Agronomic measures for better utilization of soil and fertilizer phosphates

European Journal of Agronomy 7 (1997) 221–233

Agronomic measures for better utilization of soil and fertilizer phosphates Konrad Mengel* Institute of Plant Nutrition Justus Liebig University, Su¨danlage 6, D 39390 Giessen, Germany Accepted 16 June 1997

Abstract Global known phosphate deposits are a finite resource which will run out in about four centuries at the present consumption rate. Since about 90% of the phosphate mined is used for fertilizer, soil and fertilizer phosphate should be efficiently used. Various agronomic measures are discussed relevant for saving phosphate and avoiding losses. Phosphate fertilizer rates should be adjusted to measured requirements for phosphate using soil tests. Particularly in areas with high livestock intensities soils frequently are much enriched in available phosphate and do not need further phosphate application whether in organic or in inorganic form. Excessively high levels of available soil phosphate, much higher than required for optimum crop production increase the hazard of phosphate loss by wind and water erosion and even leaching. Loss of plant available phosphate in soils occurs by phosphate fixation which is especially strong in acid mineral soils. Such losses can be dramatically reduced by liming soils to a pH of 6–7. In tropical areas where lime frequently is not available row placement of phosphate fertilizer is recommended. Oxisols with a very low pH liming, however, may promote phosphate fixation due to the formation of phosphate adsorbing Al complexes. Biological assimilation of phosphate may prevent inorganic phosphate from fixation by soil particles. Organic anions produced during the decomposition of organic matter in soils as well as the excretion of anions by plant roots depress phosphate adsorption by competing with phosphate for binding sites at the adsorbing surface. Hence farming systems and rotations which bring much organic matter into soils contribute to a better use of soil and fertilizer phosphate. Mycorrhization of plant roots with appropriate fungi ecotypes may essentially improve the exploitation of soil phosphates. The choice of the appropriate phosphate fertilizer type is crucial for its efficient use. This applies particularly for apatitic fertilizers of which the availability is poor in weakly acid to neutral and calcareous soils.  1997 Elsevier Science B.V. Keywords: Phosphate availability; Phosphate fertilizer; Livestock; Farm yard manure; Phosphate reserves; Phosphate fixation; Ca phosphates; pH; Liming; Mycorrhiza; Cropping systems

1. Introduction Phosphate deposits are finite resources. According to Sheldon (1982) known deposits of phosphate rock will last about 400 years at current rates of exploitation. Werner (1982) distinguishes between * Tel.: +49 641 9939161; fax: +49 641 9939199.

three categories of phosphate resources as shown in Table 1. Reserves are phosphate deposits which under the prevailing economic and technological conditions are worth mining. Phosphate resources comprise all known global phosphate deposits including those which under the present conditions cannot be mined for economic and technological reasons. Technological reasons are mainly the contamination of phosphate

1161-0301/97/$17.00  1997 Elsevier Science B.V. All rights reserved PII S1161-0301 (97 )0 0037-3

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rock with Fe, Al, and/or Mg which disturb the processing, or logistic reasons such as the far remoteness of deposits. High Cd concentrations in phosphate rock is today also a reason for European countries not to use it as fertilizer source (Baechle and Wolstein, 1984). Speculative resources (line 3 in Table 1) are phosphates not yet discovered and the existence of which is based on geological hypothesis. This latter category also comprises deposits at great depth and deposits with low P concentrations. Since this category is very hypothetical, it should not be considered as a realistic phosphate reserve. The longevity of phosphate deposits shown in Table 1 is based on the assumption that the phosphate consumption rates per year still increase during the last two decades of the 20th century and then remain constant from the beginning of the year 2000. According to the FAO Fertilizer Yearbook the phosphate consumption rates increased until 1988/89 with a peak consumption of 38 × 106 t/ year and then declined with a minimum consumption of 29 × 106 t in 1994 followed by an increasing tendency. From this trend it is clear that the longevities of the reserves and the reserves plus resources are very short as compared with the history of mankind and therefore mining and consumption of phosphates should be handled with much care and any waste of this resource should be avoided. About 90% of the phosphate mined is used for the production of fertilizers (Werner, 1982). The fertility of European soils being exhausted of available phosphate in the last century by cropping without compensating for phosphates removed from the soil by crops, was much improved by phosphate fertilizer application at the end of the last century and in the first half of the 20th century (Boulaine, 1992). In developing countries there are still large areas of agricultural land with insufficient available phosphate and hence require phosphate fertilization particularly under the pressure of an increasing world population. This precarious situation demands very careful and economic use of phosphates. The flow of phosphate goes from the deposits to agricultural land and from here partially into the crops which may be eaten by humans and animals. Phosphate in crops consumed by farm animals is largely recycled with farmyard manure or slurry to the soils. Phosphate in plant parts or in animals and in animal products exported from the farm is lost for

