Crop Response To Fertilizers In Relation To Content Of “Available” Phosphorus

CROP RESPONSE TO FERTILIZERS I N RELATION TO CONTENT OF "AVAILABLE" PHOSPHORUS

G. L. Terman, W. M. Hoffman, and B. C. Wright Tennessee Valley Authority, Muscle Shoals, Alabama; United States Department of Agriculture, Beltsville, Maryland; and Mississippi State University, State College, Mississippi

I. 11. 111. IV. V. VI.

VII.

Page 59 Introduction ............................................... 60 Status of Chemial Methods in the United States and Other Countries Chemical and Physical Nature of Fertilizers Marketed in the United 66 States ..................................................... 73 Crop Response Results Prior to 1950 .......................... 77 Recent Crop Response Results ................................ A. Water Solubility of the Phosphorus and Granule Size Effects . . 78 B. Quality of the Water-Insoluble Phosphate Fractions of Fertilizers 83 Problems Concerned with Nonorthophosphates and Other Fertilizers 93 93 A. Liquid and Suspension Fertilizers ......................... B. Solid Ammonium Polyphosphates ......................... 94 94 C. Fused Potassium Phosphates ............................. D. Calcium Polyphosphates ................................. 96 E. Bulk Blends ............................................ 96 In Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 References ................................................. 98

I. Introduction

The description of the nutrient content of fertilizers is of prime importance to both producers and consumers of commercial fertilizers. For this reason, fertilizer control laws have been enacted in nearly all countries that pay particular attention to the promotion of agriculture. In the United States, regulations have been adopted by the individual States, but not by the federal government. They require guarantees of the minimum percentages of each of the three primary nutrients-total nitrogen ( N ) , available phosphorus (expressed as Pz05), and soluble potassium (expressed as KZO)-and other constituents if they are listed on the fertilizer label or tag. The development and publication of standardized methods of analysis, necessary for the successful operation of fertilizer control laws, is a function of the Association of m c i a l Agri59

60

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

cultural Chemists (AOAC). Some States specify a choice of AOAC or State methods in their fertilizer laws. The AOAC method for the evaluation of phosphorus in fertilizers is based on the quantity of phosphorus dissolved by extraction with water followed by extraction with neutral ammonium citrate solution. The soluble phosphorus determined by this method is comnionly considered to be usable by plants and is termed “available” phosphorus (expressed as the pentoxide, P205) . Fertilizer technology has changed greatly in recent years, and many new fertilizers have been introduced. Meanwhile, the assemblage of salts in many of the fertilizers has also been altered greatly. Farmers also have recently made great changes in fertilizer practices. These various changes in fertilizers and in fertilizer practices raise the question how adequately the present AOAC methods for phosphorus characterize the fertilizers that are currently being used. It should be recognized, of course, that no one laboratory method may give results in agreement with crop response over a wide range of kinds of fertilizers and of growth environments. Physical characteristics, such as granule size and placement of the fertilizer, for example, may greatly affect crop response, but cannot be reflected in a laboratory analysis of a ground fertilizer sample. The chief purpose of this chapter is to discuss the effects of various fertilizer characteristics on crop response to phosphorus in applied fertilizers, with emphasis on chemical composition and dissolution in water and ammonium citrate solutions. II. Status of Chemical Methods in the United States and Other Countries

It was known as early as 1808 that sulfuric acid will decompose mineral phosphates. However, great interest in phosphorus solubility did not occur until after Liebig’s proposal (1840) that the insoluble phosphorus in bones be rendered soluble by means of sulfuric acid. The quality of the superphosphate produced was based mainly on the phosphorus in water-soluble form. In 1842, a patent was granted to Lawes, who began the commercial manufacture of superphosphate by treating phosphate rock with sulfuric acid. It was soon discovered that the water-soluble phosphorus in this superphosphate returned to less soluble forms quite rapidly. However, this reverted phosphorus had a more beneficial effect on vegetative growth when applied to the soil than unacidulated mineral phosphates. While searching for analytical methods to distinguish between the two forms of phosphorus of low solubility, European agricultural chemists found that the reverted type was largely soluble in am-

‘‘AVAILABLE’’ PHOSPHORUS

IN FERTILIZERS

61

monium citrate solution, whereas unacidulated phosphate rock was only slightly soluble. During this same period, vegetative tests had indicated the reverted, or ammonium citrate-soluble, phosphorus to be equal in value to water-soluble phosphorus. Thus, the value of superphosphate, as far as phosphorus was concerned, was based on the quantity of soluble phosphorus, i.e., water-soluble plus citrate-soluble phosphorus, that it contained. Procedures were developed for the evaluation of phosphate materials using neutral (Fresenius et al., 1871) and alkaline ( Joulie, 1873; Petermann, 1880) ammonium citrate solutions in addition to the method based on solubility in water. The method based on the use of 2 per cent citric acid (Wagner et al., 1903) was developed specifically for basic slag in order to prevent its adulteration with raw phosphate rock. In the United States, fertilizer control chemists directed their efforts almost exclusively to the procedure of Fresenius, Neubauer, and Luck. Their investigations culminated in the incorporation of neutral ammonium citrate into the official method for the determination of available phosphorus adopted at the organizational meeting of the Association of Official Agricultural Chemists (1884). The method called for digesting the water-insoluble residue from 2 g. of the phosphate fertilizer, prepared by washing the sample with water, in 100 ml. of neutral ammonium citrate solution at 65°C. for 30 minutes with frequent shaking; washing the residue with water at room temperature; and then analyzing it for phosphorus. Direct citrate digestion, without prior washing with water, for nonacidulated fertilizers was adopted at the second meeting of the Association of Official Agricultural Chemists ( 1885). These methods were used for regulatory purposes and quality control until 1931, except that in 1922 the sample size for nonacidulated phosphates was reduced to 1g. In 1931, this modification plus a change in citrate digestion time from 30 minutes to 1 hour was extended to all nonacidulated phosphates except basic slag, and to all acidulated fertilizers. The change was made as a result of evaluation studies on ammoniated superphosphates. In 1949, basic slag, which had been evaluated by the use of 2 per cent citric acid, was included under the neutral ammonium citrate procedure. In 1950 continuous agitation by mechanical means during the citrate digestion was adopted as an official procedure. Manual shaking at 5minute intervals was deleted in 1960. A procedure for the direct determination of available phosphorus in the combined water- and neutral ammonium citrate-soluble extracts rather than by an indirect determination (total phosphorus minus the

62

C. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

citrate-insoluble phosphorus) was adopted by the Association of Official Agricultural Chemists in 1961. The methods of the Association of Official Agricultural Chemists (1960) are the official methods of the United States, Canada, and Mexico. The Association’s methods or adaptations thereof are also used wholly or in part in Colombia, Chile, India, Israel, Republic of the Philippines, and some of the states of Australia and Brazil. In the European countries, as in the United States, a great lack of uniformity in the methods of analysis existed prior to 1880. The actions taken in Germany to introduce standardization of fertilizer analysis are typical of those used in other countries. In the various Germanic kingdoms, fertilizer control by the agricultural experiment station over the fertilizer plants in their immediate vicinity was entirely voluntary, even though the fertilizer industry had asked several times to have the government establish compulsory control and uniform regulations applicable to the whole trade. Several conventions of agricultural chemists, directors of experiment stations, and fertilizer manufacturers-beginning with one at Magdeburg in 1872, followed by those in Danzig and Munich, and the last one at Halle in 1881-resulted in the adoption of standard methods of analysis of fertilizers. The method for the evaluation of phosphorus soluble in ammonium citrate was based on the use of neutral ammonium citrate. Some of the meetings of directors of agricultural experiment stations were international in scope. At Karlsruhe, Germany, Petermann ( 1880) director of the station at Gembloux, Belgium, reported on the similarity of the availability to crops of various water-soluble and water-insoluble phosphates to their chemical solubility as measured by alkaline ammonium citrate. Because of his findings, the work on evaluating soluble phosphates in the European countries shifted to the use of alkaline citrate solution and resulted in the adoption of the Petermann method, or modifications thereof, as an official procedure in most countries of Europe. Twelve of 14 countries whose methods appear in a publication of the Organization for European Economic Cooperation (1952) use alkaline ammonium citrate. Nearly every country in the world has regulations governing the marketing of fertilizers. These regulations include an analytical examination of the material for determining its nutrient content. Among the nutrients, phosphorus presents the most complex problem. Total phosphorus in a fertilizer is a definite fixed quantity and different methods for its determination should give comparable results. Inasmuch as watersoluble and citrate-soluble phosphorus are arbitrary quantities and are dependent on factors such as sample weight, time of digestion, fineness

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

63

of sample, kind of solvent, agitation, which have been chosen to give values that harmonize with crop response tests, methods for their determination vary among materials and from country to country. The bases for guarantee and quality control of phosphorus in fertilizer materials and mixtures in 31 countries appear in Table I. This table contains updated information on 21 countries surveyed by Jacob and Hill (1953) plus similar data on 10 other countries, (Note: for the sake of continuity, the same form and abbreviations are used. ) All countries except Belgium, China (Taiwan), Denmark, and India require a guarantee for total phosphorus in at least one phosphate material. These are materials such as basic slag, bone meal, guano, or nonacidulated phosphate rock for direct application to the soil. In Japan and Switzerland, mixed fertilizers in which the phosphorus is solely in the forms of these materials require only a total phosphorus guarantee. All fertilizer materials and mixtures are analyzed for total phosphorus in Australia, Canada, New Zealand, and Sweden. There is no couniry in which water-soluble phosphorus is the sole basis of guarantee for all fertilizer materials and mixtures. However, many countries do require only this determination for superphosphates. Many countries, including Australia (except New South Wales), Brazil (Bahia and S5o Paulo), Canada, Chile, China (Taiwan), Colombia, India, Israel, Italy, Republic of the Philippines, and the United States, o5cially control phosphorus solubility with neutral ammonium citrate. Alkaline ammonium citrate is used in Japan as well as in most European countries. Either citrate solution may be used in the Netherlands. Neither procedure is o5cial for any fertilizer material or mixture in New South Wales, New Zealand, Republic of South Africa, or the United Kingdom. New South Wales and New Zealand are the only constituencies that specify the use of 2 per cent citric acid for all products. Most other countries apply this procedure to the control of phosphorus quality in basic slag. According to the AOAC neutral ammonium citrate method (Association of OfficialAgricultural Chemists, 1960), 1 g. of fertilizer is placed on a filter paper and leached with successive small portions of water until 250ml. of filtrate is collected in an hour’s time, The residue from the water extraction is then extracted with 100ml. of neutral ammonium citrate under prescribed conditions, The citrate-insoluble content of the residue is then determined. Total phosphorus content of the original fertilizer is determined in a separate sample. The “available” phosphorus content of the fertilizer is defined as the difference between the total and the citrate-insoluble phosphorus contents, or it may be determined

