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Chapter 9
Ways of improving the agronomic effectiveness of phosphate rocks

Chapters 5 and 7 show that the agronomic performance of phosphate rocks (PRs) applied directly as phosphate fertilizers depends on various factors and their interactions. The main factors include: (i) the physical and chemical properties of PRs; (ii) soil and climate factors; (iii) plant species and the cropping system; and (iv) farming management practices. There are situations where directly applied PRs are not effective. In these cases, it is possible to use various means to increase their agronomic effectiveness and thereby make the products more economically attractive. This chapter discusses the various possibilities under the headings of biological, chemical and physical means.

Biological means

The biological means of enhancing the agronomic effectiveness of PRs applied as phosphate fertilizers are: (i) composting organic wastes with PR (phospho-composts); (ii) inoculation of seeds or seedlings with phosphorus-solubilizing micro-organisms (fungi, bacteria and actinomycetes); and (iii) the inclusion in the cropping system of crop genotypes that exhibit greater root growth and thus increase the extent of soil exploration, exude proton and organic acids that increase the solubility of sparingly soluble phosphates by decreasing pH and/or chelation, and produce elevated levels of phosphatase enzymes that can break organic phosphorus (P) down to inorganic P.


Treating PRs with organic materials and composting them is a promising technique for enhancing the solubility and the subsequent availability to plants of phosphorus (P) from PRs. The technology is particularly attractive where: (i) ‘moderate to high’ reactive PRs are available but unsuitable for the production of fully acidulated fertilizers such as single or triple superphosphate; (ii) organic manures are applied routinely to maintain the organic fraction of soils and supplement their nutrient requirement (as in most tropical countries); (iii) organic farming is practised, which excludes the use of chemically processed fertilizers; and (iv) city and farm by-products need to be disposed of in an environmentally friendly manner. The PR-composted products are usually referred to as phospho-composts.

Principles of phospho-composting

Phospho-composting is based on sound scientific principles. During the decomposition of organic materials, intense microbial activity occurs. This results in numerous types of bacteria and fungi that produce a large number of organic acids and humic substances. Some of the most commonly produced organic acids are: citric, malic, fumaric, succinic, pyruvic, tartaric, oxaloacetic, 2-ketogluconic, lacticoxalic, propionic and butyric (Stevenson, 1967).

The term humic substances is a generic name given to a large number of amorphous, colloidal organic polymers formed during the decomposition of organic matter. Humic substances are of high molecular weight and generally more stable than organic acids. Humic substances can be divided into three main fractions based on their solubilities in acid and/or alkali. The fraction that is soluble in acid and alkali is called fulvic acid, that soluble in alkali but precipitated in acid is humic acid, and that insoluble in acid and alkali is humin. Of these, the fulvic acid fraction has the lowest molecular weight, followed by humic acid and humin.

The enhancement of P release from PRs seems to be a function of the acidification of PR by organic acids and more importantly their chelating ability on calcium (Ca), iron (Fe) and aluminium (Al) (Pohlman and McColl, 1986). The greater ability of organic acids, compared with mineral acids of comparable strength, to release P from PR and the direct evidence of their chelating ability have been documented (Johnston, 1954a, 1954b; Kpomblekou and Tabatabai, 1994a). Another important factor in the release of P from PR is the participation of the OH groups in the organic acids. For example, it has been shown that citric acid with three carboxyl (COOH) groups and an OH group was able to dissolve more P from PR than cis-aconitic acid with three carboxyl groups but without the OH group (Kpomblekou and Tabatabai, 1994a).

Fulvic acid is the most reactive of the humic substances in adsorbing significant amounts of Ca2+ and releasing H+ ions, thereby enhancing PR dissolution. Humic acid may form complexes with P and Ca, and create a sink for further dissolution of PR (Singh and Amberger, 1990). The application of humic substances to soil also makes more P available to plants by competing for, and by forming a protective coating over, soil phosphate-sorption sites. An additional benefit that accrues from the application of phospho-compost is the movement of dissolved P to a greater soil depth, which provides a larger soil volume for P uptake by plants.

