Effectiveness of two partially acidulated rock phosphates,coastal superphosphate and Ecophos,compared with North Carolina reactive rock phosphate and superphosphate

The effectiveness of coastal superphosphate, a partially acidulated rock phosphate (PARP) made from apatite, and Ecophos, a PARP made from calcium iron aluminium (crandallite millisite) rock phosphate, was compared in pot experiments with the effectiveness of ordinary superphosphate (OSP) and North Carolina reactive apatite rock phosphate (NCRP). There were three experiments using different lateritic soils collected in Western Australia. Fertilizer effectiveness was measured using yield of dried wheat (Triticum aestivum) tops grown for 28 days. Three successive crops were grown. The phosphorous (P) fertilizers were applied and mixed with the soils before sowing the first crop. In addition, OSP was added to extra pots before sowing crops 2 and 3 in order to measure the effectiveness of the original P fertilizers relative to freshly-applied OSP for these crops.As measured using plant yield, coastal superphosphate was the most effective P fertilizer for three crops on an acidic peaty sand (pH water 5.0). Relative to freshly-applied OSP, it was 154% as effective for crop 1, 75% as effective for crop 2, and 36% as effective for crop 3. Corresponding values for Ecophos were 44, 29 and 19%, and for NCRP, 77, 67 and 29%, with the original OSP treatment being 61 and 56% as effective for crops 2 and 3. For three crops on a lateritic gravel loam (pH 6.5), both coastal superphosphate and OSP were the most effective fertilizers, and were equally effective for crop 1, and relative to freshly-applied OSP, were about 31% as effective for crop 2, and 16 and 21 % as effective for crop 3. Corresponding values for Ecophos were 47,15 and 11%, and NCRP, 33,15 and 5%. For two crops in a loamy sand (pH 5.4), OSP was the most effective fertilizer, and, relative to fresh OSP, it was 36% as effective for crop 2. Relative to fresh OSP, the effectiveness for crops 1 and 2 of coastal superphosphate was 57 and 18%, for Ecophos 71 and 27%, and for NCRP 50 and 36%.     

部分酸化磷矿一步法制复肥研究

介绍了部分酸化磷矿一步法制复合肥的肥效及水溶磷在土壤中的退化机理。说明控制化肥中适当的水溶磷比例,可以减少土壤对磷的固定,提高施肥效果。     

Partially acidulated reactive phosphate rock (PAPR) fertilizer and its reactions in soil

PAPR was made by partial acidulation of North Carolina phosphate rock with H₃PO₄. The PAPRs were incubated in bands in columns of two soils of contrasting P retention. The columns were sampled after freezing and sectioning with a cryomicrotome. The movement of P in soil incubated with³³P-labelled PAPR was followed by autoradiography of polished epoxy impregnated sections of the freeze-dried soil column. PAPR solubility was also studied by a sequential dialysis process using distilled deionised water. The acid solution resulting from the dissolution of monocalcium phosphate (MCP) in PAPR moved into the surrounding soil, solubilizing soil minerals and creating a low-pH front with a high concentration of P. Depending on the soil, phosphorus moved 6–14 mm away from the fertilizer/soil interface by mass flow and diffusion in two days. The increase in 0.5 M NaOH extractable P above that of untreated soil showed a maximum at the same position as the pH minimum in the soil. In both soils, the total P movement from the fertilizer band after a two day period for 50% PAPR was comparable to that for 100% acidulation (triple superphosphate), indicating that acidulations above 50% did not necessarily increase the movement of soluble P from the fertilizer pellet. Variations in pH in the fertilizer-affected soil could be explained by the net balance of acidity resulting from incoming acid P solution and release of OH^– during P sorption. The rock residue exhibited a transient loss in solubility which was reversed on subsequent dissolution, suggesting a possible surface alteration.     

