Partial acidulation of an ‘unground’ phosphate rock: II. Plant availability of phosphate

Four greenhouse experiments were conducted using three soils to determine the availability to plants of P from unground North Carolina phosphate rock (PR) treated with 20% to 50% of the H₃PO₄ required for complete acidulation. The influences of soil P retention, P status, the method of preparation of partially acidulated phosphate rocks (PAPRs) and the granule size of the products were investigated. Perennial ryegrass was grown as the test plant for up to 8 months. Triple superphosphate (TSP) was used as the standard fertilizer and unground North Carolina rock was included for comparison.The dry matter yield and P uptake response curves showed that in all experiments PAPRs were markedly superior to the PR. P status of soils appeared to influence the effectiveness of PAPRs to a greater extent than P retention. In soils of low P status the degree of acidulation required for PAPR to be nearly or as effective as TSP was 50% whereas in a soil of high P status even 30% PAPR applied as a maintenance fertilizer was effective. There was a significant positive correlation between water soluble P of fertilizers and P uptake by ryegrass. However, in general PAPRs were more effective per unit of water soluble P than TSP. Granule size (< 1 mm and 1–2 mm) and method of preparation of PAPRs did not alter the effectiveness of PAPRs.     

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.     

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.     

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.     

Evaluation of the agronomic effectiveness of natural and partially acidulated phosphate rocks in several soils using32P isotopic dilution techniques

The agronomic effectiveness of two natural phosphate rocks (PRs) from North Carolina (USA) and Togo and their 50% partially acidulated products (PAPRs) was evaluated in two greenhouse experiments using³²P isotopic dilution techniques, namely L and A_L values.In the first experiment rye grass was grown in a soil from Ghana. While the proportion of P in the plant derived from the P fertilizer (Pdff) ranged on. the average from about 10% for the PRs up to 80% for the PAPRs, the P fertilizer recovery was less than 1% for a 60-day growth period. In the second experiment, average values of P in the maize plants derived from the PAPRs ranged from 35% to 75% in 3 different soils. Both PRs were ineffective with the exception of North Carolina PR in the Seibersdorf soil. The P fertilizer recovery was 0.25% for the North Carolina PR in this soil whereas the recovery values ranged from 1.2% to 1.6% for the PAPRs.Mean values of the relative fertilizer efficiency estimated from the L values of each soil were less than 1% for the PRs whereas the values for the PAPRs which were dependent on soil type ranged from 20% up to 45%. The coefficient of relative effect of partial acidulation, that was calculated from the ratio of A_L values for PR and PAPR in each soil indicated that partial acidulation increased the effectiveness of the natural PRs in all soils under study.This study showed that the use of³²P isotope dilution techniques allows an accurate measurement of the P availability from natural and modified PR products to crops. Another advantage is that quantitative comparison of the P sources under study, PRs and PAPRs in this case, can be made even in soils where there is no response to the applied P sources.     

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.     

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.     

Comparison of five soil testing methods to establish phosphorus sufficiency levels in soil fertilized with water-soluble and sparingly soluble-P sources

Field experiments were conducted in Niger with pearl millet (Pennisetum glaucum [L] R. Br.) in which the crop was fertilized with phosphate rock (PR) from two deposits from Niger (Tahoua and Parc W). The PR was applied either as ground rock or as partially acidulated phosphate rock (PAPR) and was compared to water soluble sources (TSP and SSP) in terms of millet yield response. The ability of five soil testing procedures (Bray P₁, Bray P₂, Mehlich 1, Olsen, and water extraction) to establish P sufficiency levels for millet was tested. The results of all soil testing methods were highly correlated amongst each other for the treatments receiving water-soluble fertilizers or PAPRs. None of the soil testing procedures which were evaluated was able to accurately measure available P when PRs were applied. Sufficiency levels were calculated for the PAPR and water-soluble fertilizers using nonlinear regression analysis and a