Practical considerations of phospho-composting

Organic manure is a broad term that comprises: manures prepared from cattle dung, excreta of other animals, crop residues, rural and urban composts, and other animal wastes. The concentration of nutrients in organic materials is variable. Although most of these materials contain significant amounts of nitrogen (N), they contain little P (Table 28). The effectiveness of the composts in solubilizing PR varies with the kind and composition of waste material and with the rate of decomposition. It is a function of the magnitude of production of organic acids and chelating substances in the compost, which in turn result from the metabolic activities of microorganisms including bacteria, fungi and actinomycetes. Reports indicate that plant materials such as chopped leaves, crop residues (e.g. cereal straw) and lawn clippings composted with animal wastes are favoured because they produce more organic acids and humic substances. Composting PR with poultry manure may not be a preferred option because poultry manures contain large amounts of calcium carbonate and other basic compounds that hinder PR dissolution (Mahimairaja et al., 1995).

Nutrient content of organic waste materials and farmyard compost (dry basis except for urine)



Nutrient content (%)




Animal wastes

Cow manure




Cattle urine




Sheep and goat dung




Night soil




Leather waste




Farmyard manure

Farmyard manure




Composts & plant residues

Poultry manure




Town compost




Rural compost




Rice straw




Lawn clippings and leaves




Subba Rao (1982b) has provided some practical hints for the phospho-composting of agricultural wastes:

The rate of decomposition of the organic materials can be accelerated by microbial inoculation of the compost (e.g. Aspergillus spp., Penicillium spp., Trichoderma viride, Cellulomonas spp. and Cytophaga spp.), and by the addition of energy sources (molasses) to the waste before composting (Singh and Amberger, 1991).

A ratio of four parts of organic waste material to one part of PR (dry-weight basis) seems to be an effective combination. Singh and Amberger (1991) investigated dissolution of two sedimentary PRs (Mussoorie and Hyperphos) applied separately. They incubated PR mixed with wheat straw at a ratio of 1:4, inoculated with soil and compost extract, and also adjusted the C:N ratio by adding urine. The authors reported that the dissolution of PR increased in 30 d to 7 percent of total P for Hyperphos PR and 15 percent for Mussoorie PR. The addition of molasses enhanced P solubility by a further 3 percent for both PRs.

Agronomic effectiveness of phospho-composts

An increase in the agronomic effectiveness of PR in phospho-composts over that of directly applied PR can be expected because of its greater water-soluble and citric-soluble P contents, which would be available to plants. Moreover, the soluble P fractions should stimulate root growth and facilitate greater exploitation of P enriched soil (Chien et al., 1987a; Rajan and Watkinson, 1992; Habib et al., 1999). Published work on the agronomic effectiveness of phospho-composts appears to be scarce.

Phospho-compost prepared by mixing farm wastes, cattle dung and soil (as a source of bacterial inoculum) has been found to be as good as single superphosphate (SSP) (Bangar et al., 1985; Palaniappan and Natarajan, 1993). The P sources were applied on an equivalent total P basis to a tropical soil and the crops were: pigeon pea, green gram, clusterbean, wheat and pearl millet. At pH values higher than 7.5, where directly applied PR is not expected to dissolve, phospho-compost was as effective as SSP (Table 29) (Mishra and Bangar, 1986). However, the P sources were applied at a single rate of P in these studies, which reduces the value of the results.

Effect of P sources on yield and P uptake for red gram and clusterbean


Red gram yield

Clusterbean yield











Mussoorie PR





Single superphosphate










Compost only



Compost + Mussoorie PR



LSD (p = 0.5)





Note: All P sources applied at equivalent P application rate of 17.3 kg/ha.

Future research needs

Phospho-composting offers the advantage of using otherwise unusable PRs, and the environmental advantage of safe disposal of organic waste. In situations where organic manures are already in use or are a viable alternative to chemical fertilizers (Mugwira et al., 2002), phospho-composting has an advantage. On the other hand, if phospho-compost is to be applied mainly as a source of P, then the benefits need to be weighed against the cost of preparation and application. Further research is needed to determine scientifically the minimum amount of compost required to solubilize PR to a level where the product would be economically as effective as water-soluble phosphate (WSP) fertilizers. The research programme should include PRs of different reactivities and various combinations of locally available organic wastes.