The agronomic effectiveness of reactive rock phosphate,partially acidulated rock phosphate and monocalcium phosphate in soils of different pH

The initial and residual fertilizer effectiveness of North Carolina RP (rock phosphate), monocalcium phosphate and partially acidulated RP (made from North Carolina RP at 30% acidulation), both granulated and non-granulated, were measured in a glasshouse experiment. Triticale (xTriticosecale) was grown for 30 days on a soil that had been adjusted to three pH values (4.2, 5.2 and 6.2). Two crops were grown with a six month interval between crops. The effectiveness of the different fertilizers was compared using relationships between (1) yield of dried tops and the amount of P applied and (2) P content (P concentration in tissue multiplied by yield) and the amount of P applied. For the first crop, relative effectiveness (RE) of the fertilizers was calculated relative to granulated monocalcium phosphate, the most effective fertilizer. Monocalcium phosphate was not applied to the second crop, so relative residual effectiveness (RRE) was estimated for each fertilizer relative to the residual effectiveness of granulated monocalcium phosphate.The relative effectiveness of granulated monocalcium phosphate (band application) was greater (RE = 1.00) than of North Carolina RP (0.01–0.02) and partially acidulated RP (0.45–0.76) for all three soil pH values for the first crop. Granulation and band application increased the effectiveness of monocalcium phosphate and partially acidulated RP, but reduced the effectiveness of North Carolina RP. Both non-granulated monocalcium phosphate and partially acidulated RP were less effective than granulated partially acidulated RP for both crops. For the second crop granulated monocalcium phosphate was most effective and the RRE of non-granulated partially acidulated RP (0.16–0.32) and North Carolina RP (0.19–0.28) was greater than for non-granulated monocalcium phosphate (0.12). For the more acidic soil the RE of non-granulated North Carolina RP was four times higher than for the high pH soil for the first crop and 60% higher for the second crop, but it was still poorly effective relative to granulated monocalcium phosphate. Granulated North Carolina RP was least effective among all the fertilizers for all soil pH values and for both crops.     

Unacidulated and Partially acidulated phosphate rock: Agronomic effectiveness and the rates of dissolution of phosphate rock

The agronomic effectiveness of unground North Carolina phosphate rock (PR) and partially acidulated phosphate rocks (PAPR) prepared by acidulation of the PR with 30%, 40% and 50% of the phosphoric acid needed for complete acidulation, was determined in a 4 year field experiment on permanent pastures. The soil developed from volcanic ash, and was highly P retentive. The rate of dissolution in soil of the PR component in PAPR and of PR applied directly was measured, together with bicarbonate extractable P. The priming effect of the monocalcium phosphate (MCP) component of PAPR on root growth was also investigated.Pasture yields showed that even the 30% acidulated PAPR was as effective as fully acidulated triple superphosphate (TSP), mainly due to the high reactivity of the PR used. The 50% acidulated PAPR tended to be superior to TSP. Soluble P in PAPR caused a marked increase in root proliferation, and dry matter yields were greater than predicted from the amounts of MCP and PR in PAPR. Directly applied PR was inferior to TSP in years 1 and 2 but was equal in year 4. (There was no pasture response to application of P fertilizers in year 3.)Dissolution rates of the PRs were determined applying a cubic model to PR dissolution data. The rate of dissolution increased with increasing acidulation and this is tentatively ascribed to increased root proliferation around PAPR granules and acidification of the clover rhizosphere during nitrogen fixation.     

An evaluation of the agronomic potential of partially acidulated rock phosphates in calcareous soil

The agronomic potential of four partially acidulated rock phosphates (PARP) made from a moderate reactive phosphate rock at 30 or 60 percent acidulation either by sulfuric acid alone or by combination of sulfuric and phosphoric acids was compared with that of monocalcium phosphate (MCP) and ground rock phosphate (RP) on a calcareous soil (Typic Hapluquent, pH 8.5) in greenhouse. Dry weight and P accumulation of successive cuttings of ryegrass shoots were used to evaluate the relative agronomic potential of these fertilizers. Results indicated that PARPs of higher water-soluble P content had similar immediate effectiveness as MCP at two earlier cuttings, however, they produced significantly less total dry matter than MCP did in overall six successive cuttings. PARPs were constantly inferior to MCP in terms of P uptake by plant in all the six cuttings. When compared to RP, on the other hand, PARPs had markedly higher relative effectiveness. RP itself affected neither the dry matter production nor the P uptake by plant as compared to control treatment.Fractionation of residual inorganic P in the soil samples at two time intervals during plant growth indicated that MCP-P mainly transformed to dicalcium phosphate and octacalcium phosphate, and to a less extent to Fe and Al associated P. These forms of P had significant correlation with P accumulation by plant. Raw RP did not subject to transformation after applied to the soil regardless the duration of culture time. No obvious dissolution of unreacted RP in PARP materials was detected. Plant dry matter production and P uptake were mainly correlated with water-soluble P added with the fertilizers. It is suggested from the experiment that although partial acidulation could substantially improved the effectiveness of rock phosphate and the immediate effect of the fertilizer was competitive with MCP, application of PARP to calcareous soils is only of short-term benefits; in a long run this fertilizer is not considered as a desirable source of P in calcareous soils since the unacidulated part in the fertilizer was unable to be solubilized in the alkaline conditions.     