the farm and in many cases also as potential sources of fertilizer phosphate. From the phosphate harvested in crops a high proportion is discharged into public waste systems and not returned to agricultural land. In Europe about 25% of the phosphate excreted by man is used as fertilizer (Winteringham, 1992). In developing countries the proportion of phosphate recycled to agricultural land with human excrements will decline with the increasing proportion of population living in primitive urban societies where recycling of phosphate in wastes is hardly possible. A considerable amount of available plant phosphate is also lost in agricutural land by a permanent transformation of soil phosphates into stable forms which are not available to plant roots. Even phosphate leaching into deeper soil layers not accessible to roots may occur particularly in organic soils (Munk, 1972). Deforestation and overgrazing leads to wind and water erosion and therefore also to a loss of phosphates bound to fine organic and inorganic soil particles. A substantial amount of phosphate in eroded particles and also in urban wastes finally flows into the ocean from where it cannot be recovered (Isermann, 1990). Saving phosphate is also a question of an efficient use of soil and fertilizer phosphate by farmers. In this paper, pertinent agronomic measures for improving phosphate efficiency are discussed. These measures are: fertilizing phosphate according to soil tests for available phosphate, providing an optimum soil pH for phosphate availability, using appropriate phosphate fertilizer types and practicing rotations and farming systems with crop species capable of mobilizing fixed or less soluble soil phosphates.

2. Phosphate fertilizing according to P soil tests The level of available soil phosphate should meet Table 1 Phosphate reserves, resources and longevity (Werner, 1982) P reserves and resources

109 t

Longevity, years

Reserves Reserves + resources Reserves + resources + hypothetical resources

35 130 1130

85 360 3400

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the demand of crops but should not be much higher than the optimum since otherwise major losses of available phosphate may occur. In this context the term available means phosphate which is accessible to and can be taken up by plant roots. Loss by run off of available soil phosphate with soil particles (Sharpley, 1993) will be higher when soils are richer in available phosphate. The same is true for the hazard of leaching of phosphate out of the rooting zone which may occur in organic soils (Munk, 1972) and in sandy soils particularly if overloaded with fertilizer phosphate (Isermann, 1990; Mozaffari and Sims, 1996; Peters and Basta, 1996). Consequently levels of available phosphate in soils which are above the optimum requirement for crop growth lead to a dissipation of phosphate and hence should be avoided. In the last five decades a remarkable number of P soil test methods has been developed which apply to various soil types (Hesse, 1971). In the following, particular investigations were carried out in Germany in which the ‘DL method’ was used. DL denotes double lactate since soils are extracted with a Ca lactate solution brought to a pH of 3.6 by the addition of HCl (Egner, 1955; Hoffmann, 1991). Numerous earlier (Schwerdt and Jessen, 1961) and more recent field trials (Bru¨ne and Heyn, 1984) with arable crops have shown that the DL-method is a reliable soil test precisely indicating the available soil phosphate. Only in cases in which soils were fertilized with rock phosphates the DL extract yields data which are higher than the actual available soil phosphate (Werner, 1969). For such soils the ‘CAL method’ is recommended. Although reliable soil tests for available phosphate are at disposition regular P soil testing covers only a limited percentage of agricultural land. In Germany with about 17 × 106 ha agricultural land about 600 000 P soil tests are made per year which represent only a small percentage of agricultural land (information from VDLUFA, Association of the German Research Stations). It is supposed that many soils are enriched in available phosphate and crops do not respond to further P fertilization as was reported by Arnold and Shepherd (1990) quoted after Bhogal et al., 1996) for UK. The same is true for areas with intensive livestock husbandry (Leinweber, 1996; Mozaffari and Sims, 1996; Peters and Basta, 1996). Fertilizing these soils wastes phosphate. Leinweber et al. (1994), taking representative soil samples from an