Inoculation of seedlings with endomycorrhizae


The term mycorrhiza refers to the symbiotic association between plant roots and fungi. In nature, most plant roots form mycorrhizal associations of one kind or another with fungi in soil, with the mycorrhizal fungi performing the function of root hairs. The type of mycorrhiza that improves P uptake by plants is vesicular-arbuscular mycorrhizae (VAM), and the commonly used spore types are Glomus fasciculatum, G. mosseae, G. etunicatum, G. tenue and Giaspora margarita. VAM fungi infect the cells of the root cortex and form both an internal network of hyphae and an external growth of hyphae. They possess special structures known as vesicles and arbuscules. The highly branched arbuscules help in the transfer of nutrients from the fungus to the plant-root cells, and the vesicles are sac-like structures, which store P as phospholipids.

VAM are geographically ubiquitous and occur over a wide ecological range from aquatic to desert environments (Mosse et al., 1981; Bagyaraj, 1990). VAM fungi colonize families of most agricultural crops. Families that rarely form VAM include Cruciferae, Chenopodiaceae, Polygonaceae and Cyperaceae.

Miyasaka and Habte (2001) have reviewed the integration of arbuscular mycorrhizal fungi into cropping systems in order to maintain yields while reducing P inputs.

Mode of action

Enhanced P uptake in VAM-infected plants seems to be facilitated by: (i) the fungal hyphae exploring a greater volume of soil for P and also intercepting a greater number of point sources of P; (ii) the fungi dissolving sparingly soluble P minerals (e.g. PR); and (iii) the infected roots increasing the rate of P uptake, by increasing the diffusion gradient by depleting P to lower P concentrations than can non-mycorrhizal roots and by enhancing the transfer of P between living roots and from dying roots to living roots (Bolan and Robson, 1987; Sylvia, 1992; Frossard et al., 1995; Lange Ness and Vlek, 2000; Brundrett, 2002). The P inflow rates of mycorrhizal roots are calculated to be 2-6 times those of non-mycorrhizal roots (Jones et al., 1998).

The effect of mycorrhizal relative to non-mycorrhizal plants on the uptake of P varies with the P application rate, with uptake generally enhanced at low to intermediate levels of P application (Ortas et al., 1996; Sari et al., 2002). The effect of infection is to alter the P response curve such that it rises more steeply and reaches a maximum at much lower levels of P applied, either as soluble or sparingly soluble P as in PR. Although some researchers (Murdoch et al., 1967) have concluded that the effectiveness is greater with low-solubility fertilizers (e.g. PR), such conclusions appear to be the result of procedures used with one or two rates of P application rather than several rates (Pairunan et al., 1980). For the mycorrhiza to be effective, some threshold concentration of P in soil solution needs to be reached (Bolan and Robson, 1987) and this could be as low as 0.02 mg/litre (Manjunath and Habte 1992). At the same time, high levels of P in solution may decrease the level of mycorrhizal infection (Kucey et al., 1989).

Application of technology to enhance PR availability

Compared with non-mycorrhizal plants, Pairunan et al. (1980) have reported that mycorrhizal plants enhance the effectiveness of fertilizers by about 30 percent, including that of PR. As mycorrhizal fungi already infect plants of most families, the aim should be to introduce the most efficient mycorrhizal endophyte into plants. Studies indicate that the introduction of appropriate VAM fungi can increase rice grain yields by three times compared with uninoculated treatments (Secilia and Bagyaraj, 1992). Arihara and Karasawa (2000) reported that, under field conditions, yield and P uptake by maize were enhanced when grown after the cultivation of mycorrhizal crops.

Various methods for introducing VAM inoculums into a field crop have been examined, e.g. coating seeds with VAM inoculums and placing inoculums in the field under seeds in furrows. It has been found that the most attractive and practical way to use VAM inoculums is with vegetatively propagated crops and/or transplanted crops. In this situation, growers need to incorporate inoculums in the seedling trays or nursery beds. Seedlings thus raised will be colonized by the introduced fungi, which can then be transplanted out in the field. This method has been used successfully in India on agronomically important crops (e.g. chilli, finger millet, tomato, citrus and mango) and on forest tree species (e.g. Tamarindus indica and Acacia nilotica). VAM fungi are commercially available in some countries such as India. VAM inoculation for raising citrus seedlings is in commercial use in the United States of America (Menge et al., 1977).