Long-term greenhouse evaluation of partially acidulated phosphate rock fertilizers

Twelve granular partially acidulated phosphate rock (PAPR) fertilizers were compared with unacidulated phosphate rocks (PR) and superphosphate at five rates of total P in the presence and absence of supplementary sulfate and plant residue recycling treatments in a long-term green-house experiment with lucerne (Medicago sativa L., cv. CUF101). The PAPRs were prepared from two PRs (Christmas Is. A grade and Duchess, Queensland) and acidulated at two rates (25% and 50% on an H₂SO₄ to single superphosphate basis) with either H₂SO₄ or H₃PO₄. Six harvests (each bulked from three cuttings) were collected over a 2-year period. It was generally found that lucerne response to PAPRs depended closely on their water-soluble plus citrate-soluble P contents which increased with increased degree of acidulation. The H₃PO₄ tended to yield more soluble P on acidulation of PR than H₂SO₄ and acidulation of Christmas Is. PR yielded more soluble P than did acidulation of Duchess PR. There was little evidence for enhanced availability of P due to action of the triple point solution in hydrolyzing granules on residual PR in those granules.     

An assessment of the agronomic efficiency of partially acidulated phosphate rock fertilisers

The literature comparing the efficiency of partially acidulated phosphate rock fertilisers with that of a single or triple superphosphate is briefly summarised and found to be conflicting. Various theories purported to explain why partially acidulated phosphate rock products are as efficient are examined. An alternative hypothesis, which appears to reconcile the conflicting evidence, is presented. This depends in part on the dissolution of the phosphate rock component of a partially acidulated product in the soil; the factors influencing the dissolution of phosphate rock in the soil are reviewed in relation to the conflicting statements about the efficiency of partially acidulated phosphate rock products.     

Long-term greenhouse evaluation of partially acidulated phosphate rock fertilizers

The agronomic effectiveness (yield and P uptake) of twelve granular, partially acidulated phosphate rock fertilizers (PAPR) and two finely ground, unacidulated phosphate rocks (PR) were compared to that of a single superphosphate in a long-term greenhouse experiment with lucerne (Medicago sativa L., cv. CUF101), grown in a low P sorbing, moderately acid, sandy loam soil of moderate P status (Paleustaf). The PAPRs were prepared from two unreactive PRs (Christmas Is. A grade and Duchess rock from Queensland) and acidulated at two rates (25% and 50% on a H₂SO₄ to single superphosphate basis) with either H₂SO₄ or H₃PO₄. Additional products included H₂SO₄ PAPRs cogranulated with elemental S (10% w/w).Superphosphate was consistently superior to all PRs and PAPRs in agronomic effectiveness throughout this two-year study. The most effective of the PAPRs were those that were 50% acidulated with H₂SO₄ and cogranulated with elemental S; this type of fertilizer from both rocks was approximately 2/3 as effective as superphosphate when relative agronomic effectiveness indices (RAE) were calculated from cumulative yields. The increase in agronomic effectiveness relative to superphosphate (RAE value) by the partial acidulation of the PR could be attributed to its effect of increasing the P solubility in the PAPR. A curvilinear relationship existed between the RAE values of PRs and PAPRs, measured from cumulative yield or P uptake data, and the percentage of the total P in each fertilizer that was in a soluble (water + citrate soluble) form. Cogranulation with elemental S (10% w/w) significantly displaced this relationship upwards by increasing the RAE of H₂SO₄ PAPRs by more than 50%. The maximum cumulative recovery of applied P by lucerne tops after five bulked harvests (fifteen consecutive harvests) was 61.5%, which occurred at the low application rate of superphosphate. The decline in the substitution value of PRs for superphosphate, that occurred with increasing P rates tended to be offset both by increasing the level of acidulation and by cogranulating the PAPR with elemental S.     