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area in north Germany with intensive livestock husbandry, found low to medium DL-P concentrations in forest soils while far more samples from arable land and grassland showed high to extremely high DL-P concentrations. The frequency distribution of DL-P of Leinweber’s investigation is shown in Fig. 1 for the various cropping systems. About 95% of the samples from grassland, arable land and special cultures, mainly raspberries and asparagus, had DL-P concentrations which were higher than the level above which crops do not respond to P fertilizer application. In these soils, heavily treated with slurries, phosphate is not only enriched in the top layer but also in deeper soil layers up to 1 m (Werner et al., 1988). Since crops also feed from phosphate in deeper layers of the root-

Fig. 1. Frequency of distribution of available phosphate in Northern Germany in an area with intensive livestock husbandry (Leinweber et al., 1994). Available phosphate measured by the extraction with a lactate solution (DL method, Egner, 1955). A concentration of 100 mg DL soluble P/kg soil may be considered as sufficient for arable crops. Special cultures are mainly raspberries and asparagus.

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ing zone this deeper located available phosphate must be taken into consideration when assessing the quantity of available soil phosphate. The problem of phosphate surplus in agricultural land is comprehensively discussed by Isermann (1993). Spiertz (1991) reported that on average, milking farms in the Netherlands had an excess of 30 kg P/ha per year which lead to an enormous accumulation of P in soils. To reduce the P fertilization in such farms is not easy since the P is imported into farms with the concentrates required for feeding the high livestock rates and farmers need this high intensity for making their living because their own acreage for forage production is not sufficient. Under environmental aspects and the aspect that phosphate is a finite resource, a solution must be found for these farms.

3. Phosphate fixation The term phosphate fixation means the transformation of plant available phosphate in soils into a nonavailable form. Two major processes may be involved in this transformation: the formation of less soluble Ca phosphates from water soluble phosphates and the adsorption of phosphate to the surfaces of soil particles. The latter process is the most important. In this context the term phosphate fixation includes phosphate occlusion brought about by adsorption by FeIII oxides and oxyhydroxides. Formation of less soluble Ca phosphates follows the sequence: Ca dihydrogen phosphate . Ca monohydrogen phosphate . Ca octophosphate . apatite, from which Ca dihydrogen phosphate is most soluble and apatite sparingly soluble in water (Olsen et al., 1977; Sposito, 1989). The reaction sequence to less soluble phosphates is promoted by high pH and high Ca2+ concentrations in the soil solution whereas high H+ and low Ca2+ concentrations have an inverse effect. It is doubtful whether even under favorable conditions such as in calcareous soils with high pH and high Ca2+ concentrations in the soil solution the crystalline apatite is formed. At least this process of crystalline Ca phosphate formation proceeds at low rates (Parfitt, 1978). According to Olsen et al. (1977) it is the octophosphate which accumulates in soils with higher soil pH. The solubility of octophosphate is high enough for optimum plant supply (Olsen et al., 1977; Sposito,

1989). Therefore, the formation of less soluble Ca phosphates does not represent a major process in phosphate fixation. Uptake and metabolization of inorganic phosphate by microorganisms means a transient reduction of plant-available phosphate. Assimilation of inorganic phosphate is paralleled by the formation of inorganic phosphates from organic phosphate which partially originates from dead microbial biomass. Particularly in the rhizosphere there is a high turnover of organic phosphates into inorganic phosphate (Helal and Sauerbeck, 1984) mediated by the relatively high phosphatase concentrations near the root surface (Tarafdar and Jungk, 1987; Helal and Dressler, 1989). The most important process of phosphate fixation is represented by the specific adsorption of phosphate to soil particles such as sesquioxides, clay minerals, allophanes, calcite as well as Al and Fe humate complexes. This so called chemi-adsorption occurs by ligand exchange in which the OH− on the adsorbing surface is exchanged by a phosphate anion (Fig. 2). In the first step the phosphate is bound only with one bond to the surface (mononuclear bond) in the following step a second anion equivalent of the phosphate is bound to the surface (binuclear bond) the latter being much more stable than the mononuclear bond (Parfitt, 1978). The adsorption process is promoted by low

Fig. 2. Principle of phosphate adsorption onto an adsorbing surface. (1) Ligand exchange between the OH− of the surface and the phosphate. (2) The mononuclear bound phosphate is deprotonated. (3) The deprotonated phosphate exchanges with another OH− of the surface and a binuclear bond is formed. The reaction sequence is reversible and phosphate desorption is driven by OH−.