TABLE I Bases for Guarantee and Quality Control of Fertilizer Phosphorus in Several Countriesa, Phosphorus soluble in:

country United States Canada Mexico Brazil Bahia,d Sao Paulo Chile Colombiad Australia New South Wales Queensland, Tasmania,d Victoria, South Australia, Western Australia New Zealandf Republic of South Africa China (Taiwan)f India6 Israeld Japan Republic of Philippines6 Belgium Denmark France Germany, Federal Republic

Total phosphorus

Ammonium citrate solutionc Water

-

Fm, R Bm, Bs, G , R

Fe

F F

-

-

Fg S

-

-

-

F F F Bm, G, R

S

R, pp, s S A, s

Bm, R

-

-

A, Fc, Fm, K,S S A, K Sa, Fm

-

Bm, Bs, R Bm, R, S

F F F Fm, S Fm, S, T p

-

2% Citric acid

-

R, s Bm, G, R G,R

R Bm, Fm, R

Alkaline

-

-

-

Neutral

-

Bm, R F Bm, G, R

-

F

-

-

-

-

Np, s R, pp, s F

-

F A, Bs, Fc, Fm, Sb

F

F

-

Pp, s

Fc, Fm, Mp, Pp. R, T p Bs, Fc Fc, Fm, Pp, S, T p Fm, Np, Pp, S, T p

P

r

-

Bs, Fm, Pp, Ps, R, Sa

-

Bs, Fm

-

Bs Bs, Fm

m

n

TABLE I (Continued) PhosDhorus soluble in: Country Hungary6 Irelandf Italy Netherlands

Total phosphorus R R Bs, R Bm, R

Ammonium citrate solutionc Water S S, Sb

-

Neutral

Fm, G, S A, Fc, Fm, Np, Sa, Pp, Tp

Alkaline

-

Fc, Fm

-

2% Citric acid

-

Bs Bs, Pp, R

A, Fc, Fm, A, Fc, Fm, Np, Np, Sa, S Sa, Pp, Tp Bs’ Np’ Fm, S Norway Bm, G, R Bs, Np, R NP7 RP R S Bs, Fm, Pp Polandd Bs,Pp Fm Bs,Fm Portugal Bin, Bs, R A, Fm, S R Fc, Fm, S Spain Al, Bm, Fc, Fm, Pp, S Bs Swedend F Fm, S Fm, Np, S Bs, R Fc, Fm, S Bm, Fm, G, R Switzerland PP, TP Bs United Kingdom Bm, Bs, G, Pp, Sb Bs Bd, Fc, Fm, S Bs, R S Fm, S Bs Y ugoslaviad a Updated and expanded Table XI1 from Jacob and Hill (1953). Sources of information: Association of Official Agricultural Chemists (1960), Organization for European Economic Cooperation (1952), official publications of the countries, and private cornmunications with officials in the governments and private industry. b The materials are identified as follows: A, ammonium phosphates; Al, aluminum phosphate; Bd, dissolved or vitriolized bones; Bm, bone meal; Bs, basic slag; F, all fertilizers; Fc, compound fertilizers; Fm, mixed fertilizers; G, guano; K, potassium phosphate; Mp, ammoniated magnesium phosphate; Np, nitric acid-phosphate rock products; Pp, precipitated calcium phosphates; Ps, phosphate rock-magnesium silicate glass; R, raw mineral phosphates for direct application to the soil; S, superphosphates; Sa, ammoniated superphosphates; Sb, basic superphosphate; and Tp, products of mixkures of mineral phosphates and alkali salts. c Includes water-soluble phosphorus. d Not included in survey of Jacob and Hill ( 1953). e Guarantee is not required for guano and rock. f Changed since survey of Jacob and Hill ( 1953). 9 Permitted but not required.

z‘

E

!: B

i

m

2

2

3

i!

6 B

66

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

directly in the combined water-soluble and citrate-soluble extracts. In the official method as usually carried out, water-soluble phosphorus content is not determined separately, but is included with the citrate-soluble fraction as available phosphorus, The water-soluble phosphorus content is not a part of the legal description in any state. 111. Chemical and Physical Nature of Fertilizers Marketed in the United States

Prior to 1930 the principal solubilized phosphate fertilizers were ordinary superphosphate ( 16 to 20 per cent Pz05),triple superphosphate (43 to 50 per cent P205),and ammonium phosphates. The chief purpose of analyzing for the content of available phosphorus in fertilizers prior to 1930 was to determine how completely the raw phosphate rock had been acidulated. About 1928, manufacturers of mixed fertilizer initiated the process of ammoniating superphosphate, which has now become almost a universal practice in the United States. Keenan (1930) found that ammoniation produced a number of phase changes in the phosphorus components. Ross and Jacob (1931) noted that absorption of 2' per cent ammonia by ordinary superphosphate did not materially reduce the citrate solubility but that 6 per cent ammonia caused the content of citrate-insoluble PzO5 to approach or even exceed 6 per cent P205. [Note: Degree of ammoniation is expressed in this chapter as the pounds of free ammonia per unit (20 pounds) of AOAC available P205. In the case of superphosphate containing 20 per cent available PzOs, percentage of ammonia (NHs) or nitrogen ( N ) in the fertilizer (frequently termed per cent ammoniation) is equal to pounds per unit of available P205.] When not carried to excess, ammoniation is a very useful and desirable process in fertilizer formulation because it neutralizes some of the free acid usually associated with superphosphate, improves the physical condition of the fertilizer, enhances granulation, and incorporates nitrogen into the fertilizers from the cheapest possible source without excessive dilution of the phosphate. Because of the lower cost of nitrogen in solutions, superphosphates are commonly ammoniated to the highest practical degree, which is the point beyond which losses of ammonia become excessive. In the first stage of ammoniation, the monocalcium phosphate is converted to watersoluble ammonium phosphates and water-insoluble basic calcium phosphates. Anhydrous dicalcium phosphate can form if temperatures rise appreciably during ammoniation. With higher ammoniation rates, the

67

"AVAILABLE" PHOSPHORUS IN FERTILIZERS

change in phase composition is quite different for ordinary than for concentrated superphosphates. With ordinary superphosphate, water solubility of the phosphorus decreases progressively with increase in degree of ammoniation to the practical maximum of about 7 pounds of ammonia Der unit of Pz05. With concentrated superphosphate, water solubility A

'OOr-----l 90

w

80

ORDINARY SUPERPHOSPHATE

J

m

a

a3

70

Y

0

z

60

0 "

nN

y

m

so

3

A 0

c;

u)

w

s

40

CON C ENT R AT SUPERPHOSPH

I-

30

20

10 0

I

2 3 4 5 6 LB. OF FREE NHs / 20 LB. AVAIL. P 2 0 ~

7

FIG. 1. Effect of degree of ammoniation on water solubility of phosphorus in ordinary (normal) and concentrated superphosphates. (Adapted from Hignett, 1956.)

decreases with increase in degree of ammoniation to a minimum of about 50 per cent of the total at 3 to 4 pounds of ammonia per unit of Pz05. With further ammoniation to the practical maximum of about 5 pounds of ammonia, water solubility of the phosphorus increases, owing to the formation of a greater proportion of ammonium phosphates. Effects of

68

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

ammoniation on the water solubility of the phosphorus in ordinary (OSP) and concentrated (CSP) superphosphates are shown in Fig. 1. Recently, Hignett and Brabson (1961) studied the reversion of available Pz05 in ordinary superphosphates as affected by ammoniation. Their results indicated that dissolution of the water-insoluble phosphorus in alkaline ammonium citrate is similar to that of mixtures of dicalcium phosphate and hydroxyapatite. Terman et al. (1962) presented the results of petrographic studies of fertilizers made by TVA indicating that monoammonium phosphate and basic calcium phosphates similar to apatites are the principal phosphate compounds in heavily ammoniated superphosphates and that dicalcium phosphate is a rather minor constituent. Unreacted apatites are also present. Monoammonium and diammonium phosphates are the principal phosphate constituents of ammonium phosphate nitrate and ammonium phosphate sulfate fertilizers. Dicalcium phosphates and monoammonium phosphate constitute the major phases in nitric phosphate fertilizers. In later X-ray identification work by Ando et al. (1964) quantitative estimates were made of the various phosphate compounds present in a series of commercial fertilizers. The results are shown in Table 11. AOAC citrate-soluble and citrate-insoluble apatites comprised 25 to 58 per cent of the phosphorus in ammoniated ordinary superphosphate-base NPK fertilizers. Monoammonium phosphate comprised 7 to 44 per cent of the phosphorus. Monocalcium phosphate was a major component in only 3 of 9 of these fertilizers. Of 4 ammonium phosphate fertilizers examined, monoammonium phosphate was the major phosphate phase in 11-48-0 and 13-13-13, and diammonium phosphate was the major phase in 16-48-0 and 18-46-0. Dicalcium phosphate and apatite were the major phases in a 20-10-0 nitric phosphate. Monoammonium and dicalcium phosphate were the major phases in a 20-20-0 nitric phosphate. As Fig. 2 shows, the trend in the production of ordinary, or normal (NSP), and enriched superphosphates was upward from 1943 to 1952 but has declined since 1952. Production of CSP rose from 1945 until 1961, but fell about 8 per cent in 1962. Production of ammonium phosphates has risen rapidly since 1958. Production of other phosphates has remained at a rather low level. Several surveys have been made to determine the water-soluble phosphorus contents of commercial fertilizers sold in the United States. Archer and Thomas (1956) found that the average water-soluble phosphorus in 250 samples of commercial fertilizers was 48 per cent of the available phosphorus content. In about 23 per cent of the samples, less than 40 per cent of phosphorus was water soluble.

TABLE 11 Phosphate Compounds Present in Commercial Fertilizersa Per cent of total P,O,b Sample no.

