amount of PR remaining in a field soil at a set time after application. The relative amounts dissolved would give an indication of their relative effectiveness as P fertilizers. However, such a method would require appreciable time and resources, and so a laboratory method that provides an acceptable estimate would be desirable. As will be shown later, the PR reactivity is the combination of several properties measureable in the laboratory, including particle size. As soils become more acidic (or plants more acidifying), differences in PR reactivity become less important; conversely, as soils become more neutral, amounts dissolved are less and differences in reactivity become more important (Khasawneh and Doll, 1978). Consequently it is important to choose a soil for measurement of dissolution that gives a good range of amounts dissolved from a range of PRs. The above definition of reactivity is useful in that it is precise and the property can be measured specifically, i.e., by measuring residual PR in a soil (Section Ill), or calculated from relevant PR properties using mechanistic dissolution rate models (Section V).

B. MEASUREMENT OF REACTMTY Several types of measurement have been proposed, more particularly: dissolution in soil; dissolution in acid or salt solutions; measurement of crystal unit cell dimensions; and calculation of dissolution in soil using parameters from mechanistic dissolution models. Since rapid results for reactivity measurements are required, only aqueous extractions in the laboratory have been used routinely (Chien, 1978, 1993; Rajan er al., 1992; Sinclair et al., 1992), except for a recent laboratory test based on fundamental PR properties from dissolution rate model parameters (Watkinson, 1994~). 1. Principles

Most existing methods are purely empirical (although dilute organic acids have been used to simulate the action of root exudates), largely because the concept of

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

8I

reactivity has not been defined explicitly. Without an exact definition, methods based on precise principles are not possible. Generally, reactivity is equated with the solubility of the PR (Khasawneh and Doll, 1978; Chien, 1993), but modified by other factors, notably free carbonate content, crystal and particle size and porosity, and intermixture with silica (Chien, 1993). However, the term solubility itself is used in two senses: in the usual thermodynamic sense of the equilibrium concentration (Chien, 1993), but mostly as the rate of solution into one of several extractants (Chien, 1993). Furthermore, different extractants give different results. For example, Watkinson ( 1 9 9 4 ~ pointed ) out that citric acid apparently dissolves more fluorapatite, but less francolite than formic acid (Rajan et al., 1992; Chien, 1993) (Fig. 1). Largely because of these difficulties Sinclair et al. (1992) put in a plea for an approach to reactivity measurement based on fundamental properties of the PR. Watkinson (1994~)attempted such an approach incorporating the above definition of reactivity, a dissolution rate model (Watkinson, 1994a), and fundamental properties of the PR measureable in the laboratory.

2. Methods a. Empirical Since the review by Khasawneh and Doll (1978), no new chemical methods seem to have been proposed, only improvements to existing methods (Chien, 1993). Those most commonly used are heated neutral ammonium citrate, 2% citric acid, and 2% formic acid (Chien, 1993). Less common methods include absolute citrate solubility and acid ammonium citrate (Chien, 1993). To overcome the problem of appreciable impurity in the PR, adding a constant mass of P

a

-

4:

CAs FAs

~

0

Formic-P Citric-P

A

0

A

10 20 30 40 50 PR dissolved in one year, DRF (“YO)

0

Figure 1 Relationship between citric-P and formic-P for carbonate- (CAS) and fluor- (FAS) apatites as shown plotted against dissolution rate function (DRF) (Rajan ef a / . . 1992; Watkinson, 1995).

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

or apatite to the extracting solution has been proposed (Axelrod and Gredinger, 1979). The preferential reaction of free carbonate with the extractant was overcome by measuring the amount of PR dissolved in the second rather than the first extract (Chien and Hammond, 1978; Mackay et al., 1984b; Mishra ef al., 1985). Rajan et al. (1992) recognized the importance of the geometric surface area in rate of solution methods, and adjusted the shaking time to include an effect of initial area or particle size. In contrast, Khasawneh and Doll (1978) treated geometric area (i.e., particle size) as being of lesser importance than the much larger specific area, which includes internal surfaces, but without supporting evidence. (Later discussion, Section V, will show that on the diffusion-controlled model, internal surface area is of small importance compared with geometric area.) b. Theoretical Olsen (1975) found that the dissolution kinetics of PRs into EDTA solution, the rate of which was increased through the chelation of the dissolved Ca2+, was consistent with a second-order reaction rate. However, he did not further test the model or use fundamental PR properties as variables. Watkinson (1995) proposed a Dissolution Rate Function (DRF), using fundamental properties of RPRs measurable in the laboratory, for estimating the reactivity of PRs. The DRF represents the amount of PR dissolved in a standard soil in a given time. It was derived from a simple rate equation in a dissolution rate model (Watkinson, 1994a,b), and the standard soil represented the properties of an average New Zealand pastoral soil used for direct application of PRs. In this soil 30% of Sechura (Peruvian) PR (also referred to as Bayovar PR) of particle size 0.075-0.15 mm dissolved in 1 year. For a PR with particles of the same diameter, do, the amount dissolved in time, t , the DRF was given by (Watkinson, 1994c) DRF

=

1 - [ I - 8D,t(C,/F)/(pdo2)]”2,

(1)

Where D, is the mean diffusion coefficient for phosphate in soil (the value for the standard soil mentioned above is 0.5 cm2 year-’), C , is the phosphate concentration at the PR surface (strictly C , - C s , where Cs is the phosphate concentration in the bulk soil, but C , Cs), p is the PR particle density, approximately 3.2 g cmP3, and F is the fractional P content of the PR. For convenience, the time, t , was 1 year, while values for do and F were measured using standard methods. The value for C , was measured as the equilibrium phosphate concentration of the PR in a simulated soil solution of constant pH (held at pH 5.5 using an automatic titrator) and set initial values of calcium (0.5 mM) and ionic strength ( 5 mM) (Watkinson, 1994~).This solution took into account the calcium from the soil and that dissolved from the PR and the calcite impurity in the PR at pH 5.5. Congruent dissolution was assumed because of the very low solubility of PRs (Kirk and Nye, 1986a), and the large ratio of solution to solid of 500: 1 used (Watkinson, 1994~).

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

83

The DRF can therefore compare a PR of high solubility and large size with a contrasting one of low solubility and small size. A large pH buffering capacity in the soil was assumed in that all the alkali generated by the dissolving lattice phosphate and carbonate was neutralized by the automatic titrator in holding the pH constant. This is generally true for New Zealand pastoral soils (Edmeades er al. 1985). The DRF for a fertilizer mixture with sieve analysis of particle sizes resulting in n successive sieve fractions, each of size range bi+, to bi and of weight n

fraction wi, where

i= I

wi = I , is given by,

n

DRF = 1 -

wJ(2 i= I

+ at/b?)(b: -

- (2

+ at/b:+,)(b?-?_,- at)”2

+ ( 3 ~ Z ) ( s i n - l ( f i / b ~-) s i n - 1 ( f i t / b ~ - ~ ) ) 1 / 2 (-b ~bi-,),

(2)

where a = 8 D, ( C , / F ) / p , and after 1 year ( t = 1) the smallest particle, b,, has not dissolved. If the smallest particles have dissolved, additional sets of similar equations are required (Watkinson, 1994c). The DRF for 1 1 PRs, ground and unground, correlated with published values of relative response of ryegrass as the test plant in three soils (Rajan et al., 1992) (Fig. 2) at least as well as acid-extractable P using citric and formic acids (Watkinson, 1994c, 1995). A comparison of the ground and unground PRs in a plot of RAE against the solubility function, C,/F, i.e., only size excluded, showed the effect of grinding (particle size) on RAE (Fig. 3). The difference between ground and unground PRs increased with increasing solubility (Watkinson, 1994~).Conversely there was a neglible difference at very low solubility, which is consistent with evidence cited earlier that it was not possible to convert an unreactive PR into a reactive PR by grinding it to a very small size. These data 1201

0

I

I

1

I

10 20 30 40 Lab test, DRF (“7)

1

50

Figure 2 Relative response of unground (UG)and ground (G) PRs (ground Sechura PR = 100) (Rajan ct a / . , 1992). in relation to the Dissolution Rate Function (DRF) (Watkinson, 1995).

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

? = 0.87 Ground

1201

:

n

0.91 Unground

r I

I

I

I

I

1

10 20 30 40 50 60 RPR solubility, CR/F (mg L-l)

figure 3 The effect of grinding on the relative response values of PRs (0,unground, and 0, ground) (Rajan et a/.. 1992) having different levels of solubility, CRIF, at pH 5.5 (Ca = 0.5 mmol liter- I ) (Watkinson, 1995).

indicate that DRF was applicable to fluorapatites with and without carbonate substitution and of different size distributions. The effect on dissolution rate of grindinig, in three stages, a PR with size distribution typical of North African PRs has been calculated from the model (Watkinson, 1994b), and is shown in Fig. 4. The soil properties are such that 30% of Sechura PR would be dissolved in the first year. The economics of grinding could be estimated from such data (Sinclair et al., 1990a,b).

C. MINERALOGY AND REACTIVITY 1. Fluorapatite Fluorapatite (FA) is much less soluble than hydroxyapatite (HA) or even the carbonate substituted fluorapatites (CAs) (Khasawneh and Doll, 1978). This very 1001

e l m m (unground) \

8.25

0

----

2 4 6 8 1 Dissolution time (years)

I

0

Figure 4 Predicted effect of grinding a PR to different sizes on its dissolution rate in a soil dissolving 30% of Sechura PR in the first year (Watkinson, 1994b).

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

85

low solubility precludes its use as a direct application fertilizer, and therefore it is classed as unreactive. It was not possible to convert an unreactive to a reactive PR, even by ultrafine grinding to a size c 0 . 0 2 mm (Khasawneh and Doll, 1978).

2. Carbonate Apatites (Sedimentary) Sedimentary fluorapatites in which carbonate substitutes for phosphate in the apatite lattice form two distinct series of phosphate rocks on the basis of their physical and chemical properties (McClellan and Van Kauwenbergh, 1992). The most common are those with an excess of fluorine over that in FAP. In this case carbonate plus fluoride together substitute for phosphate to preserve the charge balance within the lattice. The second series comprise those PRs in which phosphate is substituted by carbonate plus hydroxide and/or fluoride leading to a deficit of fluorine over that in FAP. a. Excess Fluorine The properties of those PRs with excess fluorine are controlled by the extent of carbonate substitution (McClellan and Van Kauwenbergh, 1992). The planar carbonate substitution for tetrahedral phosphate makes the lattice less stable, resulting in increasing solubility with increasing carbonate content (Khasawneh and Doll, 1978). Increasing carbonate also decreases the unit cell a-dimension and increases the solubility, and therefore the reactivity compared to FA, all other things being equal (Section 1V.A). b. Deficit Fluorine Where there is a deficit of fluorine, the combinations of substitutions are more complex (McClellan and Van Kauwenbergh, 1992), and the correlations with other properties more diffuse. The unit cell a-dimension is controlled more by the fluorine than the carbonate content, and decreases with increasing fluorine. In contrast to the excess fluorine series, the solubility decreases with decreasing unit cell size and increasing fluorine, the latter in line with the decreasing hydroxyl. The fluorine content ranges from zero (equivalent to hydroxyapatite) to that in FAP, so as a class they are generally more soluble and akin to hydroxyapatite, and more reactive than the excess fluorine class. c. Carbonate Impurity Calcite and, less commonly, dolomite impurities are often present as discrete minerals (McClellan and Van Kauwenbergh, 1992). Both dissolve to completion because the product is evolved as carbon dioxide increasing the local soil pH and calcium. Calcite dissolves much more rapidly (Sverdrup and Bjerle, 1982; Watkinson and Kear, unpublished data). These effects lower the amount of PR dissolved, and therefore the apparent reactivity. For PRs with excess fluorine, lattice carbonate is usually inversely related to calcite impurity (Watkinson and

86

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

Kear, unpublished data), so that these two effects also accentuate differences in reactivity.

111. MEASUREMENT OF PHOSPHATE ROCK DISSOLUTION IN SOIL PR for direct application has been advocated almost exclusively for noncalcareous acid soils. Consequently the techniques developed to measure dissolved PR have notably been for use in acid soils, although the inorganic fractionation scheme of Baifan and Yichu (1989) may pave the way for developing methods for calcareous soils. The extent of PR dissolution, and therefore the release of PR-P, can be measured (i) directly by determining the PR remaining in soil and (ii) indirectly by determining the reaction products released, P and Ca. A third category of methods measure apparently a constant fraction of PR dissolved. Under this category fall NaHCO, (pH 8.5) extractable P, anion resin (with or without cation resin) extractable P and isotopically exchangeable P which is measured either at a given interval after PR application to soil or at intervals. The third category is for estimating plant available P in soils and not for measuring PR dissolution per se. For that reason these methods are discussed in Section VIII. The direct measurement of residual PR is applicable in all circumstances, including field, greenhouse, and incubation studies. The indirect methods are suitable only for closed incubation systems, where the reaction products are not removed from the soil. They are not suitable for field or greenhouse studies unless the amounts of P or Ca removed from the soil by plant and microbial uptake and/or leaching are also measured. The prerequisites for any method used for estimating PR dissolution are that PR should not dissolve during preextraction and, in the case of indirect methods, the extractant should remove all of the reaction product(s) (Bolan and Hedley, 1989).

A. MEA~URFMENT IN ACIDSOILS 1. Measurement of Phosphate Rock Remaining by Inorganic P Fractionation Methods used for measuring the amount of PR remaining in soil are based on the inorganic P fractionation procedure of Chang and Jackson (1957) and later modifications (Petersen and Corey, 1966; Williams el al., 1967; Syers et al., 1972). The fractionation procedure was originally developed to characterize the

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

87

distribution of soil phosphate in various chemical forms, but has been subsequently applied to measure the dissolution products of P fertilizers and the residual PR to calculate the extent of PR dissolution (Chu et al., 1962; Shinde et al., 1978; Chaudhary and Mishra, 1980; Grigg, 1980a,b; Rajan, 1983, 1987a,b; Chien et al., 1987b; Bolan and Hedley, 1989; Perrott, 1992; Perrott et al., 1992; Rajan and Watkinson, 1992; Tambunan et al., 1993; Perrott and Kerr, 1994). Briefly, the method generally adopted to measure PR dissolution consists of prewashing soil (30 min) with NaCl or BaCI, solution buffered to more than pH 7.8 to remove soil-exchangeable Ca, followed by extraction (17 h) with NaOH (0.5- 1 M ) to extract nonoccluded Fe-P and AI-P, and then with an acid (HCI or H,SO,, 0.5-1 M )solution (4 or 17 h for HISO,) to extract Ca-P. Although some modified methods included Chang and Jackson’s technique of citrate dithionate extraction prior to acid extraction (Peterson and Corey, 1966; Williams et a l . , 1967; Syers et al., 1972), omission of this step has not been found to affect the amount of acid extractable P (Rajan, 1983). The amount of PR remaining is calculated from the increase in the Ca-P fraction (acid extractable P) of the PRtreated soil over that of the untreated control. In applying the procedure it is considered that (i) the P in PR is present as calcium apatite, (ii) the apatite P is not soluble in NaOH but is dissolved by HCI and HISO, (Williams, 1937), and (iii) the P dissolved in soil is transformed into AI-P and Fe-P. In the fractionation procedure a prewash with NaCl or similar electrolyte is necessary to remove soil exchangeable Ca which, if present, could result in precipitation of calcium phosphate during NaOH extraction (Syers et al., 1972). Dissolution of the calcium phosphate in the subsequent acid extract overestimates the Ca-P fraction and therefore the PR present (Perrott , 1992). Hughes and Gilkes (1984) reported that a prewash with unbuffered solutions of NaCI, KCI, or NH,CI of soil, incubated with PRs for a week resulted in the release of exchangeable acidity from soil and therefore dissolution of apatite. They concluded that the extracting solution pH should remain above 7.3 to prevent PR dissolution. These authors recommended prewashing with Bascomb solution (Bascomb, 1964) which consists of 2 M BaCI, solution buffered with triethanolamine (TEA) at pH 8.1. Perrott and Ken (1994), using both soil/PR mixtures and field soils collected 8 months after surface application of PRs, found that prewashing mineral soils with 1 M NaCl resulted in significantly less recovery of PR (70-95%) than when using NaCl solution buffered with 0.1 M H,BO, and NaOH to pH 7.8. The loss was significantly related to soil pH(H,O) (Fig. 5A) but not to the exchangeable acidity (Fig. 5B). H,BO,/NaOH buffers are preferred over TEA because they do not interfere with the molybdenum blue/ascorbic acid method used for P analysis and also are easier to prepare. To improve Ca extraction, Perrott and Kerr (1994) added EDTA to the buffered solution in amounts equivalent to the concentration of exchangeable Ca. Rajan (personal communication) determined the residual PR in a volcanic ash soil

88

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

0

"V

-

0

10

20

Titratable acidity (Cmol kg-l) figure 5 Effect of (A) soil pH(H20), and ( B ) titratable acidity on recovery of Sechura PR-P from soil-PR mixture using NaCl prewash. Recovery values are expressed as percentages of those using buffered NaCl prewash (Perrott and Kerr, 1994).