Comparison of efficiency of Mussoorie partially acidulated phosphate rock and single superphosphate in a shallow Alfisol of the Indian semiarid tropics

The agronomic effectiveness of a partially acidulated phosphate rock (PAPR) was measured in a field experiment with sorghum (Sorghum bicolor cv. CSH-6) in a shallow Alfisol at the ICRISAT farm, Patancheru (Hyderabad), India. The experiment compared PAPR with single superphosphate. The PAPR was made by acidulating an indigenous Indian phosphate rock (Mussoorie) with H₂SO₄ at 50% acidulation level. P response was evaluated at a single relatively high N rate (120 kg ha^–1) with five rates of P (0, 2.2, 4.4, 8.8, and 17.6 kg P ha^–1). A significant response to P was obtained at rates up to 17.6kg P ha^–1.There was no significant difference due to source of P in terms of sorghum grain yield or total P uptake. Both Olsen and Bray 1 soil tests underestimated P availability from PAPR with respect to that from SSP.A rapid rate of P uptake was observed during grain filling to maturity (81–102 days), when 40% of the total P was taken by the plant. The internal efficiency of both P sources was the same.     

Use of partially acidulated phosphate rocks as phosphate fertilizers

Partially acidulated phosphate rocks (PAPRs) are manufactured by acidulation of PRs with less than the stoichiometric amounts of, usually, phosphoric or sulphuric acids. Products of similar composition to PAPRs are also prepared by cogranulating superphosphate with PRs. For most crops the agronomic value of PAPRs is determined by the availability to plants of their water-soluble P as well as their PR P component. The acid unreacted PR present in the directly acidulated PAPR, is considered to be less reactive than the original PR. This is probably the result of surface coatings of chemical compounds formed during acidulation. Under some soil conditions, in the presence of plants, the PR component probably dissolves faster than the original PR. For seasonal crops, except for fast growing ones such as squash (Cucurbita maxima), reactive PRs partially acidulated so that the final products contain about 50% of its total P in water-soluble form, are generally as effective as fully acidulated superphosphate. For permanent pastures the water P content may be reduced to about 40% of total P without reducing their agronomic effectiveness of the product. In medium P retentive soils pH seems to have little or no influence on the agronomic effectiveness of PAPRs. In highly P retentive soils increasing soil pH reduces the agronomic effectiveness of phosphoric PAPRs apparently by reducing the solubility of the PR component of PAPRs. Even at low pH the dissolution of unreacted PR in sulphuric PAPRs is less than that in phosphoric PAPRs, probably due to the possible coating of calcium sulphate on the residual PR in sulphuric PAPRs. Results on the agronomic effectiveness of PAPRs prepared from unreactive rocks were highly variable and no generalisation could be made regarding the degree of acidulation needed for the products to be consistently effective. Single superphosphate (SSP) cogranulated with reactive rocks (SSP/PR) was agronomically less effective than SSP, and also than phosphoric PAPRs of similar water-soluble P.     

Investigations on interactions of phosphorus compounds in partially acidulated phosphate rock and fertilizer effectiveness

Khouribga phosphate rock was partially acidulated with 50 and 70% of the required H₂SO₄ for complete acidulation. The unreacted rock residue was isolated by subsequent extractions with water and alkaline ammonium citrate solution. P solubility in 2% formic acid of this residues was reduced as compared to the original Khouribga phosphate rock. This loss in reactivity consistently increased with the degree of acidulation. Plant response to fertilizer application emphasized the negative effect of partial acidulation in an acid soil. Mixtures of superphosphate and phosphate rock were more effective than partially acidulated phosphate rock.Applications of apatitic P did not affect P efficiency on a neutral soil. Differences between mixed and partially acidulated phosphate rock could therefore not be observed. The effectiveness of the products was due to their content of acidulated P.Hydrolysis of monocalciumphosphate caused a further acidulation of the residual apatite in moist incubated granules. The extent of these reactions, however, was too low to improve P efficiency significantly.     