Grade

Ammoniated superphosphate B-4 4-12-12 B1-4 4-12-12 D-3 3-12-12 1-4 4-12-12 1-6 6-8-8 W-4 4-12-12 5-10-15 w-5 W-8 8-16-0 w-10 10-10-10 Ammonium phosphate A-11 1148-0 A-16 1648-0 C-13 13-13-13 18-46-0 U-18 Nitric phosphate R-20-10 R-20-20 a b 0

d

MonoMonoamcalcium monium phosphate phosphate 0 8 20 0 0 0 16 0 0

25 13 22 14

0 0 0 0

85 45

Apatite

Other Citrate (by differinsolubled ence)

MonopoDiamtassium monium Dicalcium phosphate phosphate phosphate

Citrate solublec

5 0 0 8 0 0 0

0 0 0 0 0 0 0 0 0

11 11 20 8 12 13 15 16 15

33 41 35 45 55 26 44 21 36

16 17 6 8 3 2 7 4 1

0 0 4 0

50 0 84

0

0 0 0

0 0 2 0

0 0

7

30 12

44

30

77 3

4 0

0

1 0

11 10 -8

25 23 21 6 15 18 l5

5 16

13

F

Mr

2 2v

B

9 2 w

M

8

1 v)

20-10-0 20-20-0

0 0

0 24

Data from Ando et al. ( 1964). Calculated from X-ray analyses except as noted otherwise. Difference of total apatite and citrate-insoluble apatite. Calculated from the results of chemical analysis.

0 0

0 0

73 76

24 0

1 0

2 0

70

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

Clark and Hoffman ( 1952) reported the solubilities (see tabulation) of 92 samples of superphosphates and 420 samples of multinutrient (mixed) fertilizers marketed in the United States in 1949 and 1950. SHORT TONS P,O,-

1,500

-

1,000

I

I‘,

OTHER

1945

1950

1960

1955

FIG.2. Production of normal and enriched superphosphates (NSP and ENSP), concentrated superphosphate (CSP), ammonium phosphate ( AP), and other phosphatic fertilizers, 1943 to 1962. (From U. S. Department Agriculture, 1963.)

Fifteen per cent of the samples contained less than 30 per cent of the total phosphorus in water-soluble forms. Percentage of total P,O, Available Fertilizer Superphosphates Mixed goods: NP grades PK grades NPK grades

Water-soluble

Range

Mean

Range

Mean

87-100 77-100 81- 99 51-100

97 95 95 93

5693 24-93 9-87 3-93

82 69 55 45

Clark et al. (1960) made a similar survey of fertilizers marketed in 1955 and 1956. The solubilities of 103 samples of superphosphates and 488 samples of mixed fertilizers are tabulated. Twenty-five per cent of the fertilizers contained less than 30 per cent of the total phosphorus in water-soluble forms.

71

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

Percentage of total P205 Available Fertilizer Superphosphates Mixed goods: NP grades PK grades NPK grades

Range 71-100 88- 99 93- 99 49-100

Water-soluble Mean 97 95 97 94

Range 34-94 43-93 40-84 1-96

Mean 82 66 72 41

These results indicate that the solubilities of fertilizers sold in these two periods averaged about the same. This is not surprising since, as indicated in Fig. 2, ordinary superphosphate was the dominant phosphate material used for mixed goods in both periods. Production of CSP was increasing durirg this period, but marked expansion in ammonium phosphate production had not yet begun. Water solubility of the phosphorus was higher in both periods in the north central and western than in the southeastern sections of the United States, probably because of the greater use of ordinary superphosphate in the southeast and the lower solubilities resulting from its ammoniation. As Fig. 3 shows, there has been a slight upward trend in the percentage of available phosphorus and a trend downward in the percentage of water-soluble phosphorus in fertilizers sold in the United States from 1880 to 1956. Rogers and Ensminger (1961) reported that about 75 per cent of analyzed samples of 4-10-7 fertilizer sold in Alabama contained less than 40 per cent of the available phosphorus in water-soluble form. Less than 20 per cent was water soluble in a third of the samples. Gilliam (1963) reported the solubilities of 157 samples of fertilizer sold in Mississippi during 1959 and 1960. Water-soluble P20s as a percentage of the available ranged from 4 to 97 per cent. Among fertilizer grades containing 10 per cent or less of PZO5,largely formulated with ammoniated ordinary superphosphate, none had a mean value of more than 43 per cent. Another important change in fertilizers sold in the United States, as well as in European countries, is in the extent of granulation. As discussed by Hignett ( 1963), granular fertilizers were commonplace in Great Britain, Sweden, Germany, and the Netherlands by 1950. In the United States, however, granulation of mixed fertilizers has reached commercial importance only since that time. Annual consumption of granular fertilizers in 1954 and 1955 was only about 9 per cent of all mixed fertilizers. In 1957 it was estimated that 24 per cent of all mixed fertilizers sold were granular. In 1963, probably more than half of all solid mixed fertilizers were in granular form. Farmers now have a strong

WATER-SOLUBLE

0A V A I L A B L E

P O R T I O N OF T O T A L P205

PORTION

OF T O T A L P205

W A T E R - S O L U B L E P O R T I O N OF A V A I L A B L E

100

P205

80 I-

z W

0

a

60

w

a 40

20

0 1880

1890

1900

1910

1920

1925

1930

1935

li

1949-50

100

80

60

40

20

0

1955-56

FIG. 3. Solubility and availability of phosphorus contsnt of solid commercial fertilizers for 1800 to 1956. (From Clark et al., 1960.)

73

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

preference for granular fertilizers, and the conversion is nearly complete in some sections of the United States and Europe. This upward trend in the use of granular fertilizers in the United States is shown in Fig. 4. The importance of granule size in relation to water solubility of the phosphorus for the fertilized crop is stressed in Sections IV and V. Laborat

I950

MORE FERTILIZERS ARE GRANULAR

I955

I960

I

1963

FIG.4. Trend in use of total and granular mixed fertilizers in the United States. (TVA chart.)

tory analyses, however, cannot reflect these relationships since, in order to obtain reproducible results, each fertilizer sample is ground finely before analysis. IV. Crop Response Results Prior to 1950

Superphosphate has been more widely used as a source of phosphorus in mixed fertilizers than any other phosphatic material. From the time of initiation of its manufacture by Lawes in 1842 until about 1928, superphosphate was used as a straight material and in mixed fertilizers without further treatment. About this time, however, ammoniation as a formulation process made its appearance and was rapidly accepted. Gerlach‘s pot experiments in Germany (1916) were cited as evidence that ammoniation does not reduce phosphate effectiveness. He found that ammoniated superphosphate applied on the basis of water-soluble Pz05 was superior for oats to a mixture of nonammoniated superphosphate and ammonium salts. A fairly large number of experiments were conducted during 1930

74

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

to 1932 to determine what changes in availability of the phosphorus to crops might occur as a result of ammoniation of ordinary superphosphate. Results from these experiments have been summarized recently by Wright et al. (1963). Buie (1931) summarized results from 17 field experiments conducted in 1930 comparing ammoniated ordinary superphosphate with nonammoniated superphosphate for cotton. He interpreted the results to indicate that ammoniated superphosphate was as effective as that without ammoniation; however, no details of the experiments or experimental fertilizers were given. Parker (1931) stated that ammoniation to the extent of 3 per cent ammonia in the product was the highest then being used by the fertilizer industry and that 4 per cent ammonia was “about the maximum possible under fertilizer plant practice.” With present technological skill, however, ammoniation to the extent of 6.5 pounds of nitrogen per unit of available PzOa is commonly achieved. Parker pointed out that fine particle size of the reverted phosphate in ammoniated superphosphate should enhance the dissolution of the phosphorus and, thereby, its effectiveness. He further noted that the acid produced during the nitrification of the ammonium salts in a mixture with slightly soluble calcium phosphates should solubilize the phosphorus and render it more available. As a result of the interest of the AOAC, an extensive cooperative pot study was conducted at several experiment stations. Ross and Jacob (1931) and Ross et al. (1932), in reporting the results of this research, concluded that ammoniated ordinary superphosphate was at least 90 per cent as effective as monocalcium phosphate in soils below pH 6.0, but was much less effective in soils at higher pH levels. Fertilizers used in these pot experiments were thoroughly mixed with the soils prior to planting the test crop, which greatly enhances the effectiveness of slightly soluble basic calcium phosphates in acid soils, as compared to band application. The authors chose to exclude the results of tests conducted on soils above pH 6.0, reasoning that 75 per cent of the fertilizer used in 1932 was applied to soils below pH 6.0. They recommended a change in the AOAC official method for determining “available” phosphorus which would give a higher rating to fertilizers containing ammoniated ordinary superphosphate. Williamson (1935) summarized 185 experiments with cotton on different soil types and with different fertilizer treatments. Relative yield increases from the various phosphorus sources over no applied phosphorus were rated as follows: ordinary superphosphate, 100; superphosphate ammoniated to 2, per cent ammonia, 100; and superphosphate

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

75

ammoniated to 4 per cent ammonia, 90. Inclusion of lime in the complete fertilizer greatly reduced the effectiveness of the ammoniated ordinary superphosphate. In these experiments, the average increase due to superphosphate was only 241 pounds of seed cotton per acre. The 60 pounds of PzO5 applied per acre may have been more than adequate to produce maximum yields with the 36 pounds nitrogen applied per acre. As pointed out by Terman (1960, 196l), there is little chance of detecting differences among sources of phosphorus under such circumstances. Salter and Barnes (1935) related soil pH to the effectiveness of various phosphorus sources for wheat grown in the field and Sudangrass in the greenhouse. Their results clearly demonstrated that ammoniation of ordinary superphosphate above 3 per cent ammonia markedly decreased its effectiveness, and this decrease in effectiveness became more pronounced as the soil pH increased from 5.5 to 7.0. At pH 7 the relative increase in yields over no applied phosphorus for superphosphates ammoniated to 0, 2.9, 5.4, and 7.1 per cent ammonia were 100, 72, 50, and 23, respectively, whereas on a soil at pH 6.0, the relative yield increases for these same fertilizers were 100, 87, 86, and 77, respectively. Gilbert and Pember (1936) concluded that for oats, barley, and millet grown in greenhouse pots, the phosphorus in superphosphate ammoniated to more than 4 per cent ammonia was always less effective than in those ammoniated to less than 4 per cent. The fertilizers in these tests were mixed thoroughly with acid soils, a procedure which, as pointed out above, tends to enhance the efficiency of basic calcium phosphates. Andrews (1942) reviewed the question of ammoniated superphosphates with particular reference to the AOAC official method for determining “available” phosphorus in fertilizers, He concluded that ( 1 ) superphosphate ammoniated to 2.4 per cent ammonia is a good source of phosphorus in acid-forming fertilizers on acid soils, and ( 2 ) superphosphate ammoniated to 3.0 per cent ammonia or higher is less valuable than nonammoniated superphosphate in acid-forming fertilizers, and much less valuable in neutral fertilizers. A further conclusion was that the AOAC official method for determining the availability of phosphate in regard to crop response gives too high a rating to basic calcium phosphates occurring in mixed fertilizers and to ammoniated ordinary superphosphate that contain as much as 3 per cent ammonia. Ross and associates (1947) reported results from pot experiments with fertilizers containing ammoniated superphosphates that had been subjected to varying storage conditions. The average relative yield increase over no applied phosphorus from fertilizers ammoniated to 0, 2, 3, 4, and 5 per cent ammonia were 100, 100, 88, 82, and 85, respectively,