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OH− concentrations (Fig. 2) which means that adsorption is particularly strong at low soil pH. This relationship is of utmost agronomic importance and is discussed in detail in the remaining part of this section. Barekzai and Mengel (1985) investigated the influence of the contact time between soil and phosphate fertilizer (superphosphate) on its P availability for Lolium perenne grown in pots. From the ten soils tested only the results of the two extreme soils are shown (Fig. 3), an acid brown earth (7% clay, DLP = 9 mg P/kg soil, in KCl solution, pH 4.6) and a subsoil from a rendzina (67% CaCO3, DL-P = 0.8 mg P/kg soil, pH 7.6). According to these characteristics the first soil should favor phosphate adsorption and the latter the formation of less soluble Ca phosphates. Both soils were very low in available phosphate. In the acid soil the contact time had a highly significant impact on the phosphate uptake of the grass. Phosphate fertilizer, given 6 months before seeding, yielded a significantly lower recovery than phosphate

Fig. 3. Uptake of fertilizer phosphate by Lolium perenne from an acid and a calcareous soil as related to the time of P fertilizer application: 6 months before seeding, 3 months before seeding, at seeding. Grass was cut two times (Barekzai and Mengel, 1985). ‘a’ denotes a significant difference at the 0.1% level between phosphate application at seeding and 6 months before; ‘b’ a significant difference at the 5% level between phosphate application at seeding and 3 months before seeding; ‘c’ a significant difference at the 5% level between phosphate application 3 months before seeding and 6 months before seeding.

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fertilizer applied just before seeding. Also a 3 months contact time still had a significantly negative effect on the efficiency of the P fertilizer. This pattern found in the first cut of the grass was also evident in the second cut. In the calcareous soil the fertilizer/soil contact time had no influence on the P uptake of the grass. Obviously there was no major formation of unavailable Ca phosphates during a period of 6 months otherwise the rates of phosphate uptake by the grass should have declined with an increase in the soil/fertilizer contact time. This statement is in line with results of Olsen et al. (1977) who found that above pH 6 it is the solubility of the octocalcium phosphate which controls the phosphate availability, and added phosphates to such soils have a very high coefficient of recovery. The fast adsorption of fertilizer phosphate in acid soils was also found by Mozaffari and Sims (1996). In the acid soil, laboratory experiments of Barekzai and Mengel (1985) showed a strong phosphate adsorption (Fig. 4). In the treatment with zero contact time the adsorption curve was much flatter than the curves obtained after a contact time of 6 and 12 weeks. At the zero contact time the highest P rate resulted in a P concentration of 32 mg P/l; the same P application rate gave only a P concentration in the soil solution of about 2 mg P/l after a contact time of 6 weeks. This demonstrates the enormous reduction in P availability most likely due to specific adsorption. Further, it could be shown in laboratory experiments that phosphate availability in this acid soil was substantially increased by the incorporation of CaO into the soil (Barekzai and Mengel, 1985). From this experiment it is clear that the efficiency of soil and fertilizer phosphates depends highly on soil pH. This probably is true for all mineral soils with a potential for phosphate adsorption. The relevance of soil pH for the efficiency of phosphate fertilizer is supported by field trials. According to Werner and Wichmann (1972) the recovery of phosphate by crop uptake was much higher on neutral and calcareous soils than on acid soils. Sturm and Isermann (1978) in evaluating the phosphate recovery in long-term field experiments also found that soil pH was of high importance for P recovery. The recovery of fertilizer P was calculated from the P uptake of crops and the change in available soil phosphate since an increase in available soil P means a corresponding increase in the recovery of fertilizer P and

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Fig. 4. Phosphate buffer curve of an acid soil. Directly after P application (0 weeks), 6 weeks and 12 weeks after P application (Barekzai and Mengel, 1985).