Future research

There is strong evidence that VAM fungi increase the uptake of P by plants from PRs (Pairunan et al., 1980; Barea et al., 1983; Toro et al., 1997; Yusdar and Hanafi, 2003). This technology is particularly promising with crops raised in nurseries, seedling trays or polythene bags and then planted out in the field. Further research is needed to integrate VAM fungi into the cropping system with the specific aim of using PR as a phosphate fertilizer. It needs to assess the quantitative effects of chosen strains of fungi on P uptake from PRs by crops relative to that in non-inoculated natural soils. Such studies should include: PRs of differing reactivities; a WSP fertilizer (e.g. SSP) as the reference product; and several rates of P application so that response curves can be drawn. The studies should be conducted under field conditions so that the cost-benefit ratio of the use of PR with and without VAM fungi in relation to the use of WSP fertilizers can be calculated. As there is evidence that VAM fungal species are highly host specific and variable in their response to the mineral environment of soils (Bever et al., 2001; Ortas et al., 2002), the research programmes should consider these parameters.

Use of ectomycorrhizae

Ectomycorrhizae are generally found on woody perennials in families such as Betulaceae, Dipterocarpaceae, Fagaceae, Myrtaceae, Pinaceae and Salicaceae. The ectomycorrhizae are characterized as a root-fungus association, in which the fungus grows as a sheath or mantle on the surface of the roots. The net of hyphae extends into the cortex of plants but is confined to the intercellular spaces (unlike VAM), which form intracellular structures and produce an interconnection network known as a Hartig net. From the mantle, the hyphae extend into the soil and enhance the transport of phosphate and water to the host plants (Duddridge et al., 1980). This should assist in enhancing the availability of P from slow-release fertilizers such as PR. The fungal partner of ectomycorrhizae can be grown on synthetic medium for inoculation. Castellano and Molina (1989) discuss detailed procedures for inoculum production and nursery inoculation.

Use of phosphate solubilizing micro-organisms

A group of heterotrophic micro-organisms have been reported to solubilize inorganic forms of P. This is achieved by excreting organic acids that dissolve phosphatic minerals and/or chelate cationic partners of the P ion directly, releasing P into solution (Halder et al., 1990; Gaur, 1990; Bojinova et al., 1997; He et al., 2002). Some important micro-organisms include the bacteria Bacillus megaterium, B. circulans, B. subtilis, B. polymyxa and Pseudomonas straita. Fungal micro-organisms include Aspergillus awamori, Penicillum bilaii, P. digitatum and Trichoderma sp. Analyses of culture filtrates have identified a number of organic acids such as lactic, glycolic, citric, 2-ketogluconic, malic, oxalic, malonic, tartaric and succinic acids, all of which have chelating properties (Kucey et al., 1989).

Field trials in India and the then USSR have shown that the use of phosphate solubilizing micro-organisms (PSMs) can increase crop yields by up to 70 percent (Verma, 1993; Wani and Lee, 1992; Subba Rao, 1982a). The crops include oats, mustard, sugar beets, cabbage, tomato, barley, Egyptian clover, maize, potato, red gram, rice, chickpea, soybean and groundnut. In vitro studies have demonstrated the dissolution of PR by PSMs (Barea et al., 1983). Results from greenhouse trials have indicated a greater response of wheat and onion to PR application when seeds or seedlings are inoculated with PSMs. The increase in growth is greater with VAM fungi and PSMs in combination than when these organisms are used singly (Young, 1990; Toro et al., 1997; Singh and Kapoor, 1999). It is likely that PSMs dissolve sparingly soluble P, which is taken up by VAM mycelia, by more than one process, including the release of organic acids (Illmer et al., 1995) and the solubilization of calcium phosphates (Illmer and Scinner, 1995).

Research needs

There is strong evidence that PSMs, especially in combination with VAM, increase the agronomic effectiveness of PRs (Barea et al., 2002). PSMs are commercially available in some countries (e.g. India), where large-scale cultivation of PSMs is underway (by growing cultures in large flasks on rotary shakers or in batch fermentors). However, it is not evident whether these micro-organisms can increase the effectiveness of PRs under standard field conditions to such a magnitude that PR can be used as an alternative fertilizer. Field experiments need to be set up at specific sites in order to provide rigorous scientific testing of the practical usefulness of PSMs and VAM in enhancing the effectiveness of PRs, and thereby promoting their increased use.