76

G . L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

on acid soils and 100, 63, 69, 38, and 5, respectively, on a Houston clay (pH 8.2). The results of these experiments further showed that inclusion of dolomitic limestone in the fertilizer formulation greatly reduced the effectiveness of ammoniated superphosphates and that the AOAC method gives much too high a rating to superphosphate mixtures ammoniated higher than 2 per cent ammonia. They concluded that for superphosphates ammoniated more heavily than 2 per cent, “the official method gives higher availability values than those indicated by the pot tests for all crops, and that the spread between the two methods increases with increase in ammoniation of the mixture.” Rogers et al. (1953) concluded from their review of earlier results that ammoniation of ordinary superphosphate higher than 2 per cent ammonia results in a relatively small, but consistent, decrease in the effectiveness of the phosphorus. Several criticisms can be made regarding many of the field and greenhouse experiments just reviewed. Specifically, many of the experiments were conducted with finely divided, mixed-salt-type fertilizers that were mixed throughout the soil. On acid soils these conditions enhance the effectiveness of slightly soluble calcium phosphates and decrease the effectiveness of soluble phosphates such as superphosphate, so that the effectiveness of the fertilizer-soil reaction products resulting from slightly soluble compounds is maximal and that of the soluble compounds is minimal. Furthermore, many of the experiments were conducted at low yieId levels, and yield increases resulting from added phosphates were small. In some experiments the phosphorus sources were tested at rates which fell on the flat portion of the response curve. These conditions make it impossible to detect anything but gross differences among phosphate sources ( Terman, 1960, 1961 ). Finally, relative yields or relative increases in yield were used almost exclusively to evaluate the experimental fertilizers which, because yields may not be linearly related to the rate of fertilizer applied, is an unsuitable method to use in comparing similar fertilizers (Cooke, 1956; Cooke and Widdowson, 1959). Thus, when judged by present-day knowledge and standards, many of the older field experiments did not evaluate phosphorus sources very precisely. From this review, it may be concluded that ammoniation of ordinary superphosphate decreases the fertilizer efficiency of the phosphorus, and the loss of efficiency becomes greater as the degree of ammoniation increases. This effect is less pronounced in acid soils when the fertilizer is mixed throughout the soil but is outstanding in neutral or calcareous soils. Ammoniated ordinary superphosphate in mixed fertilizers containing lime is much less effective than in acid-forming fertilizers.

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

77

V. Recent Crop Response Results

A source of confusion in the interpretation of results from water solubility-granule size and placement experiments with phosphates has been the difference often found between early growth response and final yields of grain or hay crops under field conditions. Most greenhouse pot experiments are conducted for two months or less, and the growth responses obtained are analogous to the early growth response in field experiments. As indicated below, marked increases in early growth response are usually found on phophorus-responsive soils with increase in water solubility of the phosphorus, with increase in granule size of highly water-soluble sources, with decrease in granule size of sources having a low water solubility, and with band placement as compared to mixing with the soil. Whether these early growth responses follow through to final yields of forage, fiber, or grain depends on numerous factors. These include the level of plant-available phosphorus in the soil, soil reaction, adequacy of supplies of other nutrients, moisture supply, kind of crop, length of season, and others. As the season progresses, the plants draw increasingly from soil phosphorus and less from the fertilizer applied for that particular crop. Final yield differences related to water solubility of the phosphorus occur more frequently with potatoes and other relatively short-season crops than with longer season crops such as corn, cotton, small grains, and forage species. Even though early growth response by a crop may not be reflected in final yields, such “starter effects” are still quite important in enabling a row crop to push ahead of weed growth and in providing a large area of leaf growth for rapid photosynthesis. Early, vigorous growth is particularly important for early market of vegetable crops and frequently results in higher profits. In addition to possible effects on yields and profits, early, vigorous growth has an intangible esthetic value to most farmers, an important point in selling fertilizers. Eight agronomists all indicated in a survey article (Webb et al., 1959) that water solubility of the phosphorus was important in various areas of the United States under certain specified conditions. In a second survey (Thomas, 1959), the majority of the agronomists contacted in 18 states indicated that fertilizers containing 40 to 60 per cent of their phosphorus in water-soluble forms were satisfactory for most soil and crop conditions. Seatz and Stanberry (1963) reviewed recent literature concerning the complex relationships among soil and fertilizer composition, granule

78

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

size, and phosphorus water-solubility effects on crop response. Mattingly ( 1963) reviewed recent research on water- and citrate-soluble fertilizers at Rothamsted and elsewhere. Van Burg (1963) similarly reviewed results obtained in the Netherlands.

A. WATERSOLUBILITY OF THE PHOSPHORUS AND GRANULE SIZE EFFECTS

1. Greenhouse Pot Experiments Martin et al. (1963) reported that ammoniation of ordinary superphosphate to 4.5 per cent reduced the water solubility of the phophorus, but had no effect on yields of lettuce grown on four acid California soils. Yields were reduced markedly on two calcareous soils, however, by high ammoniation and the resulting reduction in water solubility. Lawton et al. (1956) found that crop response was closely related to the content of water-soluble phosphorus in granular 12-1212 fertilizers mixed with the soil or in banded pulverant fertilizers. If the pulverant fertilizer was mixed with the soil, however, crop response was not related to phosphorus water solubility. These fertilizers were prepared from slurries of ammonium nitrate, ammonium phosphate, dicalcium phosphate, and potassium chloride. Terman et al. (1956) similarly found that early growth response to phosphorus, as exhibited by oats and Sudangrass grown in greenhouse pots, increased with increase in granule size of NPK fertilizers high in water-soluble phosphorus (prepared with diammonium phosphate and ammoniated concentrated superphosphate). Early response also increased with decrease in size of granules low in water-soluble phosphorus (prepared with dicalcium phosphate or ammoniated ordinary superphosphate). In other greenhouse pot experiments, Terman et al. (1960) found that crop response to band-applied nonammoniated and ammoniated superphosphates was closely related to their content of water-soluble phosphorus. Other than with dicalcium phosphate, granule size was of little importance when the fertilizers were banded. With the phosphates mixed through the soil, both water solubility and granule size greatly influenced yields on most soils. Decrease in response with time of reaction with the soil prior to cropping was much less with granular than with fine superphosphates and dicalcium phosphate. Bouldin et al. (1960) studied the response of oats in greenhouse pot experiments to various granule sizes of monoammonium phosphate (water soluble) and dicalcium phosphate (low water solubility) and to mixtures of the two, which are commonly found in some ammoniated superphosphates and in nitric phosphates. The important fertilizer properties affecting response were found to be geometric surface area of

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

79

the granules of dicalcium phosphate and water-soluble phosphorus content as monoammonium phosphate per granule. Granule size effects of mixtures of the two phosphates were intermediate between those of the single components, and there was no measurable interaction between them. With decrease in granule size, response to both phosphates approached a common level, indicating rather complete reaction of the fine phosphates with the soil.

2. Field Experiments Terman et al. (1956) reported that the early growth response of wheat forage and other crops grown in field experiments increased with increase in granule size of water-soluble phosphorus fertilizers and with decrease in granule size of those low in water solubility. These early responses did not persist in final yields of corn or wheat grain and of vegetables in most experiments. DeMent and Seatz (1956) similarly found that high-alumina nitric phosphates having higher phosphorus water solubility were more effective than phosphates lower in solubility for increasing yields of wheat or oat forage and as starter fertilizers for corn. The degree of water solubility was of minor importance in final yields of long-season crops grown in southeastern United States. Jordan (1964) also noted that the most consistent effects of water-soluble phosphorus content, granule size or placement of fertilizers occur in the early growth stages of crops grown in this region. In a series of experiments with hill-placed phosphates for corn in Iowa, Webb and Pesek (1958) reported that all 20 experiments showed consistent trends toward larger yield increases with increasing water solubility of the phosphorus. About 90 per cent of the yield increase attributable to water solubility was attained with fertilizers having 60 per cent of the phosphorus in water-soluble form. Early season growth response was correlated very closely with amount and water solubility of the applied phosphorus. With broadcast application of phosphorus for corn, however, Webb and Pesek (1959) found that yields increased with amount of phosphorus applied but were not affected by water solubility of the phosphates. Sources very low in water solubility tended to be less effective in a few experiments. For corn grown on calcareous soils, Webb et al. (1961b) concluded that highly water-soluble phosphates applied broadcast were more effective than most slightly soluble sources. Increasing the granule size of less soluble sources tended to reduce their effectiveness. Webb et al. (1961a) found that water solubility of the phosphorus was an important factor in effectiveness of phosphates applied for oats grown on calcareous soils. On acid soils, placement of the phosphates

80

G. L. TERMAN, W. M. HOFFMAN, AND B. C . WRIGHT

was more important than water solubility. Drilling the fertilizer with oat seed was significantly superior to broadcasting on acid soils but only slightly so on calcareous soils. A comprehensive investigation was carried out in Mississippi by Wright et al. (1963) to measure the effectiveness of ammoniated ordinary superphosphates as sources of phosphorus for corn, cotton, and wheat. Results were expressed in terms of superphosphate equivalents, i.e., the I20

I

I

I

e- WHEAT

1

0

I

I

I

I

I

I

I

I

I

I

FORAQE

I

I

I 2 3 4 5 6 7 AMMONIATION. LB. NH3 PER 2 0 LB. AVAILABLE PgOs

I

FIG.5. Percentage superphosphate equivalents for various crops, as affected by degree of ammoniation of ordinary superphosphate. (From Wright et al., 1963.)

amount of phosphorus in noammoniated 20 per cent superphosphate expressed as a percentage of the amounts of phosphorus in ammoniated superphosphates (or other phosphorus sources) required to produce the same yield of crop. A response curve obtained with several rates of superphosphate was included in all experiments. As an average for the 43 field experiments conducted, effectiveness of the ammoniated superphosphates decreased as follows: Degree of ammoniation: Pounds of NH, per 20 pounds of P,O, Per cent superphosphate equivalents