vice versa a decrease of available P a reduction of P recovery. Available soil phosphate was determined by the DL- method. In the field trials quoted by Sturm and Isermann (1978) no rock phosphate was applied and therefore the data in Table 2 give a good indication of the fertilizer phosphate recovery. The most interesting results of this investigation are shown in Table 2 which clearly demonstrate the enormous impact of soil pH on phosphate use efficiency and from which the conclusion is drawn that much phosphate can be saved if soil pH is appropriate. A high recovery of phosphate on Luvisols with a neutral to alkaline pH was also found by Jungk et al. (1993). Humates may be involved in phosphate adsorption which is particularly true for large areas of representative arable soils derived from loess. According to investigations of Gerke and Hermann (1992) and Gerke et al. (1995) Fe and Al may be adsorbed by

carboxylic groups of humic acids and then adsorb phosphate as shown in Fig. 5. In the experiments of Gerke et al. (1995) phosphate adsorption was somewhat higher at pH 6.2 than 5.2. This surprising pH effect presumably is due to a higher deprotonation of humate carboxylic groups at the higher pH which may promote the adsorption of Fe hydroxides. Gerke and Hermann (1992) suggest that these P-Fe–humate complexes play a role in the turnover of fertilizer phosphate particularly in Luvisols derived from loess. Whether this kind of phosphate complex has a stronger impact on phosphate availability than that associated with sequioxides is not yet clarified. Phosphate adsorption is a particular problem in highly weathered soils of the tropics (Oxisols and Ultisols) because of their high phosphate adsorption potential. For phosphate melioration they require high P fertilizer rates in the range of 170 kg P/ha (Haynes, 1984). Most of these soils are acid and require liming which does not improve phosphate availability in all cases. Liming may induce polymerization of Al cation species which because of their high positive charge are strong phosphate adsorbers (Haynes, 1984). According to Hauter (1983) the decrease of phosphate availability due to liming of Oxisols is associated with their very low pH (3.7–4.4) while at a soil pH of 5.5 liming had a beneficial effect on phosphate availability. Sims and Ellis (1983) reported that liming an Ultisol increased the available soil P and enhanced P uptake by oats considerably. In order to save fertilizer phosphate on these strongly phosphate fixing soils band placement of fertilizers is recommended (Werner and Scherer, 1995). As shown earlier (Fig. 2) adsorption is an exchange of ligands and with an increase in soil pH the OH−

Table 2 Percentage recovery of fertilizer phosphate in long-term field trials on representative agricultural soils in relation to the lime status of soils. Recovery = P uptake of the crop + change in DL-P in the soil (Sturm and Isermann 1978) Lime status Arable soils, Arable soils, Arable soils, Arable soils, Arable soils, Grassland

% Recovery very well supplied with lime well supplied with lime moderately supplied with lime poorly supplied with lime poorly supplied dry locations

80 70 65 60 50 80

Fig. 5. Adsorption of phosphate to an humate Fe complex. The FeIII is adsorbed onto the humate with a covalent bond and a coordinate bond (modified after Gerke and Hermann, 1992).

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concentration increases, OH− competing with phosphates for binding sites at the adsorbing surface. Organic anions may also compete with phosphate for binding sites (Parfitt, 1978). Fox et al. (1990) found a number of organic anions capable of replacing phosphates from adsorbing surfaces. Citrate seems to be a very potent competitor for adsorbed phosphate (Gerke, 1994). Under anaerobic conditions soluble phosphate increased in the soil solution (Welp et al., 1983) mainly due to the reduction of complex bound FeIII associated with the release of soluble phosphate. As was shown by Sah and Mikkelsen (1986) occluded phosphates may be solubilized under anaerobic conditions because of the reduction of FeIII to Fe2+ (the Roman superscript indicating an Fe complex, the Arabic superscript a dissolved Fe ion). The process is of particular importance for flooded rice soils.

Fig. 6. Water soluble phosphate originating from various phosphate fertilizers in the rhizosphere of young rape. The phosphate fertilizer had been dressed in a 10 years lasting field trial. The horizontal lines designate the level of water soluble phosphate in the bulk soil. PARP, partially acidulated rock phosphate (Steffens, 1987). Thomas slag is a non-crystalline, non-water soluble phosphate fertilizer which gradually dissolves in soils and therefore is well available to plant roots. Rock phosphate is a crystalline phosphate fertilizer (apatite), non-soluble in water which is dissolved in acid soils. PARP, partially acidulated rock phosphate which consists of about to 50% of rock phosphate and 50% of water soluble phosphate.