Use of plant genotypes

P-uptake efficiency can be enhanced by selecting plant species or genotypes that exhibit several mechanisms for increased P absorption under low P conditions, such as: (i) greater root growth and, thus, increased extent of soil exploration; (ii) the exuding of proton and organic acids that increase the solubility of sparingly soluble phosphates by decreasing pH and/or chelation; and (iii) the production of elevated levels of phosphatase enzymes that can break down organic P to inorganic P (Miyasaka and Habte, 2001). Another approach is to use plants that are tolerant to Al toxicity (Ishikawa et al., 2000).

Increased soil acidity in the rhizosphere can enhance PR dissolution and its availability to plants. This has been observed directly as increased PR dissolution but more often indirectly as increased P uptake by those plants that acidify the rhizosphere (Bekele et al., 1983; Hedley et al., 1983; Moorby et al., 1988; Gahoonia et al., 1992; Haynes, 1992; Nakamaru et al., 2000). Hinsinger and Gilkes (1995) and Habib et al. (1999) have also found enhanced PR dissolution by the rhizosphere of some crop species in alkaline soils. Proton secretion by roots occurs when the equivalent sum of cation uptake by plants (K+, Ca2+, Mg2+ and Na+) exceeds that for anions (usually, NO3-, H2PO4-, SO42-, and C-). Soil acidification in the rhizosphere is greater for N-fixing legumes, which accumulate N as NH3 through symbiosis with Rhizobium micro-organisms. For this reason, leguminous plants are particularly suited for PR use.

The roots of some plants (e.g. rape) may also enhance PR dissolution by secreting organic acids, such as malic, citric, oxalic and 2-ketogluconic acids, that can be expected to complex cations of PR (Ca, Al and Fe) in addition to lowering rhizosphere soil pH (Moghimi and Tate, 1978; Hoffl and et al., 1989; Zapata et al., 1996; Nakamaru et al., 2000; Montenegro and Zapata, 2002).

Strategies to improve PR effectiveness involve the use of crop genotypes with increased acidity in the rhizosphere, such as N-fixing and acid-secreting legumes as intercrops (grain legumes) or mixed swards (pastures), which will enhance PR dissolution and make P available to nearby crop plants. An alternative is to include these crops in the crop rotation so that additional PR dissolves and enters the soil labile pool P.

A new strategy is to improve existing plant species genetically in order to increase root secretion of organic acids and protons. De la Fuente et al. (1997) reported that increased secretion of citrate occurred from roots of tobacco as a result of insertion of a citrate synthase gene from Pseudomonas aeruginosa. More research needs to focus on the possibility of inserting organic acid-producing genes into plants, which are poor utilizers of PR. Recently, a combined approach, i.e. to search for Al-tolerant and P-efficient genotypes, has been proposed in order to develop sustainable cropping systems for the acid soils of the tropics and subtropics (Hocking, 2001; Keerthisinghe et al., 2001).

Chemical means

Partial acidulation of phosphate rock

Partially acidulated phosphate rocks (PAPRs) are prepared by reacting PRs, usually with H2SO4 or H3PO4, in amounts less than that needed to make SSP or triple supersphosphate (TSP), respectively. The use of PAPRs has been widespread in Europe and South America since Nordengren (1957) first reported their use. PAPRs may offer an economic means of enhancing the agronomic effectiveness of indigenous PR sources that may otherwise be unsuited for direct application. For this reason, extensive studies have been, and continue to be, conducted internationally (Hammond et al., 1986b; Rajan and Marwaha, 1993; Chien and Menon, 1995a; Chien, 2003; Zapata, 2003). PAPRs are cheaper than fully acidulated WSP fertilizers because less acid and energy is required per unit of P in the product. In addition, PAPRs are often more concentrated than SSP. Thus, in some situations partial acidulation may be a preferred way of improving the effectiveness of imported PRs.

Choice of level and acid for partial acidulation

The level of acidulation of PR is usually referred to in percentage terms. For example, when one-fifth of the sulphuric acid needed to make superphosphate from that particular PR is used, the product is referred to as PAPR-20 percent H2SO4. The term PAPR-20 percent H3PO4 refers to phosphoric-acid-based PAPR using 20 percent of the H3PO4 required for TSP.