0 100

2.1 85

4.2 67

6.5 39

7.2 28

Results obtained with individual crops are shown in Fig. 5. Decreases in

81

“AVADLABLE”PHOSPHORUS IN FERTILIZERS

effectiveness found in the field experiments with increase in degree of ammoniation agree quite closely with results obtained in greenhouse pot experiments with a similar series of ammoniated ordinary superphosphates (Terman et ul., 1960). No evidence was found in the Mississippi study that corn, cotton, and wheat differed appreciably in their relative responses to ammoniated superphosphates. Granule size of the fertilizers was of minor importance in these experiments, in all of which the phosphates were band-applied. As shown in Fig. 6, AOAC-available

I

0

I

I 2 AMMONIATION,

I

I

I

I

I

4 I 6 7 LB. NHs PER 20 LBS. AVAILABLE PzOs

3

FIG.6. Effectiveness of ammoniated ordinary superphosphates, as related to dissolution in neutral ammonium citrate ( AOAC), alkaline ammonium citrate ( NAAC) , and water. (From Wright et al., 1963.)

phosphorus content of the test fertilizers decreased only slightly with increase in ammoniation, which resulted in a very low correlation with crop response, Decreases in solubility with increase in ammoniation, both in water and in alkaline citrate solution (NAAC method), however, were highly correlated with crop response. In a series of experiments comparing sources and rates of phosphorus for vegetable crops in western Washington during the 1951 to 1959 period (Mortensen et ul., 1964), the importance of water solubility for various crops decreased as follows: cucumbers > pole beans > potatoes > sweet corn. At a given rate of applied phosphate fertilizer, vegetable yields increased with content of water-soluble phosphorus. In some experiments limiting yields of the crops obtained with increasing rates of

a2

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

applied phosphorus were higher with sources high in water solubility (Fig. 7 ) . Contents of phosphorus in the leaves during the early growth period were rather closely correlated with water solubility of the fertilizers and with final crop yields. Lewis (1962) grew snap beans in central Maine for two seasons with ammoniated superphosphates varying in content of water-soluble phosCUCUMBERS

8

4l

6

4 W

K

22

i,"i

0-DAP 0 - CSP A - OSP X DCPO 0 DCPA

-

2

-

!:F 2t 0

w

20

4

POLE BEANS

-

0

POTATOES

I

0

50

0 50 100 I50 P 2 0 APPLIED ~ LB. PER ACRE

-

I

I

100

150

I

FIG.7. Yield response of vegetables to sources and rates of phosphorus in westem Washington, 1959. DAP (diammonium phosphate); CSP and OSP (concentrated and ordinary superphosphates) ; DCPD and DCPA ( dicalcium phosphate, dihydrate and anhydrate). (From Mortensen et al., 1964.)

phorus. Early growth response increased in all experiments with increase in phosphorus water solubility. Only in one experiment on a soil low in soluble phosphorus, however, did water solubility affect the yield of harvested product. Lingle ( 1960) obtained similar results with tomatoes. Yields of the first harvest were higher with fertilizers higher in phosphorus water solubility, but this advantage had disappeared by the time of the second harvest. Van Burg (1963) concluded that 50 per cent water solubility of the phosphorus was adequate in fertilizers for cereals and grassland in the Netherlands but that potatoes required a water solubility close to 100 per cent.

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

83

3. Immediate us. Residual Efectiveness Several investigators ( Cooke, 1956; Ensminger and Pearson, 1957; Schmehl et al., 1955; Nelson and Stanford, 1958; Mattingly, 1963) have reported similar residual effects from acidulated phosphates differing widely in initial solubility and effect on the immediate crops. Responses by corn, millet, oats, and wheat (Terman et al., 1961b) increased in a similar manner at all seasons of the year with increase in the water-soluble phosphorus content of applied phosphate fertilizers. Banding resulted in greater response by corn than mixing of the phosphates with the soil just prior to planting. Effects of placement and of water solubility were less for a second corn crop, and there were no appreciable differences in response by a third crop to these variables. Residual phosphates have accumulated in large acreages of soils in Europe and the United States as a result of fertilizer applications continued for many years. On such soils, little or no yield response to phosphorus by the fertilized crop is obtained, and the chief purpose of continued applications of phosphorus fertilizers is to maintain the soluble soil phosphorus at a high level. For such maintenance applications any acidulated phosphate which will react with the soil is considered by Cooke (1963) to be a satisfactory fertilizer. Thus, as more soils reach rather high levels of residual phosphorus available to crops, the need for fertilizers having a high proportion of their phosphorus in water-soluble forms may decrease. B. QUALITY OF THE WATER-INSOLUBLE PHOSPHATE FRACTIONS OF

FERTILIZERS

Bouldin and Sample (1959) found a direct relationship between plant response and the geometric surface area of granules per unit of phosphorus in dicalcium phosphates. Size of crystals composing the granules was of much less importance than granule surface area. Terman et al. (1961a) found a similar relationship between crop response and granule surface area of a series of water-insoluble phosphates prepared by water leaching of ammoniated superphosphates and nitric phosphates. Fine granules and particles of these phosphates, however, tended to dissolve in and react with the soil, so that the plants obtained much of their phosphorus from the fertilizer-soil reaction products. In a later experiment, Bouldin and Sample (1963) found that plant response to phosphorus was correlated with geometric surface area of granular fertilizers prepared from mixtures of dicalcium phosphate with either glass beads or several nonphosphatic salts. Calcium phosphates more basic than

84

G . L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

dicalcium phosphate, which occur in heavily ammoniated ordinary superphosphates (AOSP) and nitric phosphates (NP), are less effective sources of phosphorus for crops than dicalcium phosphate (Beaton and Gough, 1962.;Rogers et al., 1953). Terman et al. (1961a) prepared a series of water-insoluble phosphate fractions varying in AOAC citrate solubility (neutral ammonium citrate) TABLE I11 Chemical Analyses of Water-Leached Phosphate Fractionsa AOAC fractions

Water-leached residue fromb (1) DCP ( 2 ) NP (3) NP ( 4 ) ACSP ( 5 ) AOSP ( 6 ) AOSP ( 7 ) AOSP ( 8 ) P-4 ( 9 ) P-1 (10) P-2 (11) P-3 (12) AOSP (13) AOSP

TVA fertilizer number 255L 76L 151L 253L 248L 249L 259L 4L 1L 2L 3L 302L 304L

Total P,O, (%)

44.0 45.1 42.1 34.8 29.3 34.1 35.5 42.4 43.0 40.1 38.4 17.9 29.1

NAACc available Avail- ( % of total able P,O,)

( % of total P,O,) Water soluble

Citrate soluble

1 1 2 2 3 2 2 <1 <1

98 92 83 70 44 37 41 53 51 31 23 74 51

<1 <1 1 1

99 93 85 72 47 39 43 53

51 31 23 75 52

71 77 67 51 31 16 18 52 30 9 2 67 25

Data largely from Terman et al. (1961a). DCP, feed-grade anhydrous dicalcium phosphate; NP, nitric phosphate; ACSP, ammoniated concentrated superphosphate; AOSP, ammoniated ordinary superphosphate; P1-4, heavily ammoniated nitric phosphate residues; 1g. = sample size. c Netherlands alkaline ammonium citrate method. a b

from 23 to 99 per cent of the total phosphorus content by leaching AOSP and NP fertilizers with water. Response by two successive crops of corn was found to be rather closely related to the fractions of their total phosphorus content which dissolved in a neutral citrate solution. Samples of these water-insoluble fractions were also extracted by the Netherlands alkaline ammonium citrate (NAAC) method (Organization for European Economic Cooperation, 1952,). Results of chemical analyses are shown in Table 111. Plots of the relative effectiveness of phosphorus per unit of surface area of -16+20 mesh granules of six of these phosphates against percentage of phosphorus dissolved by the AOAC and NAAC

“AVAILABLE” PHOSPHORUS IN

85

FERTILIZERS

methods are shown in Fig. 8. Effectiveness of the phosphates increased with degree of dissolution in both extractants. Dissolution of all of the phosphates was lower in alkaline citrate. Results shown in Fig. 9 indicate that dissolution by the AOAC and NAAC methods (1.0-g, samples) of a number of water-insoluble phosphate fractions of AOSP and NP fertilizers and feed-grade dicalcium phosphate (DCP) are essentially linear and highly correlated for most of

l o ‘

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W 2 W

-wc 0 W LL LL W

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? c a J

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01

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1

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I

the fractions, At zero dissolution in alkaline citrate, 20 per cent or more of the phosphorus is soluble in neutral citrate. Percentage dissolution of phosphorus in several of the original fertilizers (from which the waterinsoluble fractions were obtained) are also shown in Fig. 9. Amounts of P dissolved from all these ammoniated superphosphates were high by the AOAC method but varied widely by the NAAC method. Hignett and Brabson (1961) reported similar results with a series of 6-12-12 fertilizers formulated with ammoniated ordinary superphosphate. As shown in Fig. 10, they found that amounts of phosphorus dissolved by the NAAC method decreased with increase in degree of ammoniation of the super-

86

G . L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

phosphate, but that phosphorus in all the fertilizers was highly soluble by the AOAC method. Actually, after subtraction of the water-soluble phosphorus, a similar amount of phosphorus was dissolved by the alkaline citrate and a variable amount by the neutral citrate. The sum of the water-soluble and citrate-soluble phosphorus contents thus leads to decreasing amounts of “available PzO{ by the NAAC methods, but not with the AOAC method. The results presented in Fig. 9 suggest that the 100

I

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I

I

J

a. I- 80

e

LL

0

s ;60 0

C

WATER- INSOLUBLE P FRACTIONS OF AOSP, NP AND D C P

w J m

2 40 a

3-12-12 AND 6 - 12-12 FERTILIZER

> a 0

9z

20

0 i

40 60 80 I00 AOAC AVAILABLE P 2 0 ~ % - OF TOTAL

FIG. 9. Relation between AOAC- and NAAC-available phosphorus in waterinsoluble phosphate fractions (Table 111) and in NPK fertilizers (Table IV) prepared with ammoniated ordinary superphosphate.

amount of phosphorus per sample had marked effects on the percentages of the phosphorus dissolved by the neutral and alkaline citrate solutions. Such effects of sample size were investigated further with 7 of the AOAC water-insoluble phosphate fractions ( TVA data obtained by D. R. Bouldin) (Fig. 11).With 1.0-g. samples, the relationship between amounts of phosphorus dissolved by the AOAC and NAAC methods is essentially the same as shown in Fig. 9. With decrease in sample size (0.5, 0.25, and 0.10 g.), percentages of the total phosphorus which dissolved in both extractants increased from all of the phosphates. The AOAC extraction dissolved 88 to 100 per cent and the NAAC extraction

87

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

63 to 100 per cent of the phosphorus in 0.10-g. samples. The actual ranges of amounts of water-insoluble Pz06per sample were as follows: 1.0-g. samples: 0.5-g. samples: 0.25-g. samples: 0.10-g. samples:

0.18 to 0.45 g. 0.09 to 0.225 g. 0.045 to 0.113 g. 0.018 to 0.045 g.