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4. Organic phosphate and mycorrhiza in soil cropping systems In arable land the concentration of organic phosphate is in the order of 50% of total phosphate in the upper soil layer and in grassland soils the proportion of organic phosphate may be even higher (Sharpley, 1985). A substantial part of organic phosphate, up to 100 kg P/ha, may be fixed in microbial biomass (Brookes et al., 1984). Phosphate thus immobilized may easily be mineralized and hence become available for crops. Sharpley (1985) reported that there is a seasonal variation in available organic soil phosphate decreasing in spring with crop growth and increasing in late autumn and winter. This pattern was particularly distinct in soils not treated with inorganic phosphate fertilizer showing that the plants drew phosphate from this organic pool. In calcareous soils, the phosphate of the soil solution is mainly present in organic form (Dalal, 1977) and therefore in these soils phosphate transport to plant roots is mainly brought about by organic phosphates which may be easily mineralized in the plant rhizosphere enriched with phosphatases (Tarafdar and Claassen, 1988; Dou and Steffens, 1993). About half of the organic phosphates in soils is present as myo-inositol-phosphates from which the inositol-hexaphosphate is adsorbed to sequioxides similar as inorganic phosphate (Dalal, 1977). The adsorption of inositol-hexaphosphate is relatively strong since the molecule has six phosphate groups which may be bound to soil particles. Diminishing the numbers of phosphate groups bound to inositol decreases the possibility of phosphate adsorption and thus improves phosphate availability (Evans, 1985). Myo-inositol-2-monophosphate is virtually not adsorbed (Evans, 1985), and quite mobile in soils (Dou and Steffens, 1993). In addition inositolhexaphosphate can also form rather insoluble salts with Ca2+ and Mg2+, a process which may affect phosphate availability. Other organic phosphates such as phospholipids and nucleotide phosphates do not accumulate in soils as they are easily mineralized (Dalal, 1977; Tarafdar and Claassen, 1988). Hence the large pool of organic soil phosphate is potentially available for plants. This is also true for the non-soluble organic phosphate from which a great part is present in the form of microbial biomass and which will be mineralized after the death of microorganisms. Therefore,

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in contrast to the fixed inorganic phosphate the immobilized organic phosphate represents a potential pool of available phosphate and measures which promote the formation of organic phosphate in soils and may restrict the fixation of inorganic phosphate and thus contribute to an efficient use of soil phosphates. Sources of organic phosphates in soils are plant residues, green manure, microbial biomass, and farm yard manure (FYM). For this reason cropping systems have a distinct impact on the content of organic phosphates in soils as well as on the assimilation of inorganic phosphates by fungi and bacteria and the mineralization of organic phosphates by phosphatases. Oberson et al., 1993, 1996; reported that regular application of FYM to soil increased the organic phosphate content. This effect may be due to organic phosphate present in the FYM but also to inorganic phosphate being assimilated by soil microorganisms after FYM application. The latter especially raised the ATP concentration which, according to the authors means an increase in microbial biomass. The impact of FYM on the concentration of ATP in the upper soil layer is shown in Table 3, from the work of Oberson et al. (1993). It is evident that in all treatments receiving FYM the ATP concentration was significantly increased which means that FYM had a beneficial effect on microbial biomass development and hence on the storage of potentially available phosphate. Parallel with the increase of microbial biomass the acid phosphatase activity was increased by FYM application which means that also the enzyme activity rendering organic phosphate into a form directly taken up by plant roots was promoted. The positive effect of FYM on the efficient use of soil

Table 3 Effect of FYM on the ATP concentration in soils. Soil samples taken at ear emergence of winter wheat (Oberson et al., 1993) P fertilizer kg P/ha

Rate of P appl. kg P/ha per year

ATP mg/kg soil*

No FYM 80% FYM + 20% mineral P 40% FYM + 60% mineral P 100% mineral P

– 28 31

843a 1217c 1160bc

47

1006abc

46

945ab

and fertilizer phosphate availability is enhanced by organic anions produced during the decomposition of organic matter. They compete with inorganic phosphate for adsorption sites and thus reduce the fixation of phosphate (Werner and Scherer, 1995). In addition FYM may improve soil structure and favor root growth and thus the exploitation of soil phosphates by roots (Keita and Steffens, 1989). Farms producing FYM frequently also grow arable forage crops such as red clover and alfalfa which not only contribute to the nitrogen status of soils by symbiotic N2 fixation but they also may exploit fixed soil phosphate by the excretion of root exudates as was shown for red clover excreting citrate which mobilizes adsorbed soil phosphate (Gerke, 1994). Rotations with diverse crop species generally will contribute to a better exploitation of soil phosphates. Mycorrhization of plant roots may considerably improve the accessibility of soil phosphate to plants mainly by increasing the contact surface between the soil matrix and the mycorrhized plant root. This is particularly true for leguminous species (Barea and Acon-Aguilar, 1983). The problem with mycorrhiza exploiting soil phosphate for the host plant is the high specificity between the host plant and the endomycorrhizal fungi (Lioi and Giovannetti, 1987; Diederichs, 1991). Inoculation of soils with the appropriate fungi still meets with difficulty (Hall, 1987). If the fungi/root symbiosis is efficient remarkable crop yield increases may be obtained due to a better exploitation of soil phosphate (Hall, 1984).