Figure 31 illustrates the relationships between the level of acidulation of a reactive PR (North Carolina PR) with H3PO4 or H2SO4 and the total and water-soluble P of the product. As H3PO4 contains water-soluble P, partial acidulation of PR with H3PO4 always results in PAPRs containing more total and water-soluble P than the unacidulated PR. Partial acidulation with H2SO4 results in a decrease in total P because of the formation of calcium sulphate in the product. However, the water-soluble P increases with the increasing degree of acidulation. The slope of the watersoluble P curve is smaller initially because of the presence of accessory minerals such as free carbonates, which are more soluble than apatite and preferentially consume the acid added. It is possible to prepare PAPRs from PRs of low reactivity. However, if the low reactivity is due to high contents of iron and aluminium oxides, the PR may not be suitable for partial acidulation with H2SO4. This is because the water-soluble P formed reverts to water-insoluble P during the preparation of PAPR (Hammond et al., 1989).

Ideally, partial acidulation of a given PR should be decided after a development study that varies: (i) the type of acid such as sulphuric, phosphoric and mixed acids and a mixture of sulphuric acid and ammoniacal salts; (ii) the quantity and concentration of acids; (iii) the temperature; and (iv) the type of mixing procedure (e.g. adding PR to bulk acid with constant mixing or spraying acid as fine droplets to a falling curtain of PR particles). The objective is to solubilize the maximum amount of PR for a given level of acid addition. Experience has shown that the most effective levels of acidulation, which give maximum soluble P for the quantity of acid used, range from 30 to 60 percent. At these rates, the acids affect the apatite but have little effect on hard gangues such as silicates.

The major P components in PAPR products are water-soluble monocalcium phosphate and sparingly soluble acid-unreacted PR. Partial acidulation and granulation of PR with H3PO4 yields products containing a soluble binder, monocalcium phosphate, that readily dissolves on application of the fertilizer to soil, so allowing the dissolution of the residual PR. On the other hand, partial acidulation and granulation with H2SO4 may yield products with CaSO4 coating PR particles. This may hinder the dissolution of the residual PR (Rajan and Ghani, 1997). It is possible to overcome this problem by controlling the drying temperature, as demonstrated by researchers of the International Fertilizer Development Center (IFDC) by preparing PAPR in a single-step process (Schultz, 1986).

Relationship between the level of acidulation with (A) phosphoric acid and (B) sulphuric acid of North Carolina PR and the total and water-soluble P of the products

The agronomic performance of PAPRs

The agronomic performance of PAPRs varies with: (i) the physical and chemical properties of the PR used; (ii) the degree of acidulation; (iii) the soil chemical properties, especially soil pH and P sorption (or retention); and (iv) the cropping system. In general, factors that enhance the effectiveness of PAPRs are: high reactivity and fine particle size of the PR, acidic soils, high soil P retention, long growth cycle, and crop rotation. The product is also likely to be more beneficial than water-soluble P fertilizers in situations where phosphate leaching may be a problem, as in sandy soils (Bolland et al., 1995). Increasing the level of acidulation increases the water-soluble P content and the agronomic effectiveness of PAPRs, but this needs to be weighed against the increased cost of fertilizer production. In all cases, the agronomic performance of the products produced should be evaluated to determine the cost-effective level of acidulation for a set of conditions (Owusu-Bennoah et al., 2002; Casanova et al., 2002a; Rodriguez and Herrera, 2002).

Research on the agronomic effectiveness of PAPRs has been reviewed (Bolan et al., 1990; Hammond et al., 1986b; Rajan and Marwaha, 1993; Chien and Menon, 1995a; Chien, 2003) and the general conclusion is that PAPRs of 40-50 percent acidulation with H2SO4 or 20-30 percent acidulation with H3PO4 are as effective as fully acidulated superphosphate. In soils of a high pH (pH 6.5-8.0), PAPRs may be as effective as superphosphate, albeit at a higher degree of acidulation with H3PO4 of up to 50-percent acidulation (about 66 percent of total P in watersoluble form) (McLay et al., 2000; Chien 2003).

The possible explanations for the high agronomic effectiveness of PAPRs are: (i) root priming effect by the monocalcium phosphate component of PAPR and the subsequent greater exploitation by roots of P-enriched soil; and (ii) dissolution of the monocalcium phosphate leading to formation of H3PO4 and the reaction of the acid on the unreacted PR (secondary acidulation). Root-growth studies have demonstrated a marked increase in root development in PAPR-implanted compared with PR-implanted soil samples (Rajan and Watkinson, 1992). A greater dissolution of the residual PR in PAPRs than unacidulated PR has also been documented under both greenhouse and field conditions (Rajan and Watkinson, 1992; Rajan and Ghani, 1997; McLay et al., 2000).