In the case of the original 6-12-12 or 3-12-12, fertilizers prepared

a

t0

t-

IL

0

F 2

K W

n

0

AVAILABLE P 2 0 ~

A . 0 A.C

N A A C A V A I L A B L E P2Os

A WATER- SOLUBLE PzOs n

1

0

I

2 4 6 A M M O N I A T I O N OF ORDINARY SUPERPHOSPHATE IN 6-12-12 F E R T I L I Z E R , LB. N H s / U N I T AVAILABLE PzOs

8

FIG. 10. AOAC-available, NAAC-available, and water-soluble phosphorus content of ordinary superphosphate-base NPK fertilizers, as affected by degree of ammoniation. (From Hignett and Brabson, 1961.)

88

G. L. TERMAN, W. M. HOFFMAN, AND B. C. WRIGHT

0" &I00

5

-

I

80-

---

I

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1

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0 . 5 0 . SAMPLES

.*

I

-

40-

--

-

20-

--

-

f

% a w

I

-

60-

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% w

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0.25g. SAMPLES

100-

I

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--

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0.10 g. SAMPLES

t

1

I

.

* *

1

20 '0

20 4 0 60 80 100 0 20 40 60 80 100 PzOs DISSOLVED BY NEUTRAL C I T R A T E - % OF T O T A L PpO,

FIG.11. P,O, in 7 water-insoluble phosphates dissolved by alkaline and neutral ammonium citrate, as affected by sample size. (TVA data obtained by D. R. Bouldin.)

TABLE 1V Chemical Analyses of Ammoniated Ordinary Superphosphate-Base 6-12-12 and 3-12-12 Fertilizersa

Fertilizer grade 6-12-12

3-12-12 @

Pounds NH, TVA per unit Total of avail- P,O, fertilizer (%) number able P,O,

Per cent of Total P,O, AOAC method

302A 303A 199A 304A 305A 300A 307A

2.0 4.1 6.5 7.2 5.5 5.7 5.8

13.5 13.4 13.9 13.3 13.2 13.6 13.1

56 35 25 10 31 30 23

38 57 64 80

301A 306A

4.1 6.9

13.4 12.0

46 13

TVA data.

NAAC method

Water Citrate Avail- Citrate Availsoluble soluble able soluble able

66 73

94 92 89 90 96 96 96

31 30 24 28 27 26 29

87 65 49 38 58 56 52

50 82

96 95

32 27

78 40

65

89

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

with ammoniated ordinary superphosphates (Table IV), 89 to 96 per cent of the total phosphorus was dissolved by the AOAC method (1.0-g. samples, water followed by neutral citrate) and 38 to 87 per cent by the NAAC method (2.0-g. samples, water followed by alkaline citrate). The ranges of amount of water-insoluble PZO5in these fertilizers per sample were 0.057 to 0.105 g. with the AOAC method and 0.114 to 0.210 g. with

01 0

I I I I I00 200 300 400 WATER- INSOLUBLE PzOe YO. PER SAMPLE

-

I

FIG. 12. Phosphorus dissolved from water-insoluble phosphate fractions by neutral and alkaline ammonium citrate, as affected by amount of water-insoluble P,05 per sample (0.10, 0.25, 0.5, and l . 0 g . of material). (TVA data obtained by D. R. Bouldin. )

the NAAC method. Thus the results shown in Figs. 9 and 11 are strongly influenced by the actual amount of water-insoluble P in the samples being extracted, as well as by the extractant. This is illustrated by Fig. 12, in which the percentage of the total Pz06 dissolved is plotted against the amount of water-insoluble PZO5per sample. Percentages of the phosphorus in both water-insoluble phosphate fractions (Table 111) decreased with increase in amount of water-insoluble phosphorus in the sample, the ammoniated ordinary superphosphate fraction ( AOSP, No. 6 ) much more so than the nitric phosphate fraction (NP, No. 3). Similar results were obtained with 5 other water-insoluble fractions.

90

G. L. TERMAN,

W.

M. HOFFMAN, AND B. C. WRIGHT

Because of the small amounts of water-insoluble PzO5 in the 1.0-g. samples of NPK fertilizers formulated with ammoniated ordinary superphosphate, 89 per cent or more was dissolved by the AOAC method. With 2.0-g. samples of these fertilizers extracted by the NAAC method, however, the percentage dissolutions were much less. As shown in Fig. 13, dissolution of the various fertilizers by the latter method de-

I

I

I

I

I

FIG. 13. Phosphorus dissolved from 3-12-12 and 6-12-12 fertilizers formulated with ammoniated ordinary superphosphates in relation to the amount of water-insoluble P,O, per sample. (TVA data.)

creased linearly with increase in amount of water-insoluble phosphorus in the sample. With constant amounts of Pz06 in the sample, percentage dissolution is somewhat different than with a constant 1.0-g. sample (AOAC) or 2.0-g. sample (NAAC). These results indicate that it would be desirable to standardize the amount of water-insoluble P206 per sample extracted by both the AOAC and NAAC methods for the values to be very meaningful. It is important to note, however, that with a constant sample weight of such fertilizers the amount of water-insoluble P205 per sample increases with degree of ammoniation. The relative abundance of basic phosphates of low solubility also increases with

“AVAILABLE” PHOSPHORUS IN FERTILIZERS

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increasing ammoniation. Thus, it is difficult to separate cause and effect in terms of dissolution, as affected by size of sample and by differences in amounts of water-insoluble Pz05 in different samples. Haskins (1921) found that dissolution of a 1.0-g. sample of dicalcium phosphate in neutral ammonium citrate more nearly reflected its fertilizer value than dissolution of a 2.0-g. sample. Ross and Jacob (1931) noted a decrease in citrate-insoluble Pz05 in ammoniated superphosphates with decrease in sample size from 2.0 to 0.5 g. Howes and Jacobs (1931) found that citrate-soluble PzOB in ammoniated superphosphates (3.5 to 6.0 per cent NHB) increased greatly with decrease in size of sample from 2.0 to 1.0 g., with decrease in pH of the extractant, with increase in amount of solution, and with increase in time of digestion. Many other investigators have noted these effects since then. For example, Brosheer (1953) studied the dissolution in neutral ammonium citrate of precipitates formed during ammoniation of nitric acid extracts of phosphate rock. He found that the amount of phosphorus dissolved from 1.0-g. samples containing 0.3 to 0.5 g. of water-insoluble PzO, increased irregularly from 72 to 95 per cent of the total P20, content with decrease in estimated content of apatite from 90 to 5 per cent. However, from 97 to 100 per cent of the phosphorus in 0.25-g. samples was dissolved and dissolution was not reIated to content of apatite. In connection with the increase in dissolution with decrease in size of sample, it is of interest to note that a change in the AOAC procedure recommended by Ross et al. (1932) for use of a 2.0-g. sample when the fertilizer contained 10 per cent or less available Pz05 and a 1.0-g. sample when the fertilizer contained more than 10 per cent available Pz05 was not approved. Instead, the AOAC Subcommittee A on Recommendations of Referees recommended that a 1.0-g. sample be taken for all fertilizers regardless of the available P205 content. This latter recommendation was later incorporated into the official method. In another study carried out by TVA, NH4N03 and KCl were granulated with phosphate rock of two degrees of fineness. Results with the AOAC method, as shown in Fig. 14, indicated that the presence of the salts had increased the availability of the rock. However, analysis of the same amount of rock in samples without the salts indicated similar availabilities. Thus, decreasing the size of sample of phosphate increases its availability, as shown by the official method. More phosphorus was dissolved by the neutral ammonium citrate from the finer than from the coarser ground rock. Results from greenhouse pot tests with oats, how-

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ever, failed to show any increase in plant utilization of phosphorus in the phosphate rock due to the presence of NH4N03or KC1. Increased dissolution of phosphorus by ammonium citrate with decrease in content of water-insoluble basic calcium phosphates per sample probably results largely from the corresponding decrease in content of calcium. Phosphorus dissolution is controlled largely by the

P205 ADDED, MO. PER 100 ML. OF CITRATE SOLUTION

FIG.14. Phosphorus dissolved by neutral ammonium citrate from phosphate rock in mixtures containing NH,N03 and KCl. (TVA data obtained by D. W. Rindt. )

extent to which calcium is complexed by the citrate. Rate of solution of the calcium phosphates also decreases with increasing size of the component crystals and content of apatite. Robertson (1914) described the citric acid test used in some countries (Table I ) as a test for calcium rather than for phosphorus. He showed that a second extraction of a mineral phosphate might dissolve more phosphorus than the first extraction. That is, the “citrate insoluble” residue was more “soluble” than the original material. Rosanow (1934) showed that the “‘insoluble” residues of certain mineral phosphates

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extracted with citric acid were more active as fertilizers in pot experiments than the untreated phosphates. The removal of calcium as a result of complexing by citric acid thus increased the availability of the phosphate. These results indicate that estimates of “available” phosphorus, as determined by extraction with citric acid (or with ammonium citrate) do not necessarily separate more-active from less-active basic calcium phosphates. It has been a common practice in agronomic experiments comparing phosphorus sources and rates to apply each fertilizer on the basis of its content of AOAC available, rather than of total, PzO,. With fertilizers containing onIy smalI amounts of citrate-insoluble phosphorus, this practice is satisfactory, but as shown above, it can be misleading with some fertilizers containing high amounts of water-insoluble phosphorus. With such fertilizers, it would seem much more accurate to make applications on the basis of total PzOs content. VI. Problems Concerned with Nonorthophosphates and Other Fertilizers

Several new materials that in recent years have entered, or may soon enter, the commercial fertilizer market have created some new problems of analyses for fertilizer anaIysts and controI officials. These incIude liquid and suspension fertilizers, particularly those that contain polyphosphates; ammonium polyphosphates; calcium polyphosphates, including calcium metaphosphate; and bulk blends.