5. Phosphate fertilizer types Phosphate fertilizer types differ in their solubility with the most important difference between amorphous and crystalline forms. The latter comprises the rock phosphates and partially acidulated rock phosphate (PARP) which still contains a portion which is crystalline and represents apatite. The solubility of fluoro-apatite is given by the following equation: Ca5 (PO4 )3 F + 4H + N 5Ca2 + + 3HPO24 − + HF From the equation it is evident that high Ca2+ and phosphate concentrations hamper, and increasing H+ concentrations in the soil solution promote the disso-

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lution of apatites. Solubility of apatites (rock phosphates) depends also on the degree of isomorphic 2− substitution of PO3− 4 by CO3 and is higher the more phosphate is substituted by carbonate (Anderson et al., 1985). Generally in acid soils pH , 5.0 (measured in CaCl2 solution) the efficiency of rock phosphate is as high as that of acidulated phosphate (Mengel, 1986). In soils with a higher pH, however, the efficiency is poorer and may be even nil. In such cases the application of rock phosphate means a waste of the phosphate resource. Steffens (1994) investigated a number of representative arable soils for their availability of phosphate originating from various fertilizer types. Phosphate release rates obtained by repeated extraction of soils with electro-ultrafiltration (EUF) followed the Elovich equation and reflected well the phosphate availability of various fertilizer types for crops. The agreement of the released phosphate with the Elovich equation means that the P release rates declined with the number of extractions. Highest release rates were obtained from basic slag (Thomas phosphate) and superphosphate and lowest rates from rock phosphate. This pattern of phosphate fertilizer solubility was also found in the rhizosphere of rape as shown in Fig. 6 (Steffens, 1987) from which can be seen that partially acidulated phosphate took an intermediate position. This means that mainly the water soluble portion contributed to P solubility. For this reason also the application of partially acidulated rock phosphates on soils with a poor solubility for apatite means a waste of phosphate (Resseler and Werner, 1989). The pretention that apatitic phosphate will render soluble in soils by time is only correct if soils are acid (Renno and Steffens, 1985). Such soils, however, if not organic soils, should be limed in order to improve the availability of adsorbed phosphate as discussed above. If lime is not available rock phosphates may be an alternative choice. The poor performance of apatitic phosphate found in representative arable soils in Europe (Mengel, 1986) is in agreement with experiences made on laterite soils in Western Australia (Bolland et al., 1988; Bolland and Gilkes, 1990). Also in these field trials the direct and residual effect of apatitic fertilizer was poor as compared with superphosphate. Only on the humic sandy podsols in south Western Australia with annual rainfall .800 mm Bolland (1996) found a superiority

of apatitic fertilizers as compared with superphosphate. Under these particular conditions superphosphate may be leached out from the top soil layer. Proton excretion and mycorrhizal colonization of plant roots may contribute to the solubilization of apatite. According to Hauter and Steffens (1985) the high proton excretion of red clover roots, typically for symbiotically living leguminous species (Mengel, 1994), contributed to the dissolution of rock phosphate. An interesting effect of mycorrhizal infection was found by Steffens (1992). A farmer having applied rock phosphate for years on a calcareous soil finally ended in a severe phosphate deficiency of sugar beets. Field trials carried out on this soil with different phosphate fertilizer types gave the results shown in Table 4. It is evident that the response of sugar beets to rock phosphate application was nil in contrast to superphosphate which produced a remarkable yield increase. In sunflowers, however, the effect of rock phosphate was as high as the effect of superphosphate; in wheat the high rate of rock phosphate was as good as the low rate of superphosphate. Sunflower roots were well colonized with mycorrhiza, sugar beet roots were not. This example shows the beneficial effect of mycorrhiza on the acquisition of rock phosphate. The decision for a farmer to apply rock phosphate depends therefore also on the crop species and on the species in the rotation. If the rotation comprises a sugar beet crop the level of available soil phosphate should be maintained by applying acidulated phosphates or amorphous forms of phosphate such as sinterphosphate (CaNa phosphate pro-