PAPR manufacture in an SSP plant

PAPRs have been manufactured in New Zealand at an SSP plant by mixing reactive PR with SSP (30:70 ratio by weight) at the ex-den stage, when the SSP was still wet. The product traded sold under the name Longlife Super. The extent to which the PR was partially acidulated depended on the acid-PR ratio used to make the SSP and, consequently, on the amount of free acid that was available to react with the PR (Bolan et al., 1987; Hedley et al., 1988). The water-soluble P was contributed mostly by the SSP component and to a minor extent by the partial acidulation of the PR added.

The agronomic effectiveness of Longlife Super applied to permanent pastures was determined in six field trials over three years (Ledgard et al., 1992). The fertilizers contained 35-50 percent of their total P in a water-soluble form and the soil pH values were about 5.7. The results showed that the pasture response to Longlife Super was generally less than that for superphosphate. Systematic studies indicated that the poor performance of Longlife Super was due to the CaSO4 continuing to cement PR particles as granules for six weeks, thereby depressing their dissolution.

Physical means

Compacted PR with water-soluble phosphate products

Fertilizers that are similar in chemical composition to PAPRs can be prepared indirectly by compacting dry PR with soluble phosphate fertilizers, such as SSP or TSP, under pressure (Chien et al., 1987a; Chien and Menon, 1995a; Menon and Chien, 1996). The water-soluble P content of the products will depend on the ratio of PR to WSP fertilizer used. Compaction technology offers the advantage of using PRs that are not suitable for direct partial acidulation with H2SO4 because of their high Al and Fe sesquioxides content. There is evidence that the effectiveness of PRs, even those of low reactivity, is increased after compacting with water-soluble P (Chien et al., 1987a; Kpomblekou et al., 1991). Under these conditions, compaction of PR with water-soluble P fertilizers at P ratios of about 50:50 can make the use of indigenous PRs agronomically and economically attractive in developing countries. However, the agronomic effectiveness of compacted fertilizer products relative to water-soluble fertilizers will depend on a number of factors, in a similar way to PAPRs.

Dry mixtures of PR with water-soluble phosphate fertilizers

There is field trial evidence that the application of PR as a dry mixture with WSP fertilizers can increase the effectiveness of the PR applied. Research conducted on an Indian calcareous soil indicated that applying fertilizers at a single rate in a 1:1 mixture of Mussoorie PR and SSP could be as effective as SSP (Siddique et al., 1986). In another field study, Mussoorie PR and SSP were applied at a P ratio of 2.2:1 using three fertilizer application rates (Singaram et al., 1995). The results showed that in an alkaline soil (pH 8.2) the PR-SSP mixture tended to be as effective as SSP. Moreover, based on a sequence of three crops, the product was economically equal to SSP. It was calculated that the dissolution of PR increased by 55 percent when applied in combination with SSP compared with that of an application of PR only.

In a greenhouse study, Chien et al. (1996) made quantitative estimates of the enhancing influence of water-soluble P (in the form of TSP) on the agronomic effectiveness of a medium-reactive central Florida PR using radioactive P tagged (32P) fertilizers. At a P ratio of 1:1, the PR and TSP were applied separately to the soil. Based on P uptake by plants, the study reported that the effectiveness of PR in combination with TSP increased by 165 percent for maize and 72 percent for cowpea. In another greenhouse study, Habib et al. (1999) found that a physical mixture of PR and TSP was as effective as TSP for growing rape in a limed alkaline soil. Zapata and Zaharah (2002) also found an enhancement effect on the agronomic effectiveness of Florida PR and sewage sludge when applied in mixture with TSP.

The increased effectiveness of PR when applied as a dry mixture can be ascribed to the root-priming effect of soluble P and, therefore, increased root exploitation of the PR added. Considering the negligible processing input and the resulting improvement in PR effectiveness, the above findings are of significant economic importance. Further research is needed under field conditions in order to evaluate the usefulness of this process.