(Typic vitrandept) collected 3 months after surface application of North Carolina PR. He found that soil prewash with unbuffered NaCl solution gave only 8% less acid extractable P than a prewash with EDTA buffer (Fig. 6). It is possible that a higher soil pH of 5.8 resulted in a smaller loss in acid extractable P in the unbuffered solution. Most authors have used H2S04 to extract Ca-P although Williams et al., (1967) and Syers er al. (1972) have used HCI. Tambunan et al. (1993) reported that, for reasons they could not explain, the recovery of PR residues using up to 4 M HCI solution was less than that when 0.5 or I M H2S04 was used and thus H,S04 is a preferred option. While the sequential extraction procedure to determine the extent of PR dissolution has generally been satisfactory, it is too lengthy for routine testing. They also require a sample of unfertilized soil to enable the calculation of the remaining PR. Perrott and Wise ( 1 995) proposed a simpler procedure to determine PR residues remaining in soil. A mild acid extraction (acetate buffer) was used to differentiate between native soil fluorapatite-P and PR-P. The procedure consists of extracting P from two subsamples as follows: the first subsample is shaken with an acetate solution of pH 4 for a specified time after which a NaOH/citrate

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS .c0

89

Y=0.08 + 1.08 X, R2=0.963

$ , -ool

$

100

300 500 700 Acid-P, NaCl prewash (rng kg-l soil)

Figure 6 Acid-extractable P of soil samples collected from Sechura PR treated plots prewashed with either NaCl or buffered EDTA solution (soil pH 5.75) (S.S.S. Rajan, unpublished data, 1992).

solution is added. The suspension is shaken for a further specified time and centrifuged, and P is determined in the supernatant solution. A second subsample is similarly treated except that the intial shaking is with a borate solution of pH 8. The PR-P is calculated by subtracting the amount of P extracted in the second subsample from that of the first subsample. Their results averaged over 1 1 different soils from New Zealand showed a very significant (0.1% level) variation of PR-P recovery with rock reactivity (Table I). The recovery of PR-P from Sechura and North Carolina PRs was complete, which illustrates the usefulness of this method with highly reactive PRs. The recovery with medium reactive (and also the unreactive PR) PR was less than that present in the soil. Soil types had no significant influence on the recovery of

Table I Percentage of PR-P Recovered from PR-Soil Mixtures (Values Averaged across 11 Soils) Phosphate rock

Recovery (96 of added PR)

Sechura North Carolina Arad Egyptian Florida SED

93.7 101.0 80.9 77.3 37.5 4.8***

90

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

the two most reactive PRs, although when the recovery values were averaged across the five PRs, soils did have a significant effect (1 % level).

2. Measurement of Phosphate Rock Dissolved from ANaOH-P Measurement of PR dissolution in soil from the increase in the amount of NaOH-extractable P determined after prewashing soil to remove exchangeable Ca was proposed by Mackay et al. (1986). The underlying principle behind this method is that P released from PR largely forms complexes with soil Fe and A1 which are extracted by NaOH solution (usually 0.5 M . Since apatite P does not dissolve to any significant extent in this reagent (Williams, 1937) the method provides a direct estimate of the PR dissolved in soil. It has the advantage of needing only two instead of three extractions as in the fractionation procedure. Bolan and Hedley (1989) concluded that this method is suitable for use in incubation studies where the reaction products are not removed and where there is no significant active net mineralization or immobilization of P. This method has been used extensively in short-term incubation studies. The usefulness of this method has not been investigated for long term incubation studies where the sorbed P may be converted from NaOH extractable to occluded forms of P (Hagin et al., 1990).

3. Measurement of Phosphate Rock Dissolved from ACa PR dissolution has also been estimated from the increase in the exchangeable Ca (ACa) content of the PR treated soil over that of the control. In the ACa method it is assumed that the Ca released on dissolution of PR accumulates in the soil as exchangeable Ca which is extracted with appropriate electrolyte solutions. Khasawneh (quoted in Khasawneh and Doll, 1978) used neutral 1 M NH,OAC to determine the increase in exchangeable Ca in the PR-treated soil over the control soil. Smyth and Sanchez (1982) measured PR dissolution using a 1 M KCl solution. As pointed out in a previous section, use of unbuffered solutions could dissolve PR during the extraction because of the release of exchangeable acidity from the soil (Hughes and Gilkes, 1984). These authors therefore advocated the use of BaCI, solution which has been buffered to an alkaline pH. Bolan and Hedley (1989b) reported poorer recovery of Ca by NH,OAC and the recovery decreased as the pH increased. The ACa method is the simplest of the three techniques. However, it is not suitable for use in greenhouse, field, or open incubation studies where the Ca released is removed by plants or by leaching. In closed incubation studies also, overestimation of PR dissolution will result where the PR contains appreciable amounts of free CaCO, because of its preferential dissolution and hence increase in the measured exchangeable Ca.

PHOSPHATE ROCKS FOR DIRECT APPLICATION T O SOILS

91

B. MEASUREMENT IN CALCAREOUS SOILS Although not commonly found, there are situations where PR application to calcareous soils could be plant effective (Edwards, 1956; Singaram et al., 1995). Measurement of PR dissolution in calcareous soils is made complicated by the dissolved P forming not only Fe-P and Al-P but also Ca-P (Holford et al., 1975; Hooker et al., 1980). Unlike methods for noncalcareous soils where Ca-P is treated as one component, methods aimed at measuring PR remaining in calcareous soils should distinguish between the calcium apatite applied as PR and the Ca-P formed after reaction with soil components. Reports indicate that at relatively low solution P concentrations (< 10 mg liter-'), which is far higher than would exist at the PR/soil interface, the Ca-P is probably present as adsorbed P on CaCO, and as Ca-P complexes formed with the exchangeable soil Ca (see Sample et al., 1980). Baifan and Yichu (1989) proposed an inorganic P fractionation scheme which included separation of dicalcium, octacalcium and apatite P. They used sequential extractions with NaHCO, (pH 8.5), NH,Ac (pH 7.0), NaOH plus Na,CO,, and H,SO,. Fractionation schemes similar to the above may be appropriate for PR measurement in calcareous soils but are yet to be investigated.

rV. FACTORS AFFECTING PHOSPHATE ROCK DISSOLUTION IN SOIL AND AVAILABILITY TO PLANTS

Several factors affect the rate of PR dissolution in soil and its availability to plants. The availability of PR-P to plants largely depends on its rate of dissolution. However, this is not always so because of the influence of soil characteristics, the plant and fertilizer management factors. This section reviews various factors influencing the rate of PR-P release and its availability to plants.

A. FACTORS AFFECTING RUE OF P RELEASEFROM PHOSPHATE ROCKAPPLIED TO SOIL 1. PR Properties

Two important properties determining the rate of PR dissolution in a given soil are chemical composition, which includes apatite lattice composition and the type of accessory materials, and particle size (Section 11).

92

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

PR deposits fall into three broad classes based upon their mineral assemblages in order of their increasing economic importance. These are Fe-A1 phosphates, Ca-Fe-Al phosphates, and Ca phosphates (McClellan and Gremillion, 1980). Commercial mineral Ca phosphates belong to the group of apatite minerals which are similar in crystal structure to fluorapatites but vary significantly in chemical composition. The Ca apatites of sedimentary origin have generally been found to be suitable for direct application as phosphate fertilizers. It has been well established that increasing substitution of C032- for PO,3- in the lattice structure increases the solubility of carbonate apatites. This occurs through decreased unit cell a-dimension and crystal instability on increased incorporation of planar CO,2- and F- for PO,,- tetrahedra (Lehr and McClellan, 1972; Chien, 1977). Unit cell a-dimensions in turn have been found to be closely correlated with the chemical extractability of P from PRs (Figs. 7A-7C) (Dash et al., 1988; McClellan and van Kauwenberg, 1992). For details on the chemistry of isomorphic substitution and its effect on crystallite properties the readers are referred to the review by Khasawneh and Doll (1978). There is a scarcity of studies relating directly measured dissolution of PRs in soil with the chemical composition of the PRs, under conditions where the reactant products were removed from the PR-soil interface. However, numerous reports have been published measuring PR dissolution indirectly as plant P uptake. Chien et a/. ( 1987b) determined residual PR remaining in a Columbian Oxisol 5 years after application. Six PRs (carbonate apatites) of varying citrate solubility were applied and the inorganic P fractionation of Chang and Jackson (1957) was employed to determine the PR remaining. The apatite dissolved, calculated as a difference between the PR applied and that remaining, ranged from 79 to 98% of that applied and there was a positive correlation between the PR dissolved and the citrate-soluble P of the PRs (Fig. 8). Rajan (1987b) reported that in 1 year after surface application to permanent pastures 27% of Florida PR (low carbonate substitution) dissolved compared with 42% for a North Carolina PR (highly carbonate substituted). Several publications provide evidence of a close positive correlation between increasing carbonate substitution in the lattice structure, determined by direct physical and chemical measurements or as indicated by chemical extractable P, and PR dissolution as measured by crop P uptake (Mackay et al., 1984a,b; Anderson et al., 1985; Leon et al., 1986; Dash et al., 1988; Rajan et al., 1992). Calcium carbonate is the most abundant accessory mineral in PRs. Because CaCO, is more soluble than the most chemically reactive apatites (Silverman et al., 1951) and since its dissolution increases the Ca concentration and pH at the apatite surface it is not surprising that accessory CaCO, can reduce the rate of PR dissolution in some soils (Anderson et al., 1985; Robinson et al., 1992a). How-

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

0 141

2{ 0

9.32

93

1

B

,

,

9.34

,

,

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9.36

9.38

Unit-ceIP Figure 7 Relationship between unit-cell a dimension of apatite sample and solubility of P in (A) neutral ammonium citrate. (B) citric acid, and (C)formic acid (McClellan and Van Kauwenbergh, 1992).

ever, under field conditions where Ca may be removed by plant uptake and or leaching this effect will be minimized. Because PRs are relatively insoluble materials their geometric surface area

94

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

0

0,5 1.0 1.5 2.0 2.5 3.0 Citrate-soluble P (“Aof PR)

Hgure 8 Percentage of PR-P dissolved 5 years after application as related to their solubility in ammonium citrate (Chien er a / ., 1987).

will have an important bearing on their rate of dissolution in soil (Section 11). Thus the finer the particle size, the greater the degree of contact between PR and soil and therefore the greater the rate of PR dissolution, provided the PR application rate is such that the zones of PR dissolution between the particles do not overlap. The results of Kanabo and Gilkes ( 1 988c) support the above reasoning. These authors conducted a laboratory incubation study in a lateritic podzolic soil to estimate the dissolution of North Carolina PR ground to four size fractions ranging from 0.15-0.25 mm to <0.04 mm. Their results show that PR dissolution increased with decreasing particle size, down to the smallest size range used. Indirect evidence of greater dissolution of PRs after fine grinding has been presented by researchers working on permanent pastures. Sinclair et al. (1993a) compared unground Sechura and North Carolina PRs with TSP on ryegrass/ clover pastures at four sites over a 6-year period. Soil pH values were less than 6 and the fertilizers were surface-applied annually. Pasture dry matter production results showed that the performances of both PRs were inferior initially and became similar to TSP in Year 6 (Sinclair er a1 ., 1993a). On the other hand reactive PRs, including Sechura and North Carolina, when applied after grinding either in powder or pelletized form, were found to be generally as effective as SSP from the year of application (Mackay et al., 1984a; Rajan, 1987; Rajan et al., 1987; Gregg el al., 1988).

2. Soil Properties a. Soil pH and Titratable Acidity Dissolution of PR may be expressed by the simple equation Ca,,(P0J6F,

+ 12H20+ 10CaZ++ 6H2P0,- + 2F- + 120H-.

(3)

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

95

Although the above reaction is for fluorapatite it applies to other members of the apatite minerals including reactive PRs (francolite). As indicated in Eq. (3), hydrolysis of the dissolving PO$ releases OH- into solution. Neutralization by soil acidity of the OH- released enables continuation of the PR dissolution process. In the case of PRs with isomorphically substituted ions, hydrogen ions may also be needed to neutralize OH- from Cog- hydrolysis and structural OHof PR released into solution (Chien, 1977). At equilibrium, [Ca2+][H2P0g]6 [F-I2 - -Ksp [HI l2 Kw'2

'

(4)

Thus with increasing proton supply one would expect greater dissolution of PR provided the reaction products are removed. A comprehensive summary of the relative influence of the various soil and plant factors presented by Kirk and Nye (1986b) shows that PR dissolution is very sensitive to changes in soil pH (Figs. 9A,B). Indeed early studies established the positive effect of soil acidity on PR dissolution (Ellis et al., 1955; Peaslee et al., 1962; Barnes and Kamprath, 1975). In these studies, however, the pH of soil or plant-growing mediums was adjusted by applying calcium compounds and thus no distinction could be made between the effect of proton concentration per se and that of Ca ions. Khasawneh (1977) (quoted in Khasawneh and Doll, 1978) separated the effect of these two parameters by adjusting soil pH either with CaCO, or SrCO,. The agronomic effectiveness of corn grown in pots was used as an index of the solubility of North Carolina PR. They reported an additional decrease (due to Ca) in the effectiveness of the PR with increasing pH in soils where the pH was adjusted with CaCO,. Kanabo and Gilkes (1987~)investigated the influence of soil pH, as measured in 0.01 M CaCI,, on the dissolution of North Carolina PR in a lateritic soil. The pH was adjusted by incubating soil samples with water alone, HCl, or solid SrCO,, to give a pH range of 3.73-6.83. The increase in exchangeable Ca (ACa method) was used as an index of the amount of PR dissolved. They found that the PR dissolved (ACa) correlated with increasing soil pH: log ACa = a - bpH. Bolan and Hedley (1 990) studied the effect of soil pH on the dissolution of three different PRs: highly reactive North Carolina, medium reactive Jordan, and unreactive Nauru PRs. They used a volcanic ash soil, the pH of which was adjusted to give a range from pH(H,O) 3.9 to 6.5 by treating either with dilute HCI or NaOH. The extent of PR dissolution, after 84 days incubation, was determined from the increase in the amount of 0.5 M NaOH extractable P in the PR treated soil over the control soil. The amount of PR dissolved, expressed either as increase in NaOH extractable P or as a proportion of that added, was

96

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

r"

0 '

S.V.

Figure 9 The effects of the most important variables on the fraction of PR (A) dissolved, and (B) that has been taken up by roots after 80 days. Standard values as in Kirk and Nye (1986b). The ranges in values are: pH (soil pH), 4.25-5.25; AH(root acidity), O-lO-y mol dm-, soil s-1; pCa (Caz+ activity), 2-4; b,, (pH buffer capacity), 0.035-0.14 mol dm-3 soil pH-1; L, (root density), 100-1500 dm d w 3 ; b, (phosphate buffer capacity as Freundlich a), 0.07-0.28 mol dm-3 soil; N (application rate), 0.08-0.32 kg P m - j soil; a, (particle size), 0.05-0.2 mm;AHCO, (bicarbonate in soil solution), 0-5 X I O - ' O mol dm-2 soil s-I; p, degree of carbonate substitation in the PR (Kirk and Nye, 1986b).

found to be almost linearly related to soil pH (Fig. 10). In addition to enhancing neutralization of the OH- released, an increase in soil acidity can increase the P adsorptive capacity of soils with pH dependent charges This can also increase PR dissolution by removing P released from the PR (Bolan and Hedley, 1990). Rajan et al.(1991b) studied the effect of soil pH, adjusted by applying either HCI or Ca(OH),, on the dissolution of Sechura PR under field conditions. The amount of PR-P dissolved, expressed as a fraction of that applied, decreased either exponentially or linearly with increasing soil pH. Soil pH is an intensity measurement and gives the instantaneous concentration of H + in the soil solution. A measurement of the ability of soil to supply H + or to remove OH- from the soil solution is the pH buffer capacity of the soil or

--

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

.-

A

5

-

97

B

'0°1

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Y

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E

v

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lu

+d

0

e

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9

200-

H

I

z

r u!