Properties of unreacted rock residues in partially acidulated phosphate rocks affecting their reactivity

The nonacidulated P fraction of partially acidulated phosphate rocks (PAPR) was obtained by extraction with alkaline ammonium citrate solution. Investigations on this unacidulated rock residues using standard analytical techniques and electronbeam microanalysis showed a surface coating with highly increased fluorine content surrounding the unreacted phosphate rock particles. The coating may be responsible for low reactivity of the residues and their inferior agronomic effectiveness as compared to the original mother rock phosphate. Furthermore, the existence of dicalciumphosphate and Fe—Al—P compounds in PAPR products make solubility tests based on alkaline ammonium citrate appropriate to gauge the acidulated and easily plant available P fraction. Only after the removal of these compounds with citrate solution is an assessment of the potential agronomic value of the nonacidulated rock residues in 2% formic acid possible.     

Effectiveness of partially acidulated phosphate rock as a source to plants in calcareous soils

In a series of greenhouse experiments granulated phosphate fertilizers prepared by mixing triple superphosphate with phosphate rock and partially acidulated phosphate rock, ranging in their content of water souble P from 95 to 17 per cent of total P were applied to neutral and slightly alkaline (pH 6.9–7.8), sandy loam to clay soils ranging in calcium carbonate content from 2 to 35 percent. Dry matter yield of clover, alfalfa, millet or maize were obtained, P uptake determined and sodium bicarbonate extractable P in soil measured. In one field experiment triple superphosphate was compared to mixture of triple superphosphate and phosphate rock on maize. X ray difraction on one triple superphosphate — phosphate rock mixture and on one partially acidulated phosphate rock showed that both fertilizers contain mainly monocalcium phosphate and fluorapatite. After incubation in soil the dicalcium phosphate content rose and the monocalcium phosphate disappeared.Parameters received in greenhouse experiments and in the field indicate that phosphate fertilizers composed of superphosphate and up to 50 percent phosphate rock are as efficient source of P to plants on calcareous and slightly alkaline soils as superphosphate. If this indication would be proven in extensive field experimentation it would lead to savings in acid consumption and in fertilizer manufacturing plant capacity for calcareous soils.     

Comparison of Olsen and Pi soil tests for evaluating phosphorus bioavailability in a calcareous soil treated with single superphosphate and partially acidulated phosphate rock

The Pi test for phosphorus (P) is a new method in which strips of iron oxide impregnated filter paper are used as a sink to sorb and extract P from a soil solution. In a greenhouse experiment, the Olsen and Pi tests were compared for their effectiveness in evaluating P availability to maize on calcareous soils. Phosphate rock from Togo, partially acidulated with H₂SO₄ at 50% acidulation level (PAPR 50% H₂SO₄) and single superphosphate (SSP) were applied at different rates to a calcareous soil (Vernon Clay, pH 8.2, CaCO₃ 18.9%) which was preincubated with KH₂PO₄ to raise plant-available P to different levels. In soils treated with SSP, dry-matter yield of maize correlated equally well with Pi-P and with Olsen-P (r = 0.96^***). P uptake correlated significantly with P_i-P (r = 0.94^***) as well as Olsen-P (r = 0.97^***). Likewise, in soils fertilized with PAPR, significant correlations were found between dry-matter yield and Pi-P (r = 0.97^***) and between dry-matter yield and Olsen-P (r = 0.94^***). When all the data were pooled, Pi-P and Olsen-P correlated equally well with both dry-matter weight (r = 0.97^***) and P uptake (r = 0.94^***). Phosphorus extracted by the Pi test correlated significantly with P extracted by the Olsen test (r = 0.99^***).     