A. LIQUIDAND SUSPENSIONFERTILIZERS Expansion of the liquid mixed fertilizer industry and problems of production were discussed by Slack (1957) and more recently by Potts (1963). With use of orthophosphoric acid, an 8-244 is about the maximum that can be used without danger of salting out. With use of superphosphoric acid, in which about half of the phosphorus is present as various nonorthophosphates, liquid grades such as 11-33-0 ( Striplin et al., 1959) or 11-37-0 (Slack and Scott, 1962) can be made. A 13-43-0 base suspension is also produced by TVA by ammoniation of superphosphoric acid and adding attapulgite clay (Slack and Scott, 1962). These liquids and suspensions result in crop yields equal to those obtained with water-soluble solids applied so as to contact the same amount of soil and to supply equal amounts of nitrogen and phosphorus. The Association of Official Agricultural Chemists’ “Official Methods of Analysis” (1960) makes no provision in the method for water-soluble

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phosphorus to hydrolyze meta-, pyro-, and polyphosphates to orthophosphate, which is determined by the official method. MacIntire et al. (1937) recommended that solutions of metaphosphate in water or in dilute acid be boiled with HN03 to convert nonortho- to orthophosphates before determination of phosphorus. Because of numerous reports of incorrect analyses of liquid fertilizers containing polyphosphates, boiling of the filtrate containing the water-soluble phosphorus fraction of the fertilizer was made official in 1960 by Association of Official Agricultural Chemists (1961).

B. SOLDAMMONIUMPOLYPHOSPHATES Ammonium polyphosphates prepared in solid form (Getsinger et al., 1962) are produced by ammoniating superphosphoric acid under pressure. These fertilizers (grades ranging from about 18-56-0 to 15-61-0) can be used for direct application, for preparation of NPK grades, or for liquid fertilizers (Slack, 1962). Ammonium pyrophosphates and short-chain ammonium polyphosphates are water soluble. Consequently there is no special analytical problem except that of hydrolysis to the orthophosphate form. Stinson et al. (1956) found that some ammonium metaphosphates made by gasphase reaction of NH3 and P205were less soluble in neutral ammonium citrate than in water. Certain long-chain polyphosphates have recently been identified (unpublished TVA data) of which large fractions are not readily soluble in water. However, some of these experimental products have greater solubility in water than in ammonium citrate solution, apparently because of the NH4+ common ion effect. With these materials there is a problem of separating AOAC water-soluble and non-water-soluble fractions as discussed below. C. FUSEDPOTASSIUM PHOSPHATES Fusion products approximating the anaIyses of potassium metaphosphate ( KP03) and calcium potassium pyrophosphate ( CaK2P20T)have been produced and evaluated by TVA (DeMent et al., 1963). In powder form, these materials were essentially equivalent to CSP and KC1 as sources of phosphorus and potassium for crops. Effectiveness for the immediate crop decreased with increase in particle size. Results with pure calcium ammonium and with calcium potassium pyrophosphates as sources of nitrogen, phosphorus and potassium for corn have been reported by Lehr et al. (1964). Difficulties are encountered (Brabson, 1963) in determining the water solubility of potassium phosphates and other fertilizers containing

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partially water-soluble nonorthophosphates. The present official method (Association of Official Agricultural Chemists, 1960) of washing a 1.0-g. sample of fertilizer on a filter paper with successive small portions of water until 250 ml. of filtrate is obtained was adopted when normal superphosphate was the primary phosphatic constituent of fertilizers. The water-soluble monocalcium phosphate hydrolyzes in dilute solution and dicalcium phosphate precipitates. HNO, is added prior to diluting to volume to dissolve the precipitate (turbidity) and obtain a true measure of water solubility. When this method is applied to potassium metaphosphate or calcium potassium pyrophosphates and probably to partially water-soluble polyphosphates in general, the filtrate is usually turbid because of colloidal material passing through the filter paper, rather than because of formation of a precipitate. Acidification of this turbid filtrate leads to high contents of “water-soluble” phosphorus. An alternative procedure which has been used by TVA in connection with development research is that of placing a 1.0-g. sample in a flask, diluting to 250 ml., and shaking for 45 minutes. The suspension is then allowed to settle for 15 minutes, and the supernatant liquid is filtered prior to analysis for content of watersoluble phosphorus. Another problem is that amounts of phosphorus dissolved from meta-, pyro-, and polyphosphates by ammonium citrate solutions is more a function of their rate of solution and hydrolysis than of a true solubility. Rate of solution is strongly influenced by particle size, kind of nonorthophosphate, and other factors. Thus only total phosphorus content is particularly meaningful by official AOAC methods. This was recognized by DeMent et al. ( 1963). Determination of citrate-insoluble phosphorus in nonorthophosphate may, however, provide information on the content of apatite. Harris (1963) also found that dissolution of potassium metaphospliates in water and citric acid was not a good measure of content of available Pz06. The AOAC method was also unsatisfactory. It was concluded that the extraction of a 1.0-g. sample with 100 ml. of either 2 per cent sodium nitrate or sodium chloride solutions for 30 minutes did give a reliable estimate of available P205,provided that little potassium chloride was present. If chloride is present, the sample can be extracted with water and then with salt solution. Harris (1963) reported that the phosphorus content of the second and third cuttings of grass was higher with potassium metaphosphate than with equivalent amounts of phosphorus from superphosphate. Lehr et a,?.(1964) found that response by a first crop of corn was greater for 100-mesh than for -6+14 mesh calcium ammonium and calcium

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potassium pyrophosphates. Residual response by a second crop to the coarse granules, however, was much greater. These results indicate a slower rate of dissolution and hydrolysis of the nonorthophosphates, particularly from large particles. Potassium metaphosphate has been produced experimentally for many years, but is not produced commercially. As a result of commercial production of superphosphoric acid, various nonorthophosphate components of fertilizers may become increasingly abundant, and there appears to be a need for development of better methods to assess their chemical availability.

D. CALCIUM POLYPHOSPHATES One calcium polyphosphate ( calcium metaphosphate ), produced by TVA by reacting Pz06 gas with phosphate rock, has found considerable use for direct application, as an ingredient of mixed fertilizers without hydrolysis, and for mixing with other ingredients after partial hydrolysis to orthophosphates ( Nelson and Terman, 1963). Hoffman and Lundell (1937) reported that digestion for 30 minutes in the HCl-HN03 mixture specified by AOAC (2.018b, 1960) was adequate for the decomposition and hydrolysis of calcium metaphosphate. Brabson and Edwards (1951) found that 20-mesh calcium metaphosphate was satisfactory with one AOAC method (2.018b, 1960) and that 35-mesh material dissolves more rapidly, Grinding of calcium metaphosphate to 35 mesh was first specified in the official methods of AOAC in 1955. Continuous agitation of the sample for 1 hour is also specified, which results in a nearly complete dissolution of the phosphates present.

E. BULKBLENDS The problem with bulk-blended fertilizers is one of sampling rather than of analytical method. Analyses of samples, even from a single lot of fertilizer, may vary greatly because of segregation of various sizes of granules. Proper mixing, together with similarly sized ingredients, reduces segregation (Hoffmeister et al., 1964) and the variability in the analyses, not only of phosphorus but of other nutrients as well. VII. In Conclusion

As indicated in the preceding sections, there is a need for serious consideration of the type of information that can be obtained from a single chemical analysis of phosphate fertilizers. The AOAC method for determining available phosphorus measures only the sum of the phosphorus soluble in water and in neutral ammonium citrate. It is un-

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fortunate that the term “available” was used to describe these soluble phosphates, as this word has fostered the feeling that chemical availability and crop availability are identical. At present, the chief use of the AOAC method for phosphorus is to establish a minimum level of reactivity that is commonly termed “availability” in the chemical sense. It is used effectively by state control officials in their surveillance of fertilizers moving from the producer to the consumer and has ensured that unacidulated phosphate rock is not marketed as an acidulated phosphate. It is aIso highly desirable, of course, that the official method should indicate the commercial value of the fertilizer as a source of phosphorus for crops, since this is the reason for the manufacture and use of phosphorus fertilizers. The present method sometimes fails to do this. Obviously, no one chemical procedure will perform satisfactorily for all fertilizer materials and all crop, climate, and soil conditions; but if only one method is used, it should give a satisfactory indication of crop availability for most of the fertilizers being marketed. If this is not true, then a new chemical method should be adopted that will perform more satisfactorily, as confirmed by results obtained under soil and crop conditions now prevailing. For example, as shown by Wright et al. (1963) and Brabson and Burch (1964a, b ) , a modification of the NAAC method may be found more useful for fertilizers based on ammoniated ordinary superphosphates. Agronomists in many States are currently recommending that farmers use water-soluble phosphates under certain conditions and for certain crops. However, water-soluble phosphorus determinations are not routinely made by State fertilizer control laboratories, and the legal description of fertilizers offered for sale contains no information relative to that portion of the available phosphorus content which is water soluble. As a result, the user must rely on other sources of information concerning the water solubility, such as sales literature and his own previous experience. Making the water-soluble, as we11 as the “available,” phosphorus content of fertilizers a part of the legal description would remedy this situation, and in our opinion this should be done. This would be valuable to users of heavily ammoniated ordinary superphosphates, for which the AOAC “available P205))content is not a particularly good indicator of the agronomic value of the phosphorus. Because of the marked effect of amount of water-insoluble phosphorus per sample on the content of “available” phosphorus in fertilizers, we also recommend that the AOAC method be modified to standardize the water-insoluble content per sample within rather narrow limits. In regard to terminology, we recommend that the term “available