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,,

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. 4.5

+

A

5.5

t

6.5

a

20-

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KH2P04 NCPR A Jordan 0 Nauru A Control

3.5

4.5

5.5

I

6.5

Soil pH (1 : 2.5 soil : water) Figure 10 Effect of soil pH on the dissolution of PRs in a New Zealand soil (Typic Dystrandept) after 12 weeks of incubation (Bolan and Hedley, 1990).

titratable acidity. Soil buffer capacity has been estimated as the differential of the soil acidity neutralization curves, &equivalent of OH added)ld(pH), after equilibration of soil for 48 h with Ca(OH), (Nye and Tinker, 1977). The pH is measured in 0.01 M CaCI, medium or water (Anderson et al., 1985; Anderson and Sale, 1993). Some researchers defined buffer capacity as the amount of OHrequired per unit of soil to raise its pH from the initial pH to an arbitrarily chosen pH of 6 (Kanabo and Gilkes, 1987d). The pH was measured in 0.1 M KCl medium. It is noteworthy that the relationship between pH and acid or alkali added is nearly linear in the pH range of most agricultural soils (pH 4.5-6.5) (Magdoff and Bartlett, 1985). Anderson et al. (1985) studied the influence of some chemical characteristics of 18 soils on the P release from four PR materials in a glasshouse experiment. They concluded that no single soil characteristic (Solution P, Solution Ca, pH, pH buffer capacity) had a consistent and predominant effect on P release. However, they found that the pH buffer capacity (Nye and Tinker, 1977) was nearly twice as important as any other soil parameter in the case of Huila PR containing 7.9% of free carbonate, but not with Sechura PR which contained a negligible amount . In Western Australian soils a linear relationship was reported between initial soil pH and titratable acidity (Kanabo and Gilkes, 1987~).Under such conditions

98

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

initial soil pH alone was a good predictor of PR dissolution. However, when a large number of soils (228 soils) of highly variable properties was used, stepwise regression indicated that titratable acidity accounted for only 35% of the variance (Hughes and Gilkes, 1994). It is noteworthy that the sensitivity studies of Kirk and Nye (1986b) indicate only a small influence of pH buffer capacity on PR dissolution (Fig. 9). b. Calcium in Soil Solution and Soil Exchangeable Calcium Following the law of mass action, the dissolution of PR [Eq. (311 is favored as long as the Ca concentration in soil solution is maintained at a lower level than that in the film surrounding the dissolving PR particle, and provided the ionic products of the reactants do not exceed the solubility product. A sensitivity analysis of the Kirk and Nye model (Kirk and Nye, 1986b) shows that the rate of PR dissolution is highly sensitive to Ca2+ activity in solution (Fig. 9). Wilson and Ellis (1984) studied in detail the influence of solution Ca activity on the dissolution of six PRs of a range of reactivity. They reported a linear relationship between the log of Ca ion activity and log P in soil solution. Similar results have also been reported by Robinson and Syers (1991). The solubility product relation requires that the Ca of the soil plays a role independent of pH, and needs to be considered in evaluating PRs. The concentration of Ca2+ in soil solution is largely influenced, in addition to other factors, by the cation exchange capacity of the soil and the degree of Ca2+ saturation of the exchange sites (Barber, 1984). Percentage Ca2+ saturation of the cation exchange complex has been identified as an important soil parameter affecting PR dissolution (Mackay et al., 1986). To be precise, the important factor is the amount of the cation exchange sites available to adsorb the Ca2+released from PR (CEC minus initial exchangeable Ca (Bolan et al., 1990; Robinson et al., 199 I , 1992). Analogous to pH buffering capacity, we propose that Ca buffering capacity be used to characterize the relationship between soil-exchangeable Ca2+ and Ca2+ in solution. The Ca buffering capacity will be the differential of the curve relating exchangeable Ca2+ to Ca2+ in solution (dCa2+exchldCa2+so,n). The greater the gradient, the greater the ability of soil to continue to dissolve PR. c. Phosphate in Soil Solution and Phosphate Buffering Capacity The reaction given in Eq. (3) shows that if the ionic product exceeds the solubility product of PR, the dissolution of PR will not proceed. Phosphate in soil solution is very low in agricultural soils (on the order of 10-5 M ) and any small fluctuations in absolute concentration, as under field conditions, will have less effect on the ionic product than Ca2+ (on the order of M).It has also been shown that compared to phosphate, Ca2+ has a much stronger retarding effect on the dissolution of PR than is expected from the influence of Ca2+ on the ionic product (Christoffersen and Christoffersen, 1979, 1982). A sensitivity analysis of the Kirk and Nye model (Fig. 9) shows that P buffer-

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

99

ing capacity has small influence on the rate of PR dissolution (Kirk and Nye, 1986b; Anderson and Sale, 1993). Nevertheless increasing P sorption capacities of soils have been positively correlated with increasing PR dissolution (Chien er al., 1980; Smyth and Sanchez, 1982; Mackay et al., 1986; Syers and Mackay, 1986). It is implicit in these results that it is not the absolute P adsorption capacity per se that was affecting PR dissolution, but rather the number of sites available to adsorb the P released from PR (P buffering capacity) and therefore maintain a lower P concentration in solution at the interface. Utilizing a technique similar to that described by Bolan et al. (1983), Kanabo and Gilkes (1987) increased the P buffering capacity of a lateritic podzolic soil by incorporating synthetic goethite into it. They found that the dissolution of ground North Carolina PR increased linearly with increasing P buffering capacity, the latter expressed as P-sorption maximum (Fig. 11). However, it needs to be mentioned that the H+ produced by ongoing Fe3+ hydrolysis may also be responsible for increased dissolution. Consistent with pH and Ca buffering capacity, we support the use of P buffering capacity defined as dP,,,ldP,,,, to estimate the effect of P sorbing properties of soil on PR dissolution. The Freundlich equation constant has been used for this purpose but the fit may not be linear at low solution concentrations unless soil P that equilibrates with solution is accounted for (Kirk and Nye, 1985). This can be estimated from the “a” value determined using isotopically labeled P (McAuliffe et al., 1947). d. Relative Importance of Soil Acidity, Soil Ca, and Soil P on PR Dissolution Considering PR dissolution as a simple chemical process, for the reaction to proceed, availability of the reactants (PR, H,O and H + ) and the removal of

-.

200 400 600 800 P-sorption maximum (mg kg-’ soil)

0

Figure 11 Effect of soil P sorption maximum, adjusted by addition of goethite, cn dissolution (AP)of North Carolina PR. Incubation times 0 (O), I (O), 7 (A), and 35 (A) days (Kanabo and Gilkes, 1987a).

100

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

reaction products (Ca, P, and other ions) from the point source of dissolution are of prime importance. Discussion on the greater importance of one factor over another (Robinson and Syers 1992b; Wright et al., 1992) appears to arise from the differences in soil chemical characteristics favoring the supply of the reactant (mainly H+) or removal of one or more reaction products. The situation can be seen as analogous to the limited availability of a particular major nutrient in a specific soil, and therefore a greater attention to that nutrient, although all major nutrients are of equal importance for plant growth. Generalizing on the relative importance of the various factors, the proton supply in acid soils should be adequate to dissolve PR applied at normally recommended rates. For example, titratable acidity between pH 5 and 6 in the top 10 cm of New Zealand soils is sufficient to dissolve between 2.3 and 7.8 t of North Carolina PR per hectare (300-1000 kg P ha-'), provided moisture is not limiting (Bolan et al., 1990). Even the lateritic soils of Western Australia, where PR application is not recommended (Bolland et al., 1988b), have sufficient titratable acidity to dissolve 0.8 t of PR (100 kg P ha-'). Mobilities of H + and OH- ions are also far greater than those of the other ions under consideration. Regarding removal of P from the surface of PR particles, the reaction of P with soil colloids and the high concentration gradient in P from the PR surface to the bulk soil solution generally results in effective removal of this ion on release from PR (Section 11). For example the P concentration at the surface of a dissolving North Carolina PR particle at a constant pH of 5.5 is about 8 mg liter-' (J. H. Watkinson, personal communication), whereas phosphate concentration in soil solutions is in the order of 0.2 mg liter-1. In comparison the Ca concentration near a dissolving PR particle is 40 mg liter-' and calcium in soil solution is on the order of 8-25 mg liter-' (Gillman and Bell, 1978; Edmeades et al., 1985). Because of the smaller Ca concentration gradient at the PR surface, the Ca buffering capacity of soil may appear to be more important in influencing PR dissolution. e . Effect of Organic Matter The positive influence of organic matter on PR dissolution has long been recognized (Johnston, 1952, 1954a,b; Drake, 1965). This seems to arise from the (i) high cation exchange capacity of organic matter and (ii) organic acids produced as a result of microbial and chemical transformations of organic debris. The cation exchange capacity of mineral soils, depending on their clay content, may range from a few to 50 or 60 cmol kg-' , whereas that of organic matter may exceed 200 cmol kg-I (Helling et al., 1964). Thus organic matter can enhance PR dissolution by enhancing the Ca buffer capacity of soils. Numerous organic acids (e.g., oxalic, citric, tartaric, gluconic) have been reported to be produced in soils as a result of microbial and chemical transformations of organic debris. Johnston (1952, 1954a,b, 1959) conducted comprehen-

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

101

sive studies on the effect of organic acids on the dissolution of Ca-phosphates, and also Fe- and Al-phosphates. He concluded that the organic acids dissolved the phosphates not only by supplying protons but also by complexing the cations. Kpomblekou and Tabatabai (1994)compared the ability of organic and mineral acids to dissolve P from two PRs: North Florida and Kodjari PRs. They concluded that organic acids dissolved more P than was accounted for by their proton supply, and suggested chelation of the metals associated with P in the PRs. Chien (1979)studying the effect of urea on North Carolina PR dissolution in two soils of contrasting organic matter presented direct evidence of the Ca chelating effect of hydrolyzed soil organic matter. He concluded that urea mixed with soil hydrolyzed the organic matter and the products of hydrolysis chelated Ca ions and enhanced the dissolution of the PR. Incorporation of Mussoorie PR (Indian PR) into compost has been reported to improve the release of P, suggesting the occurrence of reactions similar to those in organic soils (Bangar et a l ., 1985). Evidence of soil organic matter enhancing the availability of Sechura PR to plants relative to TSP, which implies a greater dissolution of the PR, was presented by Chien et al. (1990)(Fig. 12). In this pot trial study two soils, both Ultisols, of the same pH (pH 4 . Q exchangeable Ca and P sorption capacity were used. However, one soil contained lower (1.8%) and the other higher (4.2%) organic matter. The greater dissolution of PR in the soil containing the higher

h c)

50

0 P

3 40

lA

t 10

2 n

0

I

0

100

200

300

400 0

100

200

300

1

400

P applied (mg P kg-l soil) Figure 12 Effect of soil organic matter on the agronomic effectiveness of Sechura PR in relation to TSP; (A) low organic matter soil, (B) high organic matter soil (source: Chien er a / . , 1990).

102

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

level of organic matter could also been due to indirect effects of organic matter such as greater cation exchange capacity and high calcium buffering capacity.

3. Site Factors a. Soil Moisture In the dissolution of PR, the rate limiting process is diffusion of the dissolved products (Ca, P, bases) away from the surface of PR particles (Kirk and Nye, 1986a,b). Increased soil moisture levels can increase the apparent diffusion coefficient of the ions by reducing the tortuosity of the diffusion path and increasing the cross-sectional area of diffusion (Nye, 1979). Therefore it is expected that increasing soil moisture, whether effected by rainfall or irrigation, will increase the dissolution of PR. Kanabo and Gilkes (1987a) found from incubation studies that the water retained at field capacity was sufficient to support near potential maximum dissolution of ground North Carolina PR in a lateritic podzolic soil from Western Australia. Weil ef al. (1994) investigated the effect of soil moisture on the dissolution of North Carolina PR in closed incubation and open plant-soil systems. Their incubation study results on two volcanic ash soils (Typic Vitrandept) showed that PR dissolution increased with increasing moisture of up to 80% of field capacity in a medium P retentive soil (P retention 79%; Saunders, 1965) and up to field capacity in another soil of higher P retention (P retention 91%). Field capacity moisture, measured at 700 mm water tension, was 75% on an oven dry basis for the lower and 66% for the higher P retentive soil. Indirect evidence exists of increasing PR dissolution with increasing moisture supply under field conditions. As early as the 1950s, data obtained in Senegal, Africa, showed that the percentage yield increase (over a control) of groundnut and cereal, resulting from application of PR, correlated linearly with the mean annual rainfall which ranged from 500 to 1300 mm (Hammond et al., 1986). In acid soils of Shillong, India, potato responded better to PR in a wet year than in a dry year (La1 et al., quoted in Tandon, 1987). Such results, however, need to be interpreted with caution because low moisture levels, corresponding to water stress to plants, could also reduce the effectiveness of soluble P fertilizers largely because of reduction in maximum yield potentials (Bolland, 1994). Soils with low P sorption, where the rainfall is also high, will promote PR dissolution by facilitating the removal of the products of dissolution in contact with the PR particles(Sanchez, 1976). But from the point of plant availability this may be an advantage if P is not leached below the rooting zone. b. Temperature There is a paucity of experimental data measuring directly the influence of temperature on the dissolution of PR applied to soil. Smith et al. (1977) mea-

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

103

sured the rate of dissolution of hydroxyapatite in dilute HCI as influenced by temperature. They found that the rate constant increased in an exponential manner with temperature for Arrhenius-type temperature dependence. Extrapolating the results to soil conditions one would expect PR dissolution rates to increase with increasing soil temperature, provided other factors are not limiting. Contrastingly, the data of Watkinson (J.H. Watkinson, unpublished data) show a negligible change in the solubility of PRs within the temperature range 5-35°C.

4. Plant Effects There are marked differences in the ability of plant species to extract P from PRs, which was recognized as early as 1898 (Merrill, quoted in Flach et al., 1987). Plants can influence the rate of PR dissolution by the following processes: (i) secretion of acid or alkali, (ii) uptake of large quantities of Ca, (iii) production of chelating organic acids (citric, malic, and 2-ketogluconic acids) which complex Ca, and (iv) depletion of P in soil solution. A wealth of literature is available on the root-induced pH changes in the rhizosphere. The causes of the pH changes are attributed to the imbalance in the proportion of the anionic (usually NO3-, H,PO4-, SO;-, and C1-) and cationic nutrient ( K + , Ca2+, Mg2+, and Na+) intake by the plants (Van Ray and Van Diest, 1979; Aguilar and van Diest, 1981; Bekele, 1983; Haynes, 1983, 1992; Nye, 1981b, 1986; Hedley et al., 1982, 1983; Moorby et al., 1988; Gahoonia, 1992a,b). If the equivalent sum of cation uptake exceeds that for anions, the plants release H+ to maintain electrical neutrality across the root-soil interface and the pH of the rhizosphere soil decreases (Fig. 13). Conversely, if the sum of anions within the plants exceeds those of cations, a net efflux of OH- and/or HC03- occurs, increasing the pH of rhizosphere (Haynes, 1992). Proton secretion is greater for legumes which accumulate nitrogen through symbiotic nitrogen fixation. Because uncharged N, molecules are a major source of plant N, there is an excess uptake of cations over anions. De Swart and Van Diest (1987) reported that about 0.50 mM of acid is excreted by Pueruriu juvmica per nM of N, fixed. In the case of plant species which depend on soil and fertilizer nitrogen, uptake of nitrogen as NH4+ results in H+ secretion whereas uptake of nitrate results in secretion of OH-. Increased soil acidity in the rhizosphere can enhance PR dissolution. This has been observed directly as increased PR dissolution (Gahoonia et al., 1992; Haynes, 1992) but often indirectly as increased P uptake by those plants which acidify the rhizosphere (Van Ray and Van Diest, 1979; Bekele, 1983; Haynes, 1983; Nye, 1981, 1986; Hedley et ul., 1982, 1983; Moorby et al., 1988; Gahoonia et al., 1992). Gahoonia et ul. (1992) measured the HCl soluble P, a measure of calcium apatite P remaining, in the rhizosphere of rye grass. They found a greater amount of apatite P remaining in the soil supplied with NO3-N compared

104

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

0 TSP

0 AI-PR

0 Apatitic-PR 4.81

@

8HI 4 . 4 . r

c ..-

c

5

W - Wheat P Paspalurn grass M Maize L Molasses grass

-

B - Buckwheat

4.44.24.0-

1 2 Total equivalents cations absorbed Total equivalents anions absorbed

0

Figure 13 pH of the soil as influenced by the ratio of the sum of equivalents of cations to the sum of equivalents of anions absorbed by plants (Van Ray and Van Diest, 1979).

with that supplied with NH,-N. (Fig. 14). The limited results of Haynes (1992) suggest a greater dissolution of Jordan PR (a medium reactive PR) by lupin (Lupinus angustifolius L), which decreased rhizosphere soil pH, than by barley (Hordeum vulgare L), which increased the rhizosphere soil pH. Since PRs are essentially calcium phosphate compounds, removal of calcium from PR will release P into solution. Thus effective utilization of PR by some plant species (e.g., buckwheat and rape) has been attributed to their high Ca uptake (Bekele, 1983; Bekele and Hofner, 1993; Van Ray and Van Diest, 1979). Flach et al. (1987) determined the abilities of maize (Zea mays),pearl millet (Pennisetum typhoides), and finger millet (Eleusine coracana) to utilize P from a Mexican (Zimapa PR) and Moroccan (Khouribga) PR in a pot experiment. They concluded that finger millet utilized most P from the PRs, followed by pearl millet and then maize. This was attributed to a greater Ca uptake by finger millet. The dry matter yields of finger millet were about twice those of maize, but the Ca uptake was about three to five times greater. The greater ability of finger millet than maize to utilize a Mussoorie PR from India has also been reported by Singaram et al. (1995). Plant roots may also enhance PR dissolution by secreting organic acids which can be expected to lower rhizosphere soil pH, some of which also complex the Ca of the PR (Moghimi and Tate, 1978; Hoffland et al., 1989). Hoffland et al. (1992) attempted to quantify the possible effect of organic acid exudation on phosphate uptake from Mali PR and measured the exudation of malic and citric acids from P-deficient rape plants (Brassica napus L). They concluded that rape

PHOSPHATE ROCKS FOR DIRECT APPLICATION T O SOILS

”

105

1

0

1 2 3 Distance from the root (mm)

Figure 14 Effect of N source (NO,-N, Nil-N, or NH,-N) on depletion of acid-soluble P (apatite

P)determined in P fractionation procedure in the rhizosphere of ryegrass grown for 10 days on a soil fertilized with ground PR for 10 years (Gahoonia et a / . , 1992).

plants increased P uptake from PR through reduction in the rhizosphere pH and also by complexation of the Ca with organic acids. Thus, Ca2+ concentration in the soil solution was reduced. Simulation calculations indicated that the exudation rates can provide the roots with more phosphate than is usually taken up. From in vitro studies evidence has been presented of organic acid secretion by Rhizobium and Brudyrhizobium strains (Halder et ul., 1990). These authors reported that the 2-ketogluconic acid secreted by the cultures was the primary factor influencing the dissolution of Mussoorie PR, implicitly through complexing of the dissolved Ca component of PR, while pH per se was less important.