Partially acidulated phosphate rocks: Controlled release phosphorus fertilizers for more sustainable agriculture

Phosphate rocks partially acidulated either with H₃PO₄ or H₂SO₄ were compared against SSP or TSP as phosphate fertilizers for permanent pasture. Eleven field trials were conducted over periods of up to 6 yrs. Fertilizers were surface applied annually. Initial soil pHw values ranged from 5.5–6.3 and Soil P retention from 25% to 97%. The PRs used for partial acidulation were unground or ground North Carolina PR, ground Khouribga PR, and a blend of ground PRs of North Carolina, Arad and Khouribga PRs. From the DM yields, fertilizer substitution values were calculated: fertilizer substitution value was the ratio of total P applied as superphosphate to total P as PAPR required to produce the same DM yield.Rates of dissolution of the PR component of PAPRs were also determined in soils collected from two trials.Agronomic results demonstrated that 30% acidulated phosphoric PAPRs (about 50% of total P as water-soluble P) were as effective as TSP, when the PR acidulated was from unground North Carolina PR. Results from one field trial indicated that when PAPR was from ground North Carolina PR, 20% acidulated product (water-soluble P 30–40% of total P) was equally effective as TSP. Replacement of ground North Carolina PR by a less reactive Khouribga PR did not appear to decrease the yield. Results indicated that per unit P released into soil solution, PAPRs were more efficient fertilizers than TSP. With annual applications, fertilizer substitution value of PAPR 30% tended to increase with time.Sulphuric PAPRs prepared from North Carolina PR were generally inferior to phosphoric PAPRs containing similar amounts of water-soluble P. This was attributed to the presence of CaSO₄ coatings.Abbreviations DM Dry matter - PAPR Partially acidulated phosphate rock - PR Phosphate rock - SSP Single superphosphate - TSP Triple superphosphate     

Differential influence of soil pH on the availability of partially sulphuric and phosphoric acidulated phosphate rocks II. Chemical and scanning elecronmicroscopic studies

Part I of this study showed that the plant availability of P from a reactive phosphate rock (PR), North Carolina PR, partially acidulated with phosphoric acid (Phos-PAPR) increased with decreasing soil pH from pH 6.1 to 5.1, whereas availability from a blend of similarly reactive PRs partially acidulated with sulphuric acid (SA-PAPR) changed little. The present study was carried out to explain the above results. Phosphate sorption maximum of soil as a function of soil pH was determined. Soil samples obtained at the completion of the pot experiment [5] were analysed for inorganic P fractions, and the amounts of PR dissolved from the PAPRs were determined. A leaching study, simulating pot experiment conditions, was conducted to determine the changes in the chemical composition and the spatial distribution of P, S and Ca in the fertilizer residues. The properties of the PAPRs were further characterised by sequential extraction of the fertilizers. Phosphate sorption isotherms indicated a smaller amount of P in solution at lower pH values, which suggested reduced P availability with decreasing soil pH. Dissolution of the residual PR-P was generally greater in Phos-PAPR treatment than in PR applied directly or in the SA-PAPR treatment. PR-P dissolution in Phos-PAPR increased with decreasing pH but not in SA-PAPR. Chemical, electron microprobe, X-ray micro-analysis and X-ray powder diffraction studies of the fertilizer residues obtained from the leaching and sequential extraction experiments showed rapid dissolution of the Ca(H₂PO₄)₂ phase of the fertilizers but the CaSO₄.XH₂O persisting as a cementing phase between the PR particles. The CaSO₄.XH₂O which intially existed mostly in an anhydrous form changed to gypsum. It was concluded that the dissolution of PR-P in the SA-PAPR was impeded by the presence of CaSO₄.XH₂O acting as a physical barrier and also by providing higher Ca in solution than that would exist in a saturated solution of the apatites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Single superphosphate - reactive phosphate rock mixtures. 3. The use of concentration ratios of elements to identify the nature and amounts of unacidulated rock residues in the mixtures