Table 4 Effect of superphosphate and hyperphos (phosphate rock) on the beet yield and grain yield of various crops. The calcareous soil had a pH of 7.4 (after Steffens, 1992) Phosphate, applied

Sugar beet, Sunflowers, Wheat, 1987 1988 1989 Yield in t/ha

No phosphate Hyperphosphate low rate Superphosphate low rate Hyperphosphate high rate Superphosphate high rate Least significant difference, 5%

32.8 32.3 46.5 32.3 50.0 7.9

3.77 4.35 4.19 4.18 4.39 0.31

6.59 6.81 7.33 7.55 7.51 0.51

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duced in soda production) or Thomas phosphate. In many cases 50% of an amorphous phosphate will give the same yield as 100% of the apatitic phosphate as shown in Table 4. Hence from the viewpoint of saving the phosphate resource 50% of the amorphous phosphate is the better choice.

6. Perspectives As shown above (Table 4) mycorrhizal symbiosis with plant roots represents an interesting mechanism for the efficient use of soil phosphates. This potential will be only used if the available soil phosphate level is not too high. The high specificity in the relationship between fungus species/host plant species (Hall, 1987; Lioi and Giovannetti, 1987; Diederichs, 1991) and the problem of inoculating soils with the proper fungus species still represents severe obstacles for the use of mycorrhiza in practical farming and need further scientific and technical efforts. Excretion of organic anions and protons by plant roots under the conditions of insufficient phosphate supply is a further mechanism which merits attention. Proton excretion may help to solubilize apatitic phosphates (Hoffland et al., 1990), and the excretion of organic anions to desorb adsorbed phosphate (Hedley et al., 1982). The latter authors were the first who found that Brassica napus was capable of responding to an insufficient phosphate supply by an enhanced excretion of H+. Protons alone, however, would rather depress the availability of adsorbed phosphate than mobilize it. In addition to the proton excretion, rape roots also excrete organic anions, especially malate which may desorb adsorbed soil phosphate (Hoffland et al., 1989). This response of Brassica napus to low phosphate supply was not found with Lolium multiflorum (Ruiz, 1992). Zhyu et al. (1990) found that rice plants excrete citrate under the conditions of insufficient phosphate supply and that Japonica species were more efficient in citrate excretion than Indica species. Ae et al. (1990) reported that pigeon peas have no particular potential to exploit Ca phosphates but they are capable of excreting a tartrate derivate (piscidic acid) which mobilizes adsorbed soil phosphate. Of particular interest are the proteoid roots of Lupinus albus capable of excreting large amounts of citrate (Gardner and Parbery, 1982) which mobilize

insoluble soil phosphate the effect being due to the citrate and not to the release of H+ (Gardner et al., 1983). According to Dinkelacker et al. (1989) citrate excreted by proteoid roots of Lupinus albus may also solubilize Ca phosphate by chelating the Ca2+ of insoluble Ca phosphate. Red clover is also a potential species in excreting citrate by roots (Gerke, 1994) the quantities being released were in the same range as citrate excreted by proteoid roots of Lupinus albus (Gerke et al., 1994).

7. Conclusions A more efficient use of soil and fertilizer phosphates demands agronomic and scientific efforts. From the agronomic measures such as the selection of the appropriate phosphate fertilizer type, adjusting fertilizer rates to soil tests and liming soils to an optimum pH level may be easily implemented by European farmers since lime and various phosphate fertilizer types are available. In areas with excessively high levels of available soil phosphate due to intensive livestock farming, cropping systems should be developed with a closer integration of crop and animal production so that phosphates excreted by farm animals are efficiently used for crop production. Such farming systems should comprise a broader diversity of crop species in the rotation including forage crops which are particularly efficient in exploiting soil phosphates by mycorrhiza and/or excretion of organic anions by roots. The implementation of such farming systems substituting the intensive livestock production needs not only agronomic but particularly political and economical measures. Scientific and technical efforts are required for selecting appropriate endomyccorhizal fungi ecotypes and practicable soil inoculation techniques. The physiological mechanism by which plants respond to an insufficient phosphate supply such as the secretion of organic anions by plant roots needs elucidation.

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