5. Method and Rate of Application a. Method of Application Increasing the area of contact of PR with soil will enhance PR dissolution by removal of dissolution products and supplying H+ . Therefore one would expect a greater dissolution of PR when PRs are mixed with soil rather than applied as a band. Banding will also enhance dissolution zones of PR particles overlapping and thus hinder continual dissolution. This has been observed indirectly as increased P uptake by subterranean clover (Trifolium subterraneum) with increasing depth of mixing PR with soil (Alston and Chin, 1974). Similarly Purnomo and Black ( 1994) reported from a greenhouse study that the dry matter yields of

106

S. S. S. RAJAN, J. H. WATKINSON,AND A. G. SINCLAIR

wheat from North Carolina PR were in the order of mixed > broadcast > banded. A greater dissolution of North Carolina PR mixed with soil compared with banded PR has also been found in laboratory incubation studies (Kanabo and Gilkes, 1988b). b. Rate of Application Increasing the rate of PR application will eventually result in PR particles being so close that the zones of dissolved Ca and P ions overlap, resulting in a slower rate of PR dissolution. Such an effect can be expected at lower rates of application when the PR is added in clumps or is surface applied. Also, the effect of application rate on PR dissolution will be accentuated in the presence of plants. This is expected because the influence of roots diminishes with increasing rates of application, brought about as a consequence of the root system affecting a smaller fraction of PR (Kirk and Nye, 1986b). Experimental evidence under laboratory (Kanabo and Gilkes, 1988d) and field conditions of decreased proportion of PR dissolution has been presented (Bolland and Barrow, 1988; Rajan et al., 1991) (Fig. 15). In the latter study Sechura PR was surface applied to a permanent pasture. It needs to be emphasized that when calculated as a fraction of that applied, PR dissolved may decrease at high rates of application. In absolute amounts, however, the PR dissolved will generally increase (Kanabo and Gilkes, 1988; Rajan et al., 1991) with increasing rates of application up to the point at which the ionic product equals the solubility product of PRs, at which time PR dissolution will cease.

Y4.47-0.36 log x, RC0.90' (6 years) Yd.39-0.28 log x, R2=0.81NS (4 years) Y=O.89-0.21 log x, R2=0.94' (3 years) 1

0

100 200 300 PR added (kg P ha-')

Figure 15 Effect of rate of application on the fraction of Sechura PR dissolved 3,4, and 6 years after application (Rajan et a!., 1991b).

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107

B. FACTORS AFFECTINGPLANT AVAILABILITY OF P FROM DISSOLVED PHOSPHATE ROCK Phosphate availability to plants in a soil is governed by concentration of P in soil solution, the sustainability of this concentration on absorption of P by plants, and the ability of crops to utilize the phosphorus. In general, increased PR dissolution is expected to result in a measurable increase in soil solution P and therefore increased plant production (Rajan et al., 1991a,b), but this is not always the case because of the influence of soil, crop, and fertilizer management factors. An example of this is illustrated in Fig. 16 (Syers and McKay, 1986), where P uptake by ryegrass was not related to PR dissolved. In the following section we discuss the factors which influence the availability of PR-P.

1. Soil Factors a. Phosphate Buffering Capacity and P Status Soil P sorption capacity has been identified as an important parameter influencing the availability of dissolved PR-P to plants (Smyth and Sanchez, 1982; Hammond et al., 1986a; Syers and Mackay, 1986). Stated differently it is the fraction of vacant sites in relation to the absolute P sorption capacity that will influence the solution P concentration and therefore P availability to plants (Rennie and McKercher, 1959; Rajan, 1973). The greater this fraction, the smaller the

$

0

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? = 0.003

7

-E"I E

p

2

25-

15-

B 10a r

0

al 5 x 0

0

20-

0

4 0 0

0 0

0

c

4 on

1

PR-P dissolved (mg kg-l soil)

Figure 16 Relationship between P uptake (PR treated - control) by ryegrass and Sechura PR dissolved when added to nine soils at 500 mg P kg-I of soil in a greenhouse experiment (Syers and Mackay, 1986).

108

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

solution P concentration. Also, the greater the P buffering capacity, the slower the rate of change in solution P concentration. As stated previously, low solution concentrations of P favor PR dissolution, but the dissolved P will be immediately adsorbed by soil colloids. Three scenarios can be visualized in such circumstances. First, if the solution P level is below the threshold concentration required for P uptake by plants, very little increase in P uptake or yield will be observed in spite of PR dissolution. This will be the case in soils of low P status. In such soils a sigmoid yield response curve for fertilizer application would be expected. A second situation is that in which the P concentration is above the threshold level. However, considering a curved relationship between P sorbed and that remaining in solution, one will find less than a proportionate increase in solution P with increasing PR-P released and subsequently adsorbed by soil. Thus the available P will still be much less than that dissolved. Third, in soils of high P status most of the PR-P is dissolved and should be theoretically available to plants. b. Effect of Soil Temperature The influence of soil temperature on the availability of dissolved PR-P may be of importance in tropical soils. An increase in soil temperature has been found experimentally to have the overall effect of increasing P adsorption by soils and decreasing P in soil solution (Fig. 17), which in turn will reduce P availability to plants. Chien et al. (1982) reported that when 100 mg kg-l P was added to an Ultisol and an Oxisol from Columbia, P concentrations in solutions decreased linearly with increasing temperature. The decrease in P concentration per degree of increase in temperature was 0.63 mg liter-' for the low P retentive Ultisol whereas the value was 0.12 mg liter- I for the high P retentive Oxisol. The low solution concentration was brought about by the high temperature accelerating 8004 Moiokai

3204 Coirnbatore

*35°C 25°C -0-

-a0

8

400

10°C

7

0

16 0 Final P concentration (rng C1)

8

16

Figure 17 Effect of temperature on P sorption by Indian (coimbatore) and Hawaiian (Molokai) Oxisols (6 days equilibration) (S.S.S.Rajan, unpublished data, 1970).

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

109

the “slow reaction” that followed the intial adsorption of P (Barrow and Shaw, 1975; Barrow, 1974).

2. Crop Characteristics Early studies from the Tennessee Valley Authority (Huffman, 1962) have shown that the P concentrations near soluble P fertilizer granules could be as high as 3.5 M . Such a high concentration is conducive to diffusion of P to a large soil volume, which could be to a distance of up to 70 mm from the point of fertilizer placement, depending on the size of the granule (Benbi and Gilkes, 1987). In contrast, the concentration of P in a saturated solution of a reactive PR at a pH of 5.5 is in the order of M ,which suggests diffusion of P to a limited volume of soil. In addition the concentration of P in soil solution following PR application could be very low, in the order of one tenth of that to which soluble fertilizer is applied as has been found by Rajan er al. (1991b) 10 months after fertilizer application. Therefore availability of P from dissolved PR will be greater to crop species which have extensive root systems such as perennial grasses (Chien et al., 1990) and those crops which can extract P from soil solution at low concentrations. Indeed the sensitivity analysis of Kirk and Nye (1986b) indicate that the density of roots in the soil influences greatly the proportion of PR-P taken up by plants. Inoculation of plant roots with mycorrhizae will facilitate extension of the root system to a greater soil volume and thus enhance P uptake (Waidyanatha et al., 1979; Tinker, 1980).

3. Management Practices The method and time of application of PR in relation to crop planting time can affect the availability of dissolved PR-P to plants. a. Method of Application Plant availability of P from PR depends on the probability of plant roots encountering the localized higher-concentration pockets of soil P around dissolving PR particles. This probability will be increased if the PR is broadcast and is uniformly incorporated into the surface soil to the required depth. In other words, the greater the volume of P-enriched soil, the greater its availability to plants, provided roots explore. Broadcasting without incorporation may not increase availability of PR-P in spite of the greater amount of PR dissolution occurring at high rates of application, because the dissolved P is likely to be restricted to a shallow depth of soil (Rajan et al., 1991). The negative aspect of incorporation of PR into soil is that it facilitates greater contact between soil colloids and solution P, resulting in greater P adsorption and therefore in low solution P concentration. The net result of increased P adsorp-

110

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLATR

tion, versus the P enrichment of a greater volume of soil, on P availability to plants will depend on soil P status (Fox et al., 1986b) and rate of PR application. In soils of medium P status, or low P sorbing capacity, such as sandy soils, mixing PR may be desirable (Alston and Chin, 1974). On the other hand this may be an unsuccessful strategy in highly P sorbing soils of low P status. b. Time of Application Since there is a time lag between PR application and significant dissolution (Barnes and Kamprath, 1975; Sinclair et al., 1993a) some authors have suggested application of fertilizers in advance of planting crops. Again the advantage of such a practice will depend on the net result of two opposing reactions that are operating: dissolution of PR and adsorption of dissolved PR-P by soil colloids and its slow conversion to nonavailable forms. In soils of low P buffer capacity, dissolution of PR will increase P in solution to a greater concentration, and probably will be maintained for a longer duration than in soils of greater P buffering capacity. Thus application of PR in advance of planting may be an advantage in soils of low P buffering capacity but not necessarily in soils of higher P buffering capacity. The results of Chien ef al. (1990) (Fig.18) and Purnomo and Black (1994) are consistent with the above reasoning. When PR is

- High P bufferingsoil

80 Low P bufferingsoil

1

J

70 P

w

.-

.c

60 50

40 z .-a,

* 30 a,

L

20

P

n 10 0

before planting 100

200

300

. - I

400 0

100

200 300

P applied (mg P kg-1 soil)

400

Figure 18 Interaction between soil P buffering capacity and time of application on the plant effectiveness of North Carolina PR in relation to TSP (source: Chien et af.. 1990).

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

I1 1

applied for maintenance purposes, as implied by Engelstad and Terman (1980), early application of PR may be beneficial also in soils of high P buffering capacity.

V. MODELING THE RATE OF PHOSPHATE ROCK DISSOLUTION IN FIELD SOIL Although a number of studies examine the kinetics of PR dissolution in soil, or model PR dissolution in field soil, most do not look at the system as a whole. For example, Chien et al. (1980) examined the kinetics of PR dissolution in soil by agitating suspensions of the two and measuring changes in soluble phosphate. The results, however, probably reflect more the kinetics of P adsorption by the soil from an initially saturated solution of the PR, since the PR was added in large excess, and adsorbed P was not measured. More recently, Robinson et al. (1994) described a conceptual model for predicting the dissolution of Gafsa PR in soil. While the model qualitatively relates soil properties important to dissolution, such as soil acidity, solution calcium, and exchange sites, the transport processes within the soil and PR particle size are not considered, so that time is not a quantitative aspect of the model. Further, it is not supported by field data; such as the rate of pasture P uptake or measurement of residual PR in soil with time which are related to the rate of PR dissolution. Only two published models appear to quantitatively examine the kinetics of dissolution in field soil: the comprehensive model of Kirk and Nye ( 1986a,b) which incorporates individual soil properties, and the simpler model of Watkinson (1994a,b) for pastoral soils, which combines all soil and site effects into an overall rate constant. The qualitative models will not be discussed further. Both the Kirk and Nye (1986a,b) and the Watkinson (1994a,b) models postulate that the dissolution rate of PR in soil is controlled by the diffusion of dissolved products from the PR surface into the bulk soil. Kirk and Nye ( 1986a,b) include all ions involved in a coupled counterdiffusion process, whereas Watkinson (1 994a,b) assumes the rate-limiting step is diffusion of calcium phosphate, which is controlled by the trace diffusion of phosphate, because of the relatively small calcium concentration gradient at the PR surface. Further, Kirk and Nye (1986a,b) consider the consequences of some important situations, in particular those in which dissolved PR mutually affects the dissolution rate of neighbouring particles (Fig. 19A). In contrast, the Watkinson (1994a,b) model considers only the simpler system of dissolution for maintenance rates of PR in New Zealand pastoral soils, which involves the independent dissolution of PR particles. Under these conditions both models give similar results (Watkinson, 1994a) (Fig. 19B). A major assumption of both models, which needs verifica-

112

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR kg P m-3 0.16 0.27 0.45

0.67

0.16

0

i

16 24 Time (days)

32

Figure 19 The dissolution rate of DCPD. (A) Predicted from the Kirk and Nye model, for different application rates (numbers on curves in units of kg P m-3 soil), showing the influence of neighboring particles at higher rates. Initial radius = 0.1 mm, other variables as in Kirk and Nye (1986a). (B) For particles dissolving independently (application rate of 0.16 kg P I I - ~ ) :comparison of Kirk and Nye model (1986a) (-) and Watkinson (1994a) SFOR model for Eq. 5(-), and Eq. 6 (----)(g = 0.19 mm and G = 0.208 day-').

tion, is that the mobility of phosphate adsorbed on the soil is not a limiting factor, i.e., is more rapid than the diffusion process (Watkinson, 1994a). Instead of the interparticle diffusion assumed, some phosphate may also diffuse very slowly into the interior of soil particles. Implicit in the diffusion controlled models is that the geometric area, not the total area including internal surfaces, is the variable controlling the total flux of dissolved PR into the bulk soil. Diffusion through the soil is much slower than that through the solution in contact with the internal surfaces, and the area of soil in contact with the PR particles is the geometric area. Hence a measure of the particle size distribution by sieve analysis is sufficient to define the area variable of a PR (Watkinson, 1994~). Soil moisture is an important field variable (Anderson and Sale, 1993; Weil er al., 1994) because of diffusion (Kirk and Nye, 1986a; Watkinson, 1994a), but soil temperature would be expected to be less important, because PR solubility has a negligible temperature dependence (Watkinson and Kear, unpublished work), and the activation energy of diffusion is small. Smith er al. (1999), however, found a temperature dependence for hydroxyapatite.

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Other rate-limiting steps are possible. For example, the dissolution rate of some complex crystals can be controlled by the rate of transfer from the crystal surface into the adjacent solution (e.g., dolomite and olivine) (Sverdrup and Bjerle, 1982). Although the rate of calcite dissolution in aqueous suspensions is controlled by Nernst film diffusion, the rate of dolomite dissolution is much slower (Sverdrup and Bjerle, 1982). This will have implications for the dissolution of free calcite and dolomite that are present in many PRs. However, the rate of dissolution of the 1 1 PRs examined by Watkinson (1994~)was consistent with Nernst film diffusion, with only a small component a little slower. This supports the earlier conclusion of Huffman et al. (1957) who investigated dissolution rates of calcium phosphates in phosphoric acid solutions. Diffusion in pure solution is much more rapid than in soil, so the basic assumption of the two models is reasonable based on present knowledge. Although Olsen (1975) found that the dissolution of PRs into EDTA solution was consistent with a second-order reaction rate model, he did not test it as he was only interested in comparing the relative dissolution rates of PRs.

A. KIRK AND NYEMODEL 1. Description Kirk and Nye (1986a) developed a model to describe the rates of dissolution in soil of the apatites in PRs, based on one developed for dicalcium phosphate dihydrate (DCPD), which was itself a successful simplification of an earlier model. The rate-limiting step for DCPD dissolution is essentially the steady-state counterdiffusion of phosphate and hydrogen ions between the solution at the mineral surface and that in the bulk soil. An equation for the rate of loss of mass from the mineral particle was formulated using the equality of the diffusive flux of ions across the mineral interface, while maintaining the solubility product of the DCPD and ionic charge balance in the interface solution. The influence of neighboring particles on the concentration of dissolved material in the bulk soil was also included. In this situation, the soil solution reaches concentrations up to the solubility of DCPD which, while consistent with high soil moisture, would not be maintained under high rainwater leaching rates. The DCPD model, involving numerical solutions to the differential equations, was then extended to describe the rates of dissolution of apatites. Increasing carbonate substitution increases the solubility of carbonate apatites (Section ll.C), and therefore the dissolution rate. In contrast, the increased alkali from hydrolysis of lattice phosphate from CAs compared with DCPD lowers the rate of dissolution. Kirk and Nye (1986a) considered that the effect of gangue material was “likely to be very

114

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

small.” However, calcite dissolves rapidly generating both alkali and calcium, so the effect of appreciable calcite impurity would need to be taken into account. In a second paper (Kirk and Nye, 1986a) quantitative applications of the model to field situations were considered. The effects of several factors on the rate of dissolution of calcium phosphates in soil were examined. They included application rates to the soil, particle size distribution, and the distribution of the calcium phosphate particles in the soil. As the size range about a mean value was extended, so the total dissolution rate decreased. This was caused by the increasing fraction of larger particles in the mixture. Clumping of particles in the soil and high application rates also decreased the dissolution rate, through the increased effect of dissolved material on neighbouring particles. The influence of

’7

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A

/0.02

U‘

“I ’0°1

f

0.32

0.13

NO.10

0

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loo

150

Time (days)

1

200

Figure 20 The predicted effect on North Carolina PR dissolution in a soil, pH 5.2, in the glasshouse of (A) the pH buffer capacity (bHs = 0.02, 0.01, 0.005) whenf = 0.13 and (B)the diffusion impedance factor cf = 0.32, 0.13, 0.10) when bHs=O.O1 compared with experimental

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

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plant roots was also investigated. Dissolution rates were increased by uptake of phosphate, depending on the rooting density and rate of fertilizer application, and by plants that secreted acid from their roots. Because of the high amounts originally present in soils, it was not considered that calcium uptake would greatly affect dissolution rates.

2. Testing Anderson and Sale (1993) reported results of an experiment to test the prediction by the Kirk and Nye model of the rate of dissolution of North Carolina PR incubated in an acidic soil in a glasshouse. Three pH values (4.8,5.2, and 6.3) of the soil (through adding sodium carbonate) were examined. Observed results were reasonably consistent with theory at the two lower soil pH values, considering all the experimental errors involved, but not at the higher value. One difficulty in fitting the data was the continual decrease in soil pH during the course of the experiment, with the effect being greater at the higher pH values. The decreases were probably the result of two factors: the increase in acidity through nitrification during incubation, and the apparent incomplete equilibration of the added sodium carbonate with the soil. Another problem was the instability of the model in fitting data at pH values greater than 6.0, values which are common in many temperate pastoral soils. Anderson and Sale (1993) concluded that the model was very sensitive to the pH buffering capacity (Fig. 20A), and more particularly to soil moisture via the diffusion impedance factor (Fig. 20B). The authors concluded that soil moisture would need to be closely monitored under field conditions, a problem that would need to be overcome for routine use of the model. They also concluded that only methods strictly pertinent to the measurement of the model parameters of soil pH and pH buffering capacity should be used.

B. WATKINSON MODEL 1. Description

The model of Watkinson (1994a) was designed for use on pastoral soils receiving maintenance rates of PR. The concentration (m)of PR particles is such that they dissolve independently of each other. Dissolution is controlled by diffusion of dissolved phosphate from the PR surface solution into bulk soil solution through a boundary layer of soil. Because the concentrations, C,, C s , at both surfaces of the boundary layer are essentially constant, the process can be regarded as a simple steady-state phenomenon. In this regard, the boundary layer is conceptual and equivalent to the Nernst film of stagnant solution surrounding

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

particles in a stirred aqueous suspension. The thickness of the layer would be a function of the rainwater leaching rate, which is unrestricted (in contrast to the Kirk and Nye model), and the rainwater may leach phosphate to lower depths. Although the diffusion coefficient of phosphate will be a function of the phosphate concentration because of the nonlinear adsorption by the soil, a mean diffusion coefficient, D,, can be used since the system is at steady-state. Differential equations relating mass of material dissolved and diffusing into the soil are simple enough to be solved analytically. Two submodels were considered; in the first (SFOR), the outer radius, g, of the boundary layer was constant, and in the second (SCT), the thickness of the boundary layer, h, was constant as the particle dissolved. For slowly dissolving PR, the situation for soils of pH greater than 6.0, the models were essentially the same. The particles were taken to compose uniform equivalent spheres of initial radius r,, which were added to topsoil at an initial concentration m,.

SFOR Model: (m/m0)2/3- (m/m, - 1)(2r0/3g) = I - 2Gt. If the boundary layer is thick, i.e., r,/g (m/m,)2/3 where G

=

< 1, then,

=

(5)

from Eq. (S),

1 - 2Gt

(6)

D,(C, - C,)/(pro2F) (See Section II.B.2.b).

SCT Model: 1 - (m/m,)1/3

+ (h/r,)log[{(m/m,)l~~+ h/ro}/(l + h/ro)] = (ro/h)Gt.

(7)

If the layer is a thin coating, i.e., hlr, 6 1, then, from Eq. (7), (m/m,)1/3 = 1 - (r,/h)Gt

(8)

On the other hand, if the layer is very thick, then the SCT model is approximated by (m/m,)2’3

= 1 - 2Gt,

(9)

which is the same as for a thick layer in the SFOR model. For fertilizer mixtures, which contain a range of sizes, the kinetic relationship is described by a set of three equations like Eq. (2) (Watkinson, 1994b). They are based on the approximation of Eq. ( S ) , i.e., Eq. (6), which is appropriate for slow dissolution (see Section II.B.2.b). The above equations can be tested readily using field data, both directly from measurements of residual PR at increasing times from application and indirectly from agronomic data from which substitution values are calculated, as shown in the next section.

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

1I 7

2. Testing Equations from the model have been found consistent with field dissolution rate data for PRs on a New Zealand pastoral soil, where the pH was near the upper limit at which PRs can be used (Watkinson, 1994a) (Fig. 21A). PR treatments included mixtures with elemental sulfur (Rajan, 1987) and the unreacted component of PAPRs (Rajan and Watkinson, 1992) (Fig. 21B). Data for the dissolution rate of Sechura PR in nearly 100 New Zealand pastoral soils, with pH (H,O) values from 4.9 to 6.4, were also consistent with the model (Watkinson, 1995; Perrott el a/., 1996). The model is also supported by agronomic data. Substitution values are directly related to soluble P applied and therefore to PR dissolution rates (Edmeades et al., 1992), provided allowance is made for an annual loss in P availability (Watkinson and Perrott, 1993). More than a dozen field trials related substitution values with PR dissolution rates from the model (Watkinson and Perrott, 1993) (Fig. 22). The relative agronomic effectiveness measured on three soils in greenhouse experiments related well to the dissolution rates of 1 1 unground and ground PRs (Rajan e t a l . , 1992; Watkinson 1994~.1995) (Fig. 2). One disadvan-

0

1

2

3

Time (years)

4

Figure 21 Fit of Eq. ( 6 )to the experimental dissolution rate of PRs in a pastoral soil. (A) North Carolina (O), Chatharn Rise ( O ) ,Florida (0) (Rajan, 3987b). and North Carolina (A)(Rajan and Watkinson, 1992).Regression forced through ( 0 , 100).and (B)the unreacted North Carolina constit40% (A).and 50% uent of partially acidulated North Carolina PR. Unacidulated (0).and 30% (0). ( 0 )acidulated (Rajan and Watkinson, 1992).

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

Time (years) Figure 22 Mean cumulative substitution values (0)for annual North Carolina PR applications in 12 field trials for each of 6 years (Sinclair ef a / ., 1993) compared with values calculated assuming an annual decrease in applied P availability of 15%. at different (cubic) rate constant values, K (year-') (Watkinson and Perrott, 1993). (K = 0.1 year-' is equivalent to 30% dissolution in the first year. )

tage of the simplicity of the model is that it does not predict rate constants directly from theory using easily measured soil properties. To overcome this, rate constants have been measured in 95 field trials (K. W. Perrott, personal communication) with the objective of developing relationships between rate constants and soil properties, which will allow the constants to be estimated from easily measured soil properties.

VI. AGRONOMIC EFFECTIVENESS OF PHOSPHATE ROCK

A. DETERMINING AGRONOMIC EFFECTIVENESS Agronomic effectiveness of PR as a P fertilizer is ultimately expressed in its ability to supply adequate P for sustaining desired levels of crop production. Thus plant growth rather than merely dissolution in the soil is the final judge of agronomic effectiveness. What matters ultimately is plant growth in the situation where the PR is to be used for practical farming, and a basis for predicting this must be established. Plant growth experiments are essential for establishing this predictive basis. Assessment of PRs as P fertilizers involves comparison of agronomic performance usually with water-soluble P fertilizers such as TSP and SSP. In such comparisons care must be taken to ensure that the ability to supply P is the only factor influencing the results. In addition to ensuring adequate levels of other

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major nutrients, trace elements may also require attention; for example, Sinclair et a / . (1990) found that the molybdenum (Mo) content of Sechura PR was sufficient to eliminate Mo deficiency in clover-based pastures, so failure to apply

Mo with the standard fertilizer could invalidate the comparison. Comparisons could also be affected by the small liming effect of PR (Sinclair ef af., 1993b), but this would be significant only at very high application rates or after a long period of regular application. Plant growth experiments may be conducted in pots or in the field. Each system has its advantages and limitations. 1. Techniques

a. Pot Experiments Pot experiments have the advantage of being relatively inexpensive so that many factors influencing agronomic effectiveness can be examined. A further advantage is that factors may be examined individually while other factors are kept constant, whereas this is often impossible in the field. However, differences in growth conditions and plant behavior between pot and field are generally so great that predicting the absolute performance of a PR in the field is not possible from pot experiments. Also, considering the slow release characteristics of PRs, long-term evaluation is essential but may be difficult to sustain while avoiding significant changes in soil conditions in pot experiments. For example, substantial soil acidification may occur in lengthy pot experiments with legumes. Whereas this may be neutralized by frequent application of bases the fluctuation in pH may still be greater than would occur with the much larger soil volume in the field. With PRs, pot experiments have their main value in ranking PRs from different sources (Mackay et a / ., 1984b). The relative agronomic performance in pots has been used for judging the ability of laboratory tests to rank PRs in order of their agronomic effectiveness (Chien and Hammond, 1978; Kucey and Bole, 1984; Mackay et af., 1984b; Rajan et af., 1992). Few experiments have directly compared PR rankings in the field with those measured in pots, using the same soil and crop, but it is the general experience that PRs which perform best in the field are those which perform best in pots. Chien and Hammond ( I 978) compared the same group of PRs in glasshouse and field experiments, although the soils and crops differed. PR rankings were similar in both experiments, but not identical. Engelstad et a / . (1974) compared six PRs in a glasshouse and two field experiments with flooded rice and found almost identical rankings in the three experiments. Rajan et af. (1992) found that different soils, similar in pH (5.5-5.7), gave similar but not identical rankings of PRs in a glasshouse experiment using the same crop. Pot experiments may also be used to compare the performance of a particular

120

S. S. S. RAJAV, J. H. WATKINSON, AND A. G. SINCLAIR

PR in different soils and thus identify the soils on which it is most likely to perform well (Anderson et al., 1985). Differences between plant species in their ability to utilize P from PRs have also been examined in pot experiments (Haynes, 1992). b. Field Experiments Field experiments are essential to provide a realistic assessment of the likely performance of a PR in practical farming situations. However, it is important to realize that the field experiments merely provide a basis for prediction, since the conditions under which the PR will be used in practice are unlikely to correspond exactly to those in any field experiment. Ideally a matrix of field experiments should define PR performance as affected by PR properties, soil and climatic conditions, and crop characteristics. Interpolation and extrapolation would then permit accurate prediction of performance in any situation. However, the cost of obtaining such detailed information would be prohibitive. Consequently, predictions must be made from the relatively few relevant field experiments available, along with insights gained from laboratory and pot experiments on the effects of site, crop, and PR properties.

2. Measurements Agronomic performance in the field of PRs is generally judged in relation to conventional soluble P fertilizers such as TSP and the measure of performance is usually crop yield. However, crop yield measurement can fail to detect real differences between fertilizers in their ability to supply P for plants for the following reasons: (i) inadequate precision in yield measurements; (ii) high fertilizer application rates, so that even the less effective fertilizer supports near maximum growth, so differences between fertilizers cannot be expressed in yield differences; and (iii) inadequate responsiveness of the trial site. Thus failure to detect a difference between P fertilizers can often arise not because there was no difference but because the experiment was incapable of detecting the difference. Johnstone and Sinclair (1 99 1) provided biometrical guidelines for the comparison of P fertilizers, emphasizing the limitations of field evaluation. They show, for example, that 40 replicates would be required to ensure a 90% probability of detecting a difference between two fertilizers which differ by 10%in P availability when they are compared in a trial with a CV of 3% in which a 100% response to P could be achieved. Crop yield measurements may be supplemented with herbage P concentration and plant P uptake measurements for comparison of fertilizer effectiveness. Generally these are more sensitive to differences in P supply (see for example Gregg et al., 1988) and can distinguish between fertilizers when soil P status or P fertilizer application rates are too high for yield differences to be detected

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

12 1

(Sinclair et al., 1994). Thus, provided it can be demonstrated that the relationship between crop production and P uptake or concentration in herbage is the same for the PR and soluble P fertilizers being compared, these alternative measurements may well be superior to crop production for evaluating PRs. Radioisotope techniques using 32P and 33P provide a further approach to the evaluation of PR fertilizers (Morel and Fardeau, 1990). Labeling is a simple process with synthesized PR, but neutron irradiation is required to label P in natural PR (Fried and MacKenzie, 1949), and this may alter PR properties. Alternatively the labile P pool in the soil may be labeled and the contribution of unlabeled P fertilizers to plant P uptake calculated. Isotope studies allow the P-supplying ability of PRs to be assessed even where there is no yield response to P fertilizer. They can also give a direct measure of the amount of plant P which is derived from the test fertilizer. However, the short half-life of 32P (14.7 days) and 33P(25.3 days) severely limits the value of the technique since PR evaluation is primarily concerned with long-term effectiveness. Measuring agronomic performance of PRs in pastoral systems presents particular problems. First, the output of these systems is in animal products rather than crops, so it could be argued that those should be the parameters to measure. However, appropriate experiments with animal measurements would be prohibitively expensive. Moreover it is generally found that efficient grazing animal production is in direct proportion to pasture production (Morton er al., 1995), so pasture dry matter (DM) production is used almost exclusively in field comparisons of P fertilizers. Pasture DM production is most conveniently measured in small, regularly mown plots, and to simulate the nutrient returns in animal excreta a large proportion of harvested herbage is often spread over the plot from which it originated.

B. QUANTIFYING COMPARATIVE PERFORMANCE OF PHOSPHATE ROCKS 1. Substitution Value, Relative Response, and Isotopic

Relative Agronomic Effectiveness It is important to recognize that the comparative performance of PRs is strongly influenced by the length of time over which plant growth is measured, by fertilizer application method and frequency, by soil conditions, and by plant species. A full account of these factors is essential when reporting agronomic performance of PRs, and they should be taken into account when making predictions from experimental data. The lower agronomic value of PR relative to soluble P may manifest itself in different ways. Two extremes are illustrated in Fig. 23. In Fig. 23A, PR and

IrP-

122

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR I d

. 0

sp

-

PR

I

I

1.o

0.5

D

P

1 0 c n c n o 0

sv

0 RR

0

P applied Figure 23 Patterns of response to P fertilizers and the effects of application rates on measures of agronomic performance. ( A ) Yield from PR is always the same as yield from half the quantity of P applied as soluble P fertilizer (SP). (B)Yield response to PR is always half the yield response to SP. (C) Fertilizer substitution values (SV) and relative response (RR) corresponding to Figure (A). (D) SV and RR corresponding to (B).

soluble P give the same maximum yield but more PR than soluble P is required for yields below the maximum. It has been drawn for the specific case in which yields from any rate of soluble P can always be achieved with twice the rate of PR. In Fig. 23B PR never achieves the same maximum yield as soluble P because maximum growth of the crop requires a higher P concentration in soil solution than the solubility product of PR can permit. In this case the fraction of PR which dissolves declines as the application rate increases. Figure 23B has been drawn for the specific case in which the yield response to PR is always half of that to soluble P applied at the same rate of P. In practice, as well as these two extremes, a full range of intermediate response patterns probably occur.

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

123

The agronomic performance of PRs relative to soluble P fertilizers is generally expressed quantitatively in one of two ways, these being substitution value (SV) or similar expressions (Black and Scott, 1956; White et al., 1956; Barrow, 1985; Colwell and Goedert, 1988; Black, 1993) and relative response (RR). In this review SV is expressed as the ratio of total P applied in standard fertilizer to total P required as test fertilizer to give the same plant yield. Expressed this way, the SV of a poorly performing test fertilizer is low and vice versa. RR is the response to the test fertilizer divided by the response to a standard fertilizer when both are applied at the same rate of P. The term relative agronomic effectiveness (RAE) is often used for this parameter, but we prefer the term RR because it is unambiguous. Both SV and RR can vary with PR application rate. In cases represented by Fig. 23A, where PR and soluble P response curves have the same asymptote, SV remains constant but RR increases toward a value of 1 at high rates (Fig. 23C). Where the PR response curve has a lower asymptote than the soluble P response curve, as in Fig. 23B, SV declines toward zero with increasing PR application rate, while RR may remain constant (Fig.23D). Values of SV and RR converge as application rates are reduced toward zero, irrespective of the type of response pattern. There are clearly dangers in expressing the agronomic performance of a PR, as either S V or RR, if these are measured at high PR application rates or at rates approaching zero. For situations like that of Fig. 23A RR will be high even for a poorly effective PR, while for situations as in Fig. 23B SV will be low even for PRs which perform well at moderate rates. PR performance at rates approaching zero has been calculated from the initial slopes of the PR and soluble P response curves (Kumar et al., 1992b). This may not provide a reliable measure of performance at practical application rates. Also PRs are known to give similar yields to SSP at low levels of crop production comesponding to low rates of PR application. From the above discussion it is clear that in many situations decisions on the use of PR would need to be based on consideration of complete, well-defined response curves. Both SV and RR can also be determined from plant P uptake and P concentration. Provided that the relationship between plant yield and plant P concentration is not affected by fertilizer form, SV based on plant P content should be identical to SV based on plant yield since in both cases one is calculating the relative amounts of the different fertilizers required to produce the same plant condition. But RR will be different for different plant parameters measured, especially at high fertilizer application rates or on unresponsive soils. In these situations there may be no yield difference between fertilizers, so RR based on yield will be 1.O; but differences in plant P content are still likely, giving RR values different from 1.0. Morel and Fardeau ( 1990) introduced the parameter Isotopic Relative Agro-

124

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

nomic Effectiveness (IRAE) to express agronomic performance as determined by their isotope procedure. This procedure involved applying labeled soluble P and PR fertilizers in separate pots at a single P application rate and calculating the amount of fertilizer-derived P in the plant. Then IRAE (%)

=

100*P~l/P~s,

(10)

where Prland P, are the amounts, respectively, of test and standard fertilizer P in the plant when both fertilizers are applied at the same rate of P. Morel and Fardeau (1990) found that IRAE for North Carolina PR compared with DAP was independent of P application rate up to 200 mg P kg-I soil. This result may well have been an artifact of the experiment rather than a basic property of the IRAE. For Fig. 23A type situations, IRAE would tend toward unity at high rates since the maximum contribution of either fertilizer to plant P is 100%.But in Fig. 23B type situations, IRAE would decline as application rates exceeded that at which no further dissolution of PR could occur. An intermediate situation could account for Morel and Fardeau’s (1990) result.

2. Mathematical Functions for Phosphate Rock Response Patterns The use of mathematical functions to describe response patterns facilitates comparisons of PRs with soluble P fertilizers. The Mitscherlich equation has been widely used for this purpose. In its simplest form, y

=

a - bet.

( 1 1)

where y is plant yield, a is the maximum yield, i.e., the asymptote of the response curve, ( a - b) is the yield without fertilizer, c is curvature factor and x is the fertilizer application rate. In situations where PR and soluble P fertilizers have a common asymptote and of course a common value for (a’- b) (Fig. 23A), SV can be calculated from the curvature (Barrow, 1985; Johnstone and Sinclair, 1991): A slightly different procedure, but with the same outcome, for Fig.23A situations is to combine data from both fertilizer response patterns into a single curve which is expressed by

y = a - b*c(xsp+

hpR),

(13)

where xsp and xPRare application rates of soluble P and PR and k is the substitution value (SV); k is then determined by best-fit procedures. Where asymptotes differ (Fig. 23B) Mitscherlich curves could still be fitted to the individual fertilizer responses but these would differ in all coefficients ( a , b,

PHOSPHATE ROCKS FOR DIRECT APPLICATION T O SOILS

125

and c). Two alternative functions with only a single material-dependent coefficient have been proposed for Fig. 23B situations. These are y =y,

+ b.ln x

(14)

y =y,

+

(15)

and

b.x”jn,

where yo is yield without P, y is yield with x units of P, m is a constant, and b is the material-dependent coefficient. Leon e t a / . (1986) used Eq. (14), and Bationo et a / . (1991) used Eq. (15) with m = 2, for comparisons of various PRs with soluble P fertilizers. Both equations assume a constant value of RR and in both cases RR

=

bpRlbsp.

(16)

Equations (13), (14), and (15) are strictly applicable only to the extreme situations of a constant SV or a constant RR and not to intermediate situations, which probably predominate. Barrow and Bolland (1990) have overcome these restraints by replacing the constant k (i.e., the SV) in Eq. (13) by a function whose value can decline as the application rate of PR increases. They used the function

k

=

(I

+

mxt)-)l,

(17)

where m and IZ are coefficients and xt is the amount of test fertilizer applied. Computer programs have been written specifically for fitting this type of model (Black, 1993).

C. RESIDUAL EFFECTIVENESS OF PHOSPHATE ROCKS Reviewing residual effects of PRs, Khasawneh and Doll (1978) concluded that the experimental evidence they presented did not confirm the common assumption that PRs have greater residual effects than soluble P fertilizers. On the contrary, it appeared that the residual effects of soluble P fertilizers were greater than those of PR in the first 3 or 4 years after application. In comparing the residual effects it must be remembered that with soluble fertilizers the residual effect derives from the soil-phosphate reaction products and the reaction of prime importance is conversion of P from labile to nonlabile forms. But with PRs, the PR-P needs to be released into solution before any residual effect can manifest itself. Therefore in the short term one is likely to find a poorer residual effect from PR application. The situation is different in coarse textured acid soils

126

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

where large and rapid leaching of soluble P will show PR giving a greater residual effect in the shorter term (Yeates and Clarke, 1993). Unlike a single application, when PR applications have continued over a period of several years, a large pool of undissolved PR can accumulate. The substantial release from this pool can result in a high residual value of PR in subsequent years. Residual effects of PR relative to those from SP therefore depend on, in addition to the rate of loss of P from the available soil P pool, the previous pattern of application, and the rate of dissolution of PR. Barrow and Campbell (1972) and Barrow and Carter ( I 978), using mathematical models, described the residual effect of soluble fertilizers in relation to freshly applied superphosphate. Probert (1985) presented a similar model, but extended it to include the measurement of the residual effect from fertilizers such as PRs from which a fraction of P is released in a given time. Building on these, Sinclair et al. (1993) and Sinclair and Johnstone (1995) proposed a compartment model to predict the residual effects for various combinations of the factors that influence the residual effect of fertilizers. The soil P part of the model is illustrated in Fig. 24. Fertilizer P applied to the soil enters the first compartment (F) which contains undissolved fertilizer P in the soil. On dissolution, P moves from F into A which is the pool containing plantavailable P. Phosphate is lost from this pool by immobilization andlor leaching processes in the soil and by nonreturn of a proportion of P taken up by crops. P , and P A are the amounts of P in compartments F and A at any time. K , and K , are the rate constants describing the transfer of P from compartment F to compartment A and the loss of P from compartment A , respectively. For simplicity, first order kinetics are applied to both processes. When the fertilizer applied is PR with a range of particle sizes its dissolution rate by the diffusion model (Section V) is given by an arcsin function. This could be approximated by a simple exponential for the first 90% or so of dissolution where the ratio of greatest to least sizes was <2 (Watkinson, 1994b). The differential equations describing the system are

dP,ldt

FertilizerP

I

-

= -

I

KIPF

1

(18)

.

1

I Pin soil I PF

Kl

'A

K2

b

Figure 24 Two compartment model of active P in soil (Sinclair et a / . . 1993).

PHOSPHATE ROCKS FOR DIRECT APPLICATION T O SOILS

127

Solution of these equations gives the recurrence relationships fF(t

+ 8t) = fF(t)e-fi'kl

(20)

+ 8f)= fA(f)e-"k2 + KIfF(t) [e--6rkl - e-s'k2] / (K2 - K l ) , (21) where t is the time since the last application of P to F. The average amount of fertilizer P in F in the time interval ( t , t application at time f is given by fF(f)(l - c K I ) / K I .

+ 1) after an (22)

Similarly the average amount of P in A in the time interval ( t , t + 1) after an application at time t is given by

+

eKz)/K, K I P F ( [ ) [ (-] e K I ) / K l - ( I - e K z ) / K 2 ] / ( K2 Kl).

fA(t)(l-

(23)

In applying the model in this review, r is in years. The DM production part of the model is based on the assumptions that DM yield must be nil when PA is nil and that DM yield is related to PA by a Mitscherlich response curve, i.e., DM yield = Y,,, (I - CPA)

(24)

or

RY/100

=

1 - CPA,

(25)

where Y,,, = DM yield when P is nonlimiting, C is a constant which defines the curvature of the response curve, and RY is the yield expressed as a percentage of Y,,,,. Numerical solutions can be derived from the above equations except in the case of identical values for K , and K 2 , in which case a very small adjustment in one of the values will allow an accurate solution to be obtained. Figure 25 shows predicted plant yield patterns following a single application of P fertilizers to a soil initially devoid of plant-available P. Yields are expressed as the percentage of yield when P is nonlimiting. At 15% per year loss from the labile P pool (Fig. 25A) the residual value of PR is predicted to exceed that from soluble P after 1 1 , 8, and 4 years for PR dissolution rates of 5, 10, and 40% per year, respectively. At 50% per year loss from the labile P pool the corresponding times are 5, 4, and 2 years, respectively (Fig. 25C). Chien and Hammond (1987b) reported that Gafsa, Sechura, and North Carolina PRs were inferior to soluble P for the first crop of beans in a field experiment on an Andisol in Colombia, but were as effective as soluble P for the second crop and superior for the third crop. These data are for single applications of P fertilizer in situations in which loss from the labile P pool was rapid (as evidenced by the rapid decline in residual effect of SP in both experiments). There-

128

S. S. S. RAJAN, J. H. WATKINSON,AND A. G. SINCLAIR 10080 -

60' 40 I

2o

1 / J

Ploss=15%lyr

0

100-

; 80.%

.-c

(II 5

a

.

60-

.

40-

' .

.

.

.

.

P loss = 50% I yr C PR dissolution

-

10080 -

5% 10%

60-

-6- 20%

40 20 -

o+

0

2

4

6

Years

8 1 0 1 2

Figure 25 Predicted relative yield (RY) of crop following single applications of 225 kg P ha- I P as soluble P fertilizer (SP) and PR to soils containing no plant-available P. Assumptions are that yields are related to plant-available P at midyear; the Mitscherlich response function applies; plantavailable P required at midyear for 90% RY is 130 kg ha-' (A.G. Sinclair, unpublished data, 1995).

fore these should be considered in relation to Fig. 25C. The good performance of the PRs for the first crop indicates rapid dissolution, in which case the model predicts the early development of a superior residual effect from PRs, as was observed. Bolland er al. (1988b) surveyed results on PR performance in field experiments in Australia. They concluded that during 8 years following single applications of PR and soluble P, the residual effect of PR improved relative to the residual effect of soluble P, but this was due to the decline in the latter. But even

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

129

8 years after application, PR was still less effective than soluble P. Other writers (Quin, 1988; Bolan et al., 1990) have pointed out that the data summarized by Bolland et al. (1988b) related largely to relatively unreactive PRs on soils with low PR solubilizing ability. The data surveyed by Bolland et a1 .would be fairly closely matched by a PR dissolution rate a little less than 5% and a 50% labile pool P loss rate as in Fig. 25C. Figure 26 shows predicted residual effects following 6 years of annual application of P fertilizers at rates of P calculated to maintain about 90-95% relative

100-

Ploss=15%lyr

8060'

40

-

PR dissolution Per Yr -5% 10% 20% -40%

*

20

-0-

80%

.

0

,

I

,

,

,

100-

E a ..9

80 -

60-

- 40Q

,

'

cT 20-

1

20 U

0

P loss = 25% I yr

P loss = 50% I yr "

2

'

4

'

6

'

Years

.

'

8 1 0 1 2

Figure 26 Predicted relative yield (RY) of crop receiving soluble P fertilizer (SP) and PR annually for 6 years and no P fertilizer for a further 6 years. Assumptions as for Fig. 25. Annual application rates are ( A ) 25, ( B ) 37.5, and (C) 75 kg ha-1 (A.G. Sinclair, unpublished data, 1995).

130

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

yield when applied as SSP. These are considered to have been imposed on a soil with P status appropriate for about 90% relative yield. The data of Perrott et al. (1992) are for experiments in which the patterns of PR and SSP used correspond to those in Fig. 26A. PR dissolution rates averaged 30% per year (Watkinson and Perrott, 1993) and labile pool P loss rates can be estimated to be about 15-20% per year (Sinclair et a l . , 1994). For those rates the prediction is that the residual value of PR would be about the same as that from SSP in the first year following cessation of annual fertilizer application. Again there is reasonable agreement between observation and prediction. The patterns of residual value of Gafsa and Nauru PR reported by Scott and Cullen ( 1 965) are compatible with model predictions illustrated in Fig. 26B. In summary, experimental data reported since the review of Khasawneh and Doll (1978), as well as some earlier data, show that residual effects of PRs can exceed those of soluble P fertilizers, and are greatly influenced by the PR dissolution rate and the rate of loss of P from the plant-available P pool in the soil. A simple mechanistic model can apparently account well for observed residual effects.

VII. ECONOMICS OF USING PHOSPHATE ROCK FERTILIZERS Several approaches have been proposed for assessing the economics of using PR instead of soluble P. These may also be used for selecting between PR fertilizers. Engelstad (1 978) fitted polynomial functions to the experimental response curves for each fertilizer and used these along with the ratio of the price/kg of P to the valuehnit of crop to calculate the economically optimum fertilizer rates and the corresponding net returndha. PRs were then assessed on the maximum returns ha- I which they could achieve in comparison with soluble P. Engelstad also considered the situation where funds for fertilizer purchase were very limited and calculated the return per dollar spent on fertilizers applied at a low common rate. A sounder comparison would have been between fertilizers applied at a low common cost. Returns per dollar spent at a low rate of application gave a more favorable assessment of PRs relative to soluble P than maximum returns per hectare. Chien et al. (1990) suggested that the SV of a PR relative to a standard fertilizer such as TSP could be used in a simple way to decide which fertilizer is more profitable to use. If the SV is greater than the price ratio (i.e., the ratio of the price kg P- I in PR to that in TSP) then the PR is more profitable than TSP, and vice versa. These calculations relate to single applications of PR and ignore the value of

PHOSPHATE ROCKS FOR DIRECT APPLICATION T O SOILS

13 1

the PR remaining undissolved in the soil at the end of the growth period which will subsequently dissolve and benefit later crops. A conceptual framework for economic evaluation of PRs which takes into account the residual value of fertilizers following a single application has been proposed by Sidhu ( 1978). However, in practice one is likely to be less interested in single applications than in regular, e.g., annual, applications. If ultimately all PR applied to the soil dissolves and, once dissolved, P from PR has the same agronomic value as P from soluble P, then economic evaluations must give equal long-term value to P from both sources. PR should therefore be penalized only because of its slower release of P. The use of PR instead of soluble P should therefore be considered as an investment. Sinclair et al. (1993a) considered that these conditions were obtained when a reactive PR was applied annually to permanent pastures in New Zealand. They assessed the economics of replacing annual applications of soluble P with sufficient PR to maintain an identical level of production. This would require establishing and maintaining a pool of PR in the soil large enough to release annually the same amount of P that would have been supplied in the annual application of soluble P. Assuming a first order dissolution rate model, the size of the pool of PR required equals the annual application rate of soluble P multiplied by the reciprocal of the fraction of PR dissolved per year. Thus, if for example the dissolution rate of PR is 33.3% per annum, annual application of X kg P ha-' as soluble P could be replaced by PR applied at 3X kgP/ha P in Year 1 and X kg P-I thereafter. This represents an initial investment equivalent to the difference in cost between 3X kg P as PR and X kg P as soluble P, followed by annual savings equal to X times the difference in cost per kilogram P between soluble P and PR. In general terms annual savings from using PR instead of soluble P expressed as a percentage of the initial investment is 100

X

( 1 - R)/(R/F - 1)

forR > F,

(26)

where F is the fraction of PR in the soil dissolving per year and R is the ratio of the cost/kg of P in PR to the cost/kg of P in soluble P. If F is equal to or greater than R there is no initial investment cost in replacing soluble P with PR, so the replacement is immediately profitable. Figure 27 shows the percentage return on investment for various combinations of F and R. In permanent grasslands on generally moist, slightly acidic soils in New Zealand F is approximately 0.3-0.4 (Sinclair et al. 1993), so return on investment would exceed 10% with R values < 0.85. A lower return on investment generally would not be considered worthwhile, since there are likely to be alternative types of investment which would yield a better return. Furthermore the farmer may perceive that there is a greater risk involved in the use of PR than in conventional P fertilizer, thus requiring a higher predicted return on investment. Because this type of economic analysis recognizes the subsequent value of

132

S. S. S. RAJAN, J. H.WATKINSON,AND A. G.SINCLAIR 8o 1

I

70 -

C

0

0.1 0.2 0.3 0.4 0.5 Fraction of PR dissolved I year

Figure 27 Effect of PR dissolution rate and ratio ( R ) of cost of P in PR to cost of P in soluble P fertilizer (SP) on the return on investment for replacing SP with PR (Sinclair et a / . . 1993).

undissolved PR in the soil the conclusions are much more favorable for PR use than the methods of Engelstad (1978) and of Chien et al. (1990) would indicate. The above approach to economic analysis is valid only when the dissolution of PR is approximately first order and when P dissolved in the soil from PR is equally effective for plant growth as P from soluble P (i.e., response curves to annual applications of PR and soluble P are identical in the long term). Where the maximum yield that can be achieved with PR is less than that with soluble P economic evaluation could be based on mean long-term response curves using the methods described by Engelstad (1978). If lime is beneficial, the liming effect of PR should be included in economic analysis. PR absorbs two protons per atom of P in the process of dissolution. Thus 31 kg P as PR has a liming effect equivalent to 100 kg CaCO,. The liming effect of PR relative to TSP has been clearly demonstrated in long-term field trials on pastures (Sinclair et al., 1993). If PR is to be used in place of single superphosphate, the sulfur (S) content of superphosphate must be recognized wherever S is required. The price of PR must be raised to include the cost of augmenting it with the minimum requirement for S which would have been met by the S content of single superphosphate.

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

1 33

VIII. SOIL TESTING WJ3ERE PHOSPHATE ROCKS ARE USED Use of PRs which are sparingly soluble brings in a new dimension in the estimation of plant available P and also in interpretation of data in terms of fertilizer requirements. This is because soils which have received PR can contain undissolved PR which usually continues to dissolve and be available to plants during the cropping season under consideration. Thus the soil test should be able to give an estimate of the availability of P from undissolved PR in addition to the availability of soil pool P at the time of testing. Also, because of their slowrelease nature, the reaction products of PR with soil and their availability to plants may differ from those obtained with soluble P fertilizers (Chien et a/., 1987b). Numerous techniques have been proposed to obtain an estimate of plant available P in soils including extraction with (i) chemical solutions, (ii) ion exchange resins, (iii) iron-impregnated paper strips, and (iv) ion exchange with isotopically labeled phosphate ions. Of these, chemical extraction procedures have been used widely, but ion exchange methods are gaining popularity.

A. CURRENT RESEARCH 1. Chemical Extraction

Chemical extraction procedures are rapid, inexpensive, and amenable to automation and handling a large number of soil samples. The extractants used in soil P tests can be grouped under the following general categories: water and weak electrolytes (e.g., 0.01M CaCI, solution), alkaline solutions (Olsen bicarbonate solution and its modifications), weak acids with complexing anions (Bray 1, acetic acid, citric acid), and strong acids (Bray 2, Truog). In this review we have not discussed all the chemical extractants in the literature. Instead we have summarized the reasons underlying the attempt to use different types of chemical extractants in soils where PR has been applied and briefly summarize the results (Table 11). Of the soil tests employed to estimate available P, the Olsen bicarbonate test or its modifications (Colwell, 1963) have been used widely. Because of its success as a P test in soils treated with soluble fertilizers this technique has also been evaluated in soils treated with PR. However, apatites are not soluble in NaOH or NaHCO, (Williams, 1937; Kumar et al., 1991; Perrott er al., 1992; Saggar er a/., 1992), and therefore Olsen bicarbonate solution cannot be expected to predict potential P release from the residual PR or PR to be applied. In fact the

134

S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR Table Il Some Soil Tests Used To Estimate Plant Available P Soil test

Extractant

SoiVsolution ratio

Shaking time 30 min

Reference

Olsen Colwell Bray- I

0.5 M NaHCO,, pH 8.5 0.5 M NaHCO,, pH 8.5 0.03 M NH,F 0.025 M HCl

+

1:20 1:lOO 1:7

60 s

Olsen et al. (1954) Colwell (1963) Bray and Krutz

Bray-2

0.03 M NH,F

+ 0. I M

1:7

40 s

Bray and Krutz

1:2OO

30 min

Truog (1930)

Citric acid Ammonium oxalate

HCI 0.001 M H,SO, buffered with (NH4)*SO,, pH 3 1% citric acid solution 0.1 M NH,-oxalate

Calcium acetate + calcium lactate

0.1 M Ca-lactate 0.1 M Ca-acetate, pH 4.1

'hog

+

16 h

( 1945)

(1945)

1:25

7 days 2h

Dyer ( 1894) Joret and Hubert

1:20

2h

Schiiller (1969)

1:lO

(1955)

literature shows that in soils which received annual applications of a reactive PR, Sechura PR, P extracted by the Olsen solution was closely correlated with the soil PR reactant product, Fe-P and AI-P (Perrott et al., 1992) (Fig. 28). When there is an appreciable amount of PR present in soil, Olsen P values underestimate plant yield in comparison with soluble P fertilizers (Fig. 29) (Cornforth et al., 1983; Bolland et al., 1988a; Yost et al., 1982; Rajan et al., 1991b) . However, when the PR applied is of the crandallite/millisite type (Fe-P and AI-P) the alkaline Olsen extract may dissolve potentially unavailable PR and thus overestimate available P (Bolland et al., 1994). Separate calibration curves with yield data may be needed for both kinds of PR and soluble P fertilizer treatments. Rajan et al. ( 1 99 la,b) obtained significant correlations between bicarbonateextractable P and plant yield at different rates of PR application under field conditions, provided the soil samples were taken at least a few months after PR application (Rajan et a/., 1991a,b). This would imply that the bicarbonateextracted P was proportional to the PR already dissolved which in turn apparently correlated to the relative amounts of PR dissolved later on and was available to plants. Perrott et a/. (1992), from results at four New Zealand sites under permanent grasdlegume pasture, suggested that multiplication of Olsen P test by 1.69 for soils with a history of PR use (six annual applications) was appropriate to equate the calibration curve with that for soluble P treatment. However, this correction

PHOSPHATE ROCKS FOR DIRECT APPLICATION TO SOILS

135

40

P

30

l J '

0

100

260

300

400

560

Alkali-P (mg kg-' soil)

Figure 28 Relationship between Olsen P and alkali extractable P (Fe-P + ALP) in TSP treated

(0) and Sechura PR treated (0 ) soils in New Zedland (Benio low P retentive and Manapouri high P retentive soils) (Perrott ef ul.. 1992).

factor will vary with time as the amount of PR in soil varies with time depending on fertilizer application practice. Mackay el a / . (1984a) indeed found that 3 years after fertilizer application, soils treated with Sechura PR and SSP could be described using a single regression relationship, without any multiplication factor for the soil test values obtained from PR-treated plots. It has been suggested that whenever slow-release fertilizers containing PRs are applied other extractants capable of dissolving fertilizer apatites, such as Bray y = -3.22 + 0.39~,R2 = 0.93 (PR) y = 2.43 + O.O6x, R2 = 0.92" (MCP)

2 0 20 40 60 80 100 Bicarbonate extractable P (mg L-l soil)

Figure 29 Relationship between pasture yield and Olsen P values in plots treated with Sechura PR and a commercial monocalcium phosphate (Rajan ef a / . . 1991b).

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S. S. S. RAJAN, J. H. WATKINSON, AND A. G. SINCLAIR

extractants or weak acetic or citric acid, may be of greater value than the Olsen extractant (Rajan, 1982). Chien et al. (1978) concluded that the P extracted from acid soils treated with reactive PRs was partially derived from the residual PR as well as from the reaction products. The amount extracted varied with the reactivity of PRs. The question arises whether the amount of PR-P extracted will be in proportion to that to be dissolved and available to plants during the crop growing season, or during a year in the case of permanent pastures. Results indicate that Bray 1 underestimates P availability in PR-treated soils, relative to P test values obtained in soils where soluble P fertilizers have been used (Reinhorn and Hagin, 1978). Some researchers have tried to overcome this problem by using more acidic extractants such as Bray 2, Truog and Dyer citric acid solutions (Bationo et a l . , 1991; Bolland e t a / ., 1994). These extractants, however, overestimated available P presumably by dissolving a greater amount of PR than would dissolve in soil in a cropping period and contribute to plant uptake. The above evaluation leads to the conclusion that in soils where substantial amounts of residual PR-P contribute to P uptake by plants, because of PR dissolution during the cropping period, weak electrolytes and alkaline solutions will underestimate available P. Weak acids may extract a greater amount of P but not necessarily correlate closely with plant available P, and strong acids may overestimate available P. Consequently one often comes across conclusions similar to that stated by Bolland er a / . (1988a) that “All soil tests were equally predictive of yield but usually for each soil test separate calibrations between yield and soil test values were required for the different fertilizers for each combination of fertilizers and plant species for each year.”

2. Ion Exchange Resins Use of ion exchange resins to estimate plant available P was first proposed in the 1950s (Amer et a l . , 1955). Ion exchange resins either used as beads (Sibbesen, 1983; Dalal, 1985) or more conveniently as membranes (Saunders, 1964; Saggar e t a / ., 1990; Schoenau, 1991) have been found to be effective in estimating plant available P for a wide range of soil conditions where soluble P fertilizers have been applied (Sibbesen, 1978). In his review of various soil tests, Sibbesen concluded that the anion exchange resin (AER) method is generally better than the chemical extraction procedures (Sibbesen, 1983). Application of AER as a P test in soil containing PR was attempted in a greenhouse study by Van Raij and Van Diest (1980). They observed a poor correlation between AER P-test values and P uptake by soybean, and attributed this to the very short growing period of 35 days allowed for the crop. The authors found that P in solution, as estimated by a CaCI, solution, was more relevant. A comprehensive study on the use of AER (HC0,- form) with or without

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CER (Na form) both in the form of membranes was conducted by Saggar et al. (1990, 1992a,b). These authors found that AER extracted less P from PR or soil PR mixtures than a combination of AER and CER membranes The greater extractability when CER was included was attributed to its capacity to act as a sink for Ca of the PR. Importantly, the mixed resin of AER + CER extracted a representative portion of residual PR in accordance with the reactivity of PRs and dissolved P. These authors evaluated the mixed resin technique in a glasshouse study using four soils of contrasting P sorption capacities and containing soluble P and residual PRs of different reactivities. The reactivity of the PRs ranged from very reactive Sechura (Peruvian) to unreactive Florida (U.S.A) PR. They found that the mixed-resin P was a good predictor of plant available P in soils to which monocalcium phosphate or PRs have been applied (Fig. 30) (Saggar et al., 1992a,b). The mixed-resin test has been evaluated recently using field soils collected from several sites to which either a reactive PR (Sechura) or TSP has been applied over a period of 6 years. The results (Saggar, unpublished data, quoted by Perrott et al., 1993) showed that this test has potential as a routine soil P test, irrespective of whether soluble or PR fertilizers have been used (Fig. 3 1).

3. Iron Oxide Impregnated Paper Strips of iron oxide-impregnated filter paper developed by Sissingh ( 1983) were used as a sink to adsorb P from a suspension with soil in a 0.01 M CaCI, solution. After separation, P adsorbed on the iron oxide was determined after

80 s p.-x 60h Y

0

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3 40-

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2000

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30

40

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AER + CER P (rng kg-l soil)

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Elgure 30 Relationship between plant dry matter yield and a mixed resin (AER CER) extractable P in four soils treated with PRs. The regression line is the best tit for monocalcium phosphate treated soils (Saggar el ul., 1992b).

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1

10

-

h

v-

'% 8 -

v-

c m s 6-

rr

.-(u

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0 TSP treated plots + control 0 SPR treated plots

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Figure 31 Relationship between pasture yield and a mixed resin (AER + CER) extractable P in plots treated with either TSP or Sechura PR (Sagpar et a / . . unpublished data; Perrott ef a / . . 1993).

extraction with H2S0, (Hammond er al., 1985, Menon et a / . , 1989b). The principles underlying this technique (usually refered to as the Pi test) are similar to those of tests using anion exchange resins. The mechanisms of P extraction by the iron oxide impregnated paper have been discussed in detail by Perrott et al. (1993). In brief, when a soil suspension is shaken with the iron oxide strip, because of the high affinity between phosphate ions and oxides and hydroxides of iron, P in the solution is adsorbed by the iron oxide. This results in depletion of P in solution which promotes the release of P mainly from the soil labile pool by desorption. In contrast, AER exchange is nonspecific and dependent only on ionic charge. It has a much lower affinity if P exists only as the monovalent species, H 2 P O ~Sharpley . (1991) studied the relationships between P extracted by iron oxide strips and the P fractions in a range of calcareous and noncalcareous soils. He concluded that the iron oxide strip extracted P equivalent to that by AER, but very little of sparingly soluble soil ALP, Fe-P, or Ca-P compounds. In soils with a history of soluble P fertilizer use, the Pi test values have been found to give a higher correlation with plant dry matter yield or P uptake than chemical extraction techniques (Menon et al., 1989; Menon, 1990). When compared with the AER method the Pi test seems to be comparable (Lin er al., 1991). The use of Pi in soils with a history of PR use has not been adequately tested. In one study, which had as one of its objectives an evaluation of the Pi test for soils containing PR, the PR used was a Florida rock (Menon et al., 1988). The unreactive nature of the PR and the very short duration of the crop growth (6 weeks) would have resulted in dissolution of very little PR during the crop growth and therefore made insignificant contribution to the plant-available pool of P. Under

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such conditions the source of P for the plants, in PR treated soils also, would have been mostly from the soil pool P. This amounts to evaluation of P tests under conditions similar to that existing in soils where only soluble fertilizers have been used. In another study (Kumar et al., 1992b) also short-term crops were grown. Without knowing either how much PR dissolved during this period or its contribution to plant uptake, a reliable assessment cannot be made of the Pi test for application in soils where PR dissolution is a significant factor. Perrott and Wise (1993) identified three problems with the technique itself: (i) soil particles which adhered to the filter-paper surface, even after careful rinsing, contributed 0-85% to the Pi test; (ii) suspension pH was lowered in some cases by 0.5-1.2 units, enhancing dissolution of phosphate rock during shaking; and (iii) PR particles adhered to the strip, contributing to the Pi values during extraction.

4. Isotopic Ion Exchange The isotope exchange method measures the amount of phosphate in soil that is isotopically exchangeable with 12P-labeled phosphate in either of two ways: by measuring the change in specific activity of the solution in a soil suspension (McAuliffe et af., 1947) or by measuring the difference in specific activity between that of labeled P equilibrated with soil and that of P in a plant later grown in it (Larsen, 1952). Isotopic exchange is usually a rapid process in the first few hours but the reaction continues for weeks. Thus the amounts of exchangeable P measured by both methods are a function of time and for comparative purposes the amount estimated is that exchanged within a specified time. Because these methods, whether employed as a single measurement or as a function of time (Morel and Fardeau, 1990; Di et al., 1994; Frossard et af., 1994; Morel et al., 1994; Zapata et af., 1994), estimate a portion of adsorbed P that correlates with plant-available P, they are similar to the AER technique. As such, when used in soils containing PR, the limitations that apply to the AER technique will also apply to the isotopic technique. The other restrictions as a routine test are the safety requirements in handling isotopes and the likely higher costs involved.

B. FUTURERESEARCHNEEDS As one reviews the numerous publications on P tests one becomes aware of the deficiencies in the design of experiments intended to evaluate the various tests for use in soils with a history of PR application. .Inadequate experimental approaches appear to have resulted in not obtaining significant correlations, albeit at different levels, for a number of P tests. Ideally the following conditions should be satis-

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fied for rigorous evaluation of P tests: (i) at the time of testing one set of soils should have a history of only soluble fertilizer use and another set should contain both adsorbed P (which is normally the case) and undissolved PR; and (ii) the cropping experiment should be such that the amount of PR dissolving and contributing to P uptake by the plants during the crop growth season should exceed (preferably by two times) the coefficient of variation obtained when the yield or P uptake function is plotted against soil test values. Information on the amount of PR that has dissolved during the cropping season will also render the results more convincing. When crops are grown for a short period of only a few weeks it is probable that very little PR is dissolved. Not surprisingly, in such circumstances even an alkaline solution such as Olsen extract or weak electrolyte solutions (e.g., CaCI,), which extract exclusively or predominantly adsorbed P, may give significant correlation with plant yield parameters. In the same way when unreactive PRs are used, their contribution to P uptake may be in such a small proportio; compared with soil pool P to distinguish between different soil testing methods. In such cases highly acidic solutions (Bray 2 or Truog solution) may overestimate available P. These solutions can dissolve unreactive PRs which may not dissolve in soil during the cropping duration. To estimate available P in soils containing PR two procedures appear to be promising. One is empirical and is based on the mixed AER + CER technique and the second one is based on a mechanistic model of PR dissolution in soil and its availability to plants. The research procedure adapted by Saggar et al. ( 1992a,b) is probably close to meeting the two criteria mentioned earlier for evaluating P tests rigorously in soils containing PR. In this study, since the mixed resin of AER + CER extracted a representative portion of residual PR in accordance with the reactivity of PRs and also dissolved P, this technique needs to be evaluated further. In addition, it would appear that a single calibration is possible between P extracted and yield function irrespective of whether the soils received soluble P fertilizer or PRs. As mentioned in other sections of this review PR availability to plants is a property not only of the reactivity of the PR but also of soil and climatic conditions and the type of plant species. Therefore, greenhouse studies should be conducted initially to determine which soils can be grouped together. This should be followed by evaluation under field conditions on sites covering a range of fertilizer histories. The ultimate aim will be to provide information on P status and translation of that into fertilizer recommendations for specific soil, cropping, and climatic situations. The AER/CER method needs to be tailored so that the P extracted reflects, in addition to plant-available P from the soil pool-P, the PR-P dissolved during the cropping season for short-term crops, or annually for permanent pastures. If a very short extraction time is used a negligible amount of even

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reactive PR may dissolve, and after a very long time all of the reactive PR will dissolve. An additional advantage of the mixed resin method is the possibility of using this technique not only to determine available P but as a comprehensive method to determine multielements (Van Raij et al., 1986 ; McLaughlin et al., 1994). The mechanistic approach to determine soil P status and availability of P to plants from the PR residues includes: (i) estimation of labile P, (ii) estimation of the residual PR in soil and their particle size distribution, and (iii) prediction of the amount of PR that will dissolve during the crop growth season and the amount that becomes available to plants. Perrott and Wise (1995) proposed a relatively simple alternative to the sequential inorganic fractionation technique to measure PR residues in soil (Section 111). According to these authors this method also had the advantage of not requiring a sample of unfertilized soil to correct for background levels of acid P largely due to native fluorapatite. Once the amount of residual PR in soil is estimated, the next step is to predict how much of the PR will dissolve in the cropping season. Dissolution of PR can be predicted using the comprehensive but rather complex PR dissolution model of Kirk and Nye (1986a,b). Alternatively a simpler model (Watkinson, 1994a,b) can be used to predict the rate of dissolution of PR (Rajan and Watkinson, 1988). If it is assumed that the effectiveness of the PR is equal to the proportion of PR dissolved relative to soluble fertilizers (Section, Vl) (Sinclair et al., 1993; Watkinson and Perrott, 1993) one can predict the fertilizer requirement or the agronomic response. This assumption encompasses two aspects: there is no difference between the sources of P once P is dissolved in the soil (i) in P uptake by plants and (ii) in P immobilization or leaching in the soil. Luxury P uptake by plants immediately following addition of Soluble P could give rise to greater P removal in plant material than occurs with the gradual release of P from PR. Under cropping, however, luxury P uptake from Soluble P at an early stage in crop growth could be followed by P translocation at a later stage giving the same overall efficiency of P use by plants as can be provided by slow release P fertilizer. Bolland et al. (1988) have reported common relationships between plant yield and plant P concentration for SSP and PR with barley, oats, triticale, and clover, thus demonstrating equal internal efficiency of P derived from the two sources. Under permanent pastures a greater forage production per unit P uptake in reactive PR applied than in soluble P applied plots was reported (Mackay et al., 1984; Rajan et a/., 1991). On the other hand, data from long-term field trials (A. G . Sinclair, personal communication) with perennial pastures suggest no difference between soluble P and PR in the relationship between pasture yield and plant P uptake. It appears that provided excessive amounts of P fertilizer are

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avoided, only small differences will arise between soluble P and PR in the efficiency with which plants use the P which they take up. The other potential difference in P efficiency between the two sources could arise from differences in their reaction with soil: diffusive migration of adsorbed P into soil (Barrow, 1985; Parfitt, 1989), decomposition of hydrous oxides and aluminum-silicate clays (Rajan, 1975; Rajan and Fox, 1975), and precipitation of sparingly soluble compounds (Chien et al., 1987; Pierzynski, 1990). Such differences are to be expected in view of the very great differences in P concentration, cation concentration and pH surrounding dissolving particles of SP and PR in the soil. The results of Rajan (1991a) and Rajan et al. (1993) suggest a greater efficiency of PR, but further studies are needed to establish different nutrient efficiencies. The fact that there is good agreement between observed and predicted values of SV (Fig. 22) indicates the usefulness of the models to predict the agronomic effectiveness (Sinclair et a / ., 1993; Sinclair and Johnstone, 1995). However, this does not necessarily establish the validity of the underlying relation. More expermental evidence for similar agronomic efficiency of P derived from the dissolution of both fertilizer forms is required before predictions of agronomic performance based on PR dissolution rates can be accepted without reservation in all situations. The mechanistic model based approach requires detailed information on the rate of dissolution of PRs as affected by several factors, namely soil properties, climatic conditions, plant species, and management practices. Although this may appear daunting, modem computer technology has opened that possibility, so that predicting fertil'izer requirement based on this approach may be viewed as a practical proposition in the near future.

IX. AMENDMENTS TO PHOSPHATE ROCKS There are only a limited number of climatic and soil situations in which PRs will be sufficiently reactive for use as direct-application fertilizers, especially for fast-growing annual crops. In view of this, numerous studies have been conducted amending PRs to increase their immediate P availability and also to possibly enhance their rate of dissolution after application to soil. In this regard three processes have often been investigated: (i) composting with organic manures, (ii) combining PR with elemental S (So),with or without Thiobaciffusspp culture, and (iii) partially acidulating with mineral acids or compacting with superphosphate. The principle underlying the first two processes is production of organic or

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mineral acids which will create a localized high acidity in the immediate vicinity of PRs and also complex Ca in the case of some organic acids. Partial acidulation results in conversion of a portion of the PR to a soluble form.

A. COMPOSTING WITH ORGANIC MANURES Cornposting of PRs with agricultural wastes is known to increase the solubility of PRs (Bangar, 1985; Mey e t a / ., 1986; Mishra el a/., 1986; Mishra and Bangar, 1986; Kothandaraman, 1987; Singh and Arnberger, 1990) The extent of solubilization of a given PR varies with the kind of waste and the rate of decomposition (Bangar et a / . , 1985; Mahimairaja et a / . , 1995). For example, Bangar et a/. (1985) reported that composting unreactive Mussoorie PR with farm wastes (chopped grasses and tree leaves) increased the citric solubility of the PR. Their results from a small plot field experiment indicate that the product applied on an equivalent total P basis gave grain and straw yields of clusterbean (Cyrnopsis tetragonoloba L) equal to those on application of SSP. Similar results have also been obtained by Mishra et a/. (1984) on red gram (Cajanus cajan L). The increased P availability from the phospho-compost could have resulted both from conversion of PR-P to water-soluble form and a greater efficiency of the dissolved P in terms of its availability to plants (Khanna et al., 1983). Cornposting PR with poultry manure may not be an attractive option because poultry manures contain large amounts of CaC03 and other basic compounds (Mahimairaja et a / ., 1995). Although phospho-composts contain low amounts of P (e.g., 3.4%; Mishra et a/., 1984) they may still be favored in organic farming systems or where farm wastes are to be utilized effectively.

B. PHOSPHATE ROCK-SULFUR ASSEMBLAGES Experiments were conducted from as early as 1916 (Lipman and Mclean, 1916, 1918) on admixing So with PRs to increase the availability of PR-P. The topic was briefly reviewed by Kucey et a / . (1989). The principle behind the process is that soil microbes oxidize So to H,SO,, which in turn dissolves the PR particles which are in close proximity to So. Germida and Janzen ( 1993) recently presented a comprehensive review of the factors affecting the rate of oxidation of S o . Of the soil bacteria which are able to oxidize S o the chemoautotrophic bacteria Thiobacillus thioxidans and Thiobacillus thioparus are considered to be the most important ones (Starkey, 1966). Inoculating soils already containing Thiobacillus spp. has not been found to be of additional benefit (Rajan, 1982).

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On the other hand, soils with inadequate bacterial activity will benefit from inoculation of the PR-So with the Thiobacillus spp. bacteria. The term “biosuper” was introduced to describe PR-So-inoculated product (Swaby, 1975, 1983). Factors that influence the effectiveness of PR-So assemblages are (i) proximity of PR particles to So (ii) reactivity of the PR, (iii) mode of application, (iv) PR:So ratio, and (v) type of crops. The effectiveness of So will be maximum if the So particles are in intimate contact with PRs since this will facilitate the reaction of the H,SO, produced on the PR. For this reason PR-So has been used after cogranulation (Swaby, 1975, 1983; Rajan, 1983). The almost complete ineffectiveness of So when applied without mixing with PR has been demonstrated (Rajan, 1983). Application of either unreactive or reactive PRs in the form of PR-So granules increases their dissolution in soil and availability to plants. However, when the PR used is unreactive the agronomic effectiveness of PR-So may not equal that of soluble fertilizers, such as superphosphate (Rajan, 1982; Swaby, 1983; Loganathan et al., 1994). This is unlike the results where the PR-So product is prepared from reactive PRs (Rajan, 1982). Application of PR as a band can be expected to conserve the H2S0, produced and permit maximum acidulation of the PR. On the other hand, this may restrict the volume of P-enriched soil that is available for root interception. Studies conducted comparing mixing versus layer application of PR-So products showed that at lower rates of application layering is preferred whereas at higher rates of application mixing gave greater yields of ryegrass (Rajan, 1983). In addition to the less capital intensive technology an advantage of PR-So assemblages is the flexibility it offers in altering the PR:S ratio according to the pH and the P and S nutrient requirement of a given soil. PR-Su of lower sulfur content will be suitable for soils of greater native acidity and vice versa. Attoe and his associates (Attoe and Olson, 1966; Kittams and Attoe, 1965; Nimgade, 1968) found that products containing a PR:S ratio of 1 : I were as effective as superphosphate for ryegrass in soils of pH 6.6 or greater. They used an unreactive Florida PR. Swaby (1983) concluded that PR:S ratio of 5: 1 was effective in areas where there was adequate rainfall (>635 mm). This ratio is similar to the proportion in which So is used as H2S0, to make SSP. The greenhouse study results of Rajan ( 1983) showed that PR-So products of 7: 1 PR:S ratio gave yields of ryegrass equal to an application of SSP (Rajan;1983). He used a volcanic ash soil of pH(water) 5.6 and a reactive Sechura PR. The experiments referred to above suggest that PR-So products, especially those prepared from reactive PRs, are effective for long term crops such as pastures. Extrapolating these results, one would expect similar results with other crops such as plantation crops where a low amount of P may be required over a long period of time. The effectiveness of PR-So on short-term crops is still unknown. Whereas Swaby ( 1 983) concluded that biosupers were inferior to SSP

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for fast-growing crops, it is noteworthy that unreactive PRs were used in his studies. PR-So assemblages, especially those without inoculum, are attractive because (i) the production is not capital intensive, (ii) they enable flexible P:S ratios, (iii) they use low-grade PRs which may be unsuitable for making soluble fertilizers, and (iv) they behave as a controlled-release P and S fertilizer. However, the product has not been commercialized for want of a suitable granulation process. Recently, acceptable-size granules of PR-So have been prepared on a small scale in Thailand (D.E. Higgins, personal communication). Studies also need to be conducted on the agronomic effectiveness of PR-So granules on short-term crops. The product should be prepared using reactive PRs and applied preferably I month before crop sowing to ensure oxidation of 3’ and reaction of the H,SO, on the PR, and therefore a ready supply of soluble P for the plants. An alternative to advanced application is to preincubate PR with So and a Thiobacilli source (Ghani et a / ., 1994).

C. PARTIALLY ACIDULATED PHOSPHATE ROCKS Partially acidulated phosphate rocks (PAPRs) are PRs which have been acidulated usually with sulfuric or phosphoric acids with less than the stoichiometric quantities of acid needed for making SSP or TSP. The products usually contain a part of the P as monocalcium phosphate, the proportion of which depends on the level of acidulation, and the rest as unreacted PR. Small amounts of dicalcium phosphate may also be present. Products similar to PAPRs can also be prepared by cogranulating or compacting soluble P with PRs. Comprehensive reviews on PAPRs have been published recently by Hammond et a / . (1986), Stephen and Condron ( 1986), Bolan ef d.(1993), Hagin and Harrison, (1993) and Rajan and Marwaha (1993). In the field, partial acidulation has been found to enhance the dissolution of the PR component in PAPR, compared with that applied directly (Rajan and Watkinson, 1992). In this study the PAPR was prepared by partially acidulating a reactive North Carolina PR with phosphoric acid. The greater dissolution of PR was attributed primarily to increased root proliferation caused by the soluble P and the ensuing increase in exploitation of PR-P. Evidence was also presented of a greater rate of dissolution of PR component with increasing levels of partial acidulation. This was explained by the soil surface being initially “saturated” with P by the soluble P component, allowing the unreacted PR to dissolve at an effectively greater mean diffusion coefficient (Watkinson, 1994). The reviews referred to above show that PAPRs prepared using H,SO, directly by partial acidulation or by cogranulating PR with SSP, not only may not enhance PR dissolution, but may actually depress it. This has been attributed to CaSO,

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present as coatings on the PR particles (Hammond et al., 1980). Recent studies using electron microscopic and energy-dispersive X-ray diffraction techniques have shown that the CaSO, coatings may delay disintegration of the PAPR granules and therefore dispersion of the PR component for maximum contact with soil acidity (SSS Rajan, personal communication).

X. CONCLUDING REMARKS Because of their controlled-release properties PRs are ideally suited for longterm crops such as permanent pastures and plantation crops. In very acidic soils they may be effective even on seasonal crops. On long-term crops there is also likely to be a time lag between the time of application of PR and its adequate availability to plants. The time lag can, however, be shortened by amending PRs; it also overcomes the need for greater soil acidity for effective use of PRs on seasonal crops. Application of PRs could be more widespread if the resulting yield increases are predictable and profitable. This would aid in the economic development of countries, especially those with indigenous PR resources, and also minimize pollution in industrialized countries. To achieve predictability of PR effectiveness a network of long-term experiments should be established at well characterized sites and the rate of PR dissolution and agronomic yield parameters should be measured. Further development and application of models similar to those described in this review should be able to relate soil and site properties with dissolution of PRs and their agronomic and economic effectiveness.

ACKNOWLEDGMENTS The authors are grateful to Dr.K.W. Perrott for useful comments on the manuscript.

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Anderson, G. C., and Sale, P. W. G. (1993). Application of the Kirk and Nye phosphate rock dissolution model. Fert. Res. 35, 61-66. Archer, F. C. ( 1978). Comparison of different forms of phosphate fertilizers 11. Grassland. J. SoilSci. 29, 277-285. Attoe, 0.J., and Olson, R. A. (1966). Factors affecting rate of oxidation in soils of elemental sulphur and that added in rock phosphate-sulphur fusions. Soil Sci. 101, 317-324. Axelrod, S . , and Greidinger, D. (1979). Phosphate solubility tests-Interference of some accessory minerals. J. Soil Sci. 30, 153-157. Baifan, J., and Yichu. G. (1989). A suggested fractionation scheme of inorganic phosphorus in calcareous soils. Fert Res. 20, 159-165. Bangar, K. C., Yadav, K. S., and Mishra, M. M. (1985). Transformation of rock phosphate during cornposting and the effect of humic acid. Plant Soil 85, 259-266. Barber, S. A. (1984). “Soil Nutrient Bioavailability. I . A Mechanistic Approach.” Wiley, New York. Bardiya, M., and Gaur. A. (1972). Rock phosphate dissolution by bacteria. Indian J. Microbiol. 12, 269-27 I . Bardiya, M. C., and Gaur, A. C. (1974). Isolation and screening of microorganisms dissolving lowgrade rock phosphate. Folio Microbiol. 19, 386-389. Barnes. J. S.. and Kamprath, E. J. (1975). Availability of North Carolina rock phosphate applied to soils. Tech. Bull. 229, 3-23. Barrow, N. J. (1974). The slow reactions between soil and anions: I . Effects of time, temperature, and water content of a soil on the decrease in effectiveness of phosphate form plant growth. Soil Sci. 118, 380-386. Barrow. N. J. (1980). Evaluation and utilization of residual phosphorus in soils. In “The Role of Phosphorus in Agriculture” (F. E. Khasawneh, E. C. Sample, and E. J. Kamprath, Eds.), pp. 333-343. Am. Soc. Agron., Madison, WI. Barrow, N. J. (1985). Comparing the effectiveness of fertilizers. Fert. Res. 8, 85-90. Barrow, N. J . , and Campbell. N. A. (1972). Methods of measuring residual value of fertilizers. Aust. J. Exp. Agric. Anim. Husb. 12, 502-510. Barrow, N. J., and Carter, E. D. (1978). A modified model for evaluating residual phosphate in soil. Aust. J. Agric. Res. 29, 101 1-1021. Barrow, N. J., and Shaw, T. C. (1975).The slow reactions between soil and anions: 2. Effect of time and temperature on the decrease in phosphate concentration in the soil solution. Soil Sci. 119, 167- 177. Bascomb. C. (1964). Rapid method for the determination of cation-exchange capacity of calcareous and non-calcareous soils. J . Sci. Fd. Agric. 15, 821-823. Bationo, A., Baethgen, W. E., Christianson, C. B., and Mokwunye, A. U . (1991). Comparison of five soil testing methods to establish phosphorus sufficiency levels in soil fertilized with watersoluble and sparingly soluble-P sources. Ferr. Res. 28, 271-279. Bekele, T..Cino, B. J., Ehlert, P. A. I., Maas, V. D., and Diest, V. A. (1983). An evaluation of plant-borne factors promoting the solubilization of alkaline rock phosphates. Plant Soil 75, 361 -378. Bekele, T., and Hofner, W. (1993). Effects of different phosphate fertilizers on yield of barley and rape seed on reddish brown soils of the Ethopian highlands. Ferr. Res. 34, 243-250. Benbi, D. K.. and Gilkes. R. J. (1987). The movement into soil of P from superphosphate grains and its availability to plants. Fert. Res. 12, 21-36. Bhujbal, B. M.. and Mistry, K. B. (1985). Studies on the dissolution of Major Indian Phosphate Rocks in an acid soil. J. lndian Soc. Soil Sci. 33, 568-573. Black, C. A. (1993). “Soil Fertility Evaluation and Control.” Lewis, Boca Raton, FL. Black, C. A,, and Scott. C. 0. (1956). Fertilizer evaluation. 1. Fundamental principles. Soil Sci. Soc. Am. Proc. 20, 176- 179.

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