Water insoluble residues (WIR) of unreactive phosphate rocks in single superphosphate-reactive phosphate rock (SSP-RPR) mixtures are considered to reduce the agronomic value of these mixtures. A technique using concentration ratios of elements to identify the quantities of WIR of ground North Carolina (NC), ground Nauru and as received NC phosphate rocks in a SSP-RPR was developed. Of 22 elements tested P/Sr ratios were found to be the only element ratios that could be used to distinguish between WIR's derived from Nauru and NC. P/Sr ratios in Nauru and NC were markedly different and provided a useful index for differentiating between the two rocks. During acidulation the P/Sr concentration ratio remained essentially constant in the WIR's from both rocks.Using the element ratio technique the percentage of total P in the SSP-RPR sample was determined to be 60-61% water soluble, 5% water insoluble Nauru, 2% water insoluble ground NC and 32-33% water insoluble as received NC.The element ratio technique developed in this study can be applied to partially acidulated P fertilizers made with rocks other than NC and Nauru provided elements which satisfy the same conditions as Sr in this study can be found.     

Agronomic evaluation of modified phosphate rock products

Phosphorus (P) is critically needed to improve the soil fertility for crop production in large areas of developing countries. The high cost of conventional, water-soluble P fertilizers constrains their use by resource-poor farmers. Finely ground phosphate rock (PR) has been tested and used as a direct application fertilizer on tropical acid soils as a low-cost alternative where indigenous deposits of PR are located. However, direct application of PR with low reactivity or with inappropriate soil/crop combinations does not always give satisfactory results. Partial acidulation of PR (PAPR) or compaction with triple superphosphate (PR + TSP) or single superphosphate (PR + SSP) represent technologies that can be used to produce highly effective P fertilizers from those indigenous deposits. Numerous field trials conducted by IFDC in Asia, sub-Saharan Africa, and Latin America have demonstrated that PAPR at 40-50% acidulation with H₂SO₄ or at 20% with H₃PO₄ approaches the effectiveness of SSP or TSP in certain tropical soils and crops. This paper discusses how the agronomic effectiveness of PAPR is affected by mineralogical composition and reactivity of PR used and by soil properties and soil reactions. The paper also indicates that if a PR has high Fe₂O₃ + Al₂O₃ content, it may not be suitable for PAPR processing because of the reversion of water-soluble P to water-insoluble P during the PAPR manufacturing process. Under these conditions, compaction of PR with water-soluble P fertilizers (e.g. SSP, TSP) at P ratio of approximately 50:50 can be agronomically and economically attractive for utilizing the indigenous PRs in developing countries.     

Differential availability of partially sulphuric and phosphoric acidulated phosphate rocks I. Plant response

A glasshouse study was conducted to determine the influence of soil pH on the agronomic effectiveness of partially phosphoric (Phos-PAPR) and partially sulphuric (SA-PAPR) acidulated phosphate rocks (PR). For Phos-PAPR ground North Carolina PR (NCPR) was acidulated with 10, 30 and 50% of acid needed for complete acidulation. For SA-PAPR a blend of NCPR, Arad and Khouribga PRs were acidulated with 60% of the acid needed. The relative agronomic effectiveness of these PAPRs were compared with superphosphate (SSP) and ground NCPR. A highly phosphate (P) retentive and P deficient pasture soil was used. Prior to addition of fertilizers to soil, the pH of soil was adjusted to 5.1 (initial soil pH) 5.4, 5.7 and 6.1 by applying varying amounts of Ca(OH)₂. Ryegrass (Lolium perenne) was grown as the test plant over a period of eight months. Fertilizers were applied at three rates plus control. Soil pH was monitored and continuously adjusted to the desired levels throughout the experimental period.The dry matter yields and P uptake in SSP treated pots were not influenced by soil pH. With increasing soil pH, agronomic performance of Phos-PAPRs and NCPR significantly (P<0.01) decreased but that of SA-PAPR was not affected. On the basis of per unit water-soluble P applied, uptake of P by plants was greater from PAPRs than SSP. Using the P uptake values of SSP and NCPR (which was used to prepare the PAPRs), the dissolution of P from the residual PR component of the PAPRs were calculated. The residual PR component of the Phos-PAPRs apparently dissolved in greater quantities than unacidulated NCPR. Dissolution of the residual PR was enhanced with increasing degree of acidulation. However, in the case of SA-PAPR, the agronomic performance of the PAPR was mostly dependent on the water-soluble P component of the PAPR. The uptake of P from the residual PR component of the SA-PAPR was insignificant.