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PzOc be replaced by “water-soluble plus citrate-soluble phosphorus,” as determined under prescribed conditions. The AOAC method was not designed for a chemical evaluation of polyphosphate fertilizers, and there is some doubt whether it can be modified for their proper evaluation. There is therefore a need for research to find suitable chemical methods for evaluating polyphosphates. ACKNOWLEDGMENT This chapter was prepared under the auspices of the Fertilizer Evaluation Committee of the Soil Science Society of America. Other members of the committee are D. R. Bouldin, 0. P. Engelstad, W. R. Schmehl, and J. R. Webb, chairman. W. M. Hoffman is the AOAC associate referee on phosphorus in fertilizers. REFERENCES Ando, J., Siegel, M. R., and Jordan, J. E. 1964. Unpublished TVA data. Andrews, W. B. 1942. J . Assoc. Ofic. Agr. Chemists 25, 498-509. Archer, J. R., and Thomas, R. P. 1956. J . Agr. Food Chem. 4, 608-613. Association of Official Agricultural Chemists. 1884. Proc. 1st Ann. Meeting, A.O.A.C., Philadelphia, 1884. Association of Official Agricultural Chemists. 1885. U.S. Dept. Agr. Diu. Chern. Bull. 7. Association of Official Agricultural Chemists. 1960. “Official Methods of Analysis,” 9th ed. Washington, D. C. Association of Official Agricultural Chemists. 1961. J. Assoc. W c . Agr. Chemists 44, 133-134. Beaton, J. D., and Gough, N. A. 1962. Soil. Sci. SOC. Am. Proc. 26, 265-270. Bouldin, D. R.,and Sample, E. C. 1959. Soit Sci. SOC. Am. Proc. 23, 276-281. Bouldin, D. R., and Sample, E. C. 1963. J . Agr. Food Chern. 11, 212-214. Bouldin, D. R., DeMent, J. D., and Sample, E. C. 1960. J . Agr. Food Chem. 8, 470-474. Brabson, J. A. 1963. Personal communication, TVA. Brabson, J. A., and Burch, W. G. 1964a. J. Assoc. Ofic. Agr. Chemists. ( I n press). Brabson, J. A., and Burch, W. G. 1964b. J . Assoc. Ofic. Agr. Chemists. ( I n press). Brabson, J. A., and Edwards, 0. W. 1951. J . Assoc. Ofic. Agr. Chemists 34, 771777. Brosheer, J. C. 1953. Unpublished TVA data. Buie, T. S. 1931. Comm. Fert. 4 2 ( 3 ) , 27-28. Clark, K. G., and Hoffman, W. M. 1952. Farm. Chem. 115(5), 17-20, 21, 23. Clark, K. G., Hoffman, W. M., and Freeman, H. P. 1960. J . Agr. Food Chern. 8, 2-7. Cooke, G. W. 1956. J. Agr. Sci. 48, 74-103. Cooke, G. W. 1963. Personal communication to T. P. Hignett, TVA. Cooke, G. W., and Widdowson, F. V. 1959. J . Agr. Sci. 53, 46-63. DeMent, J. D., and Seatz, L. F. 1956. 1. Agr. Food Chem. 4, 432-435. DeMent, J. D., Terman, G. L., and Bradford, B. N. 1963. J. Agr. Food Chem. 11, 207-212.

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Ensminger, L. E., and Pearson, R. W. 1957. Soil Sci. SOC. Am. Proc. 21, 80-84. Fresenius, R., Neubauer, C., and Luck, E. 1871. Z . Anal. Chem. 10, 133-158. Gerlach. 1916. Z. Angwew. Chem. 29( l ) , 13-14, 18-20. Getsinger, J. G., Siege], M. R., and Mann, H. C. 1962. J . Agr. Food Chem. 10, 341-344. Gilbert, B. E., and Pember, F. R. 1936. Rhode Island Uniu. Agr. Expt. Sta. Bull. 256. Gilliam, J. W. 1963. M. S. thesis. Mississippi State Univ., State College, Mississippi. Harris, F. J. 1963. Fertiliser SOC. (Engl.) Proc. 76. Haskins, H. D. 1921. J . Assoc. Opt. Agr. Chemists 4, 64-66. Hignett, T. P. 1956. Com. Fertilizer 9 2 ( 5 ) , 23-24, 26, 67. Hignett, T. P. 1963. Farm Chem. 126( l ) , 34-35. Hignett, T. P., and Brabson, J. A. 1961. J . Agr. Food Chem. 9, 272-276. Hoffman, J. I., and Lundell, G. E. F. 1937. J . Res. Natl. Bur. Std. 19, 59-64. Hoffmeister, G., Watkins, S. C., and Silverberg, J. 1964. J . Agr. Food Chem. 12, 64-69. Howes, C. C., and Jacobs, C. B. 1931. Ind. Eng. Chem. Anal. Ed. 3 , 70-72. Jacob, K. D., and Hill, W. L. 1953. In “Soil and Fertilizer Phosphorus in Crop Nutrition” (W. H. Pierre and A. G. Norman, eds.), Vol. 4, pp. 299-345. Academic Press, New York. Jordon, H. V. 1964. U . S. Dept. Agr. Tech. Bull. (In press), Joulie, H. 1873. Monit. Sci. 3, 563-584. Keenan, F. G. 1930. Ind. Eng. Chem. 22, 1378-1382. Lawton, K., Apostolakis, C., Cook, R. L., and Hill, W. L. 1956. Soil Sci. 82, 465-476. Lehr, J. R., Engelstad, 0. P., and Brown, E. H. 1964. Soil Sci. SOC. Am. Proc. (In press). Lewis, D. T. 1962. M. S. thesis. Univ. Maine, Orono, Maine. Liebig, J. 1840. “Organic Chemistry in Its Application to Agriculture and Physiology.” Lingle, J. C. 1960. Proc. Am. SOC. Hort. Sci. 76, 495-503. MacIntire, W. H., Hardin, L. J., and Oldham, F. D. 1937. Ind. Eng. Chem. 29, 224-234. Martin, W. E., Vlamis, J., and Quick, J. 1953. Soil Sci. 75, 41-49. Mattingly, G. E. G. 1963. Fertiliser SOC. (Engl.) Proc. 75, 55-97. Mortensen, W. P., Baker, A. S., and Tennan, G. L. 1964. Wash. State Uniu. Agr. Expt. Sta. Bull. p. 652. Nelson, W. L., and Stanford, G. 1958. Aduan. Agron. 10, 67-141. Nelson, W. L., and Terman, G . L. 1963. In “Fertilizer Technology and Usage.” ( M . H. McVickar, G. L. Bridger, and L. B. Nelson, eds.), pp, 379-427. Soil Sci. SOC.Am., Madison, Wisconsin. Organization for European Economic Cooperation. 1952. “Fertilizers: Methods of Analysis Used in OEEC Countries.” Paris. Parker, F. W. 1931. Corn. Fertilizer 4 2 ( 5 ) , 28-44. Petermann, A. 1880. Landwirtsch. Vers. Sta. 24, 310-350. Potts, J. M. 1963. Fertilizer Solutions Magazine 5 ( 2 ) , 18-21. Robertson, G. S. 1914. J. SOC. Chem. Ind. (London) 33, 9. Rogers, H. T., and Ensminger, L. E. 1961. Chem. Farming (Spring).

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Rogers, H. T., Pearson, R. W., and Ensminger, L. E. 1953. In “Soil and Fertilizer Phosphorus in Crop Nutrition” (W. H. Pierre and A. G. Norman, eds. ), Vol. 4, pp. 189-242. Academic Press, New York. Rosanow, S. N. 1934. Phophorsaeure 4, 641. Ross, W. H., and Jacob, K. D. 1931. J. Assoc. Ofic. Agr. Chemists 14, 182-196. Ross, W. H., Jacob, K. D., and Beeson, K. C. 1932. J . Assoc. Ofic. Agr. Chemists 15, 227-266. Ross, W. H., Adams, J. R., Hardesty, J. O., and Whittaker, C. W. 1947. J . Assoc. Ofic. Agr. Chemists 30, 624-640. Salter, R. M., and Barnes, E. E. 1935. Ohio. Agr. Expt. Sta. Bull. 553. Schmehl, W. R., Olsen, S. R., Gardner, R., Romsdal, S. D., and Kunkel, R. 1955. Colo. Agr. Expt. Sta. Tech. Bull. 58. Seatz, L. F., and Stanberry, C. 0. 1963. In “Fertilizer Technology and Usage” (M. H. McVickar, G. L. Bridger, and L. B. Nelson, eds.), pp. 155-187. Soil Sci. SOC.Am., Madison, Wisconsin. Slack, A. V. 1957. Com. Fertilizer 95, 28-29, 33, 35-37, 39-40. Slack, A. V. 1962. Farm Chem. 125( l l ) , 16, 18, 20. Slack, A. V., and Scott, W. C. 1962. Com. Fertilizer 105( 11), 24-26. Stinson, J. M., Striplin, M. M., Brown, N. A., and Seatz, L. F. 1956. I. Agr. Food Chem. 4, 248-254. Striplin, M. M., Stinson, J. M., and Wilbanks, J. A. 1959. J, Agr. Food Chem. 7, 623-628. Terman, G. L. 1960. Soil Sci. SOC. Am. Proc. 24, 356-360. Terman, G. L. 1961. Soil Sci. SOC. Am. Proc. 25, 49-52. Terman, G. L., Anthony, J. L., Mortensen, W. P., and Lutz, J. A. 1956. Soil. Sci. SOC.Am. Proc. 20, 551-556. Terman, G. L., DeMent, J. D., Clements, L. B., and Lutz, J. A. 1960. J. Agr. Food Chem. 8, 13-18. Terman, G. L., Bouldin, D. R., and Webb, J. R. 1961a. J. Agr. Food Chem. 9, 166-170. Terman, G. L., DeMent, J. D., and Engelstad, 0. P. 1961b. Agron. J. 53, 221224. Terman, G. L., Bouldin, D. R., and Webb, J. R. 1962. Aduan. Agron. 14, 265319. Thomas, R. P. 1959. Croplife 6 ( 2 6 ) . U. S. Department of Agriculture. 1963. “The Fertilizer Situation.” Govt. Printing Office, Washington, D. C. van Burg, P. F. J. 1963. Fertiliser SOC. (Engl.) Proc. 75, 5-54. Wagner, P., Dorsch, R., Aschoff, F., and Kunze, R. 1903. Mitt. Ver. Deut. Landw. Vers. Sta. 1. Webb, J. R., and Pesek, J. T. 1958. Soil Sci. SOC. Am. Proc. 22, 533-538. Webb, J. R., and Pesek, J. T. 1959. Soil Sci. SOC. Am. Proc. 23, 381-384. Webb, J. R., Lathwell, D. J., Caldwell, A. G., Terman, G. L., Schmehl, W. R., and Mortensen, W. P. 1959. Crops Soils 12( l ) , 12-15. Webb, J. R., Pesek, J. T., and Eik, K. 1961a. Soil Sci. SOC. Am. Proc. 25, 222-226. Webb, J. R., Eik, K., and Pesek, J. T. 1961b. Soil Sci. SOC. Am. Proc. 25, 232-236. Williamson, J. T. 1935. J. Am. SOC. Agron. 27, 724-728. Wright, B. C., Lancaster, J. D., and Anthony, J. L. 1963. Mississippi State Uniu. Agr. Expt. Sta. Tech. Bull. 52.