Phosphate rock elemental sulphur assemblages

Numerous studies have shown that the agronomic effectiveness of PRs can be improved when they are applied after admixing or cogranulating with sulphur (S). In some instances, the products were inoculated with sulphur-oxidizing bacteria Thiobacillus spp. Thus, the product was referred to as ‘biosuper’ (Swaby, 1975). Kucey et al. (1989) reviewed the role of microbes in increasing plant-available P.

The principle behind the use of phosphate rock elemental sulphur assemblages (PR/S) is that the inoculated or native population of soil bacteria oxidizes S to H2SO4 when the product is applied to the soil. This acid in turn reacts with the PR particles that are in close proximity to S and forms monocalcium and dicalcium phosphates. Thus, the dissolution of PRs in soil is assisted by localized acidulation, in addition to that caused by ambient soil acidity. The important sulphur-oxidizing bacterial species are Thiobacillus thioxidans and Thiobacillus thioparus. Inoculating soils already rich in Thiobacillus spp. may not be essential (Rajan, 1982a) but inoculation is preferred for speedy multiplication of the bacteria and PR dissolution after application to soil.

PR/Ss are attractive because: (i) the production is not capital intensive; (ii) they enable flexible PR:S ratios; (iii) low-grade PRs that may be unsuitable for making soluble fertilizers can be used; and (iv) they behave as controlled-release P and S fertilizers. On the negative side, for speedy oxidation of S, the S needs to be present as fine particles (95 percent minus 0.15 mm), either ground with PR or ground separately and mixed. Grinding S without admixing PR or in a dry state, without moisture addition, is a fire hazard and it is not encouraged. Nevertheless, in New Zealand, a pilot plant has recently produced PR/Ss containing Thiobacillus spp. bacteria (EnviroPhosTM) using a cost-effective technique.

Factors that influence the effectiveness of PR/Ss are: (i) reactivity of the PR; (ii) PR:S ratio; (iii) type of crops; and (iv) soil environment. The agronomic effectiveness of PRs applied as PR/S can be expected to increase with either increasing reactivity of PRs or with increased fineness of PR particles (Rajan, 1982b; Loganathan et al., 1994). Research shows that the PR/S prepared from a finely ground reactive North Carolina PR, at a PR:S ratio of 5:1, was as effective as SSP, whereas that prepared from an unreactive Florida PR was inferior. Nevertheless, there was an increase in the agronomic effectiveness of both PRs when they were applied as PR/S, the values being 18-30 percent for the North Carolina PR and 50-70 percent for the Florida PR depending on the rate of application (Rajan, 1982b).

As the H2SO4 produced on oxidation of S affects the dissolution of PR in PR/Ss, it is logical to conclude that increasing the S content in PR/Ss would increase their agronomic effectiveness. On the other hand, a balanced approach is needed as the greater the enrichment of the product with S, the greater will be the cost of the product. Results from early studies show that PR/Ss could be as effective as SSP when the PR:S ratios are between 1:1 and 5:1 (Kittams and Attoe, 1965; Attoe and Olson, 1966; Swaby, 1975). The 5:1 ratio is similar to the proportion in which PR and S in the form of H2SO4 are used for making SSP. However, mostly unreactive PRs such as Florida PR (the United States of America) and Queensland PR (Australia) were used. When finely ground reactive PRs are used, this ratio can be increased to 7:1 without losing agronomic effectiveness (Rajan, 1983). On long-term crops such as permanent pastures, PR/Ss prepared at a PR:S ratio of 14:1 can be agronomically as effective as superphosphate (Rajan, 2002). On the other hand, for short-term crops that require a high rate of P supply, the PR/Ss prepared from reactive PRs with a narrow PR:S ratio may be required. As for the influence of soil environment, the agronomic effectiveness of PR/Ss will be greater in sites that favour PR dissolution. Under such conditions, PR/Ss prepared from less reactive rocks, or with wide PR:S ratios if the PRs are highly reactive, can be used as effectively as phosphate fertilizers.

The effectiveness of S will be highest if the S particles are in intimate contact with PRs as this will facilitate the maximum reaction of the H2SO4 produced on the PR. For this reason, PR/Ss have been obtained by cogranulating PR with elemental S (Swaby, 1975; Rajan, 1982b). On the negative side, granulation restricts the surface area for soil acid to react on the PR. For this reason, granules of less than 2 mm are preferred. Moist physical mixtures of PR/S containing Thiobacillus spp. have been found to be as effective as SSP for pasture production (Rajan, 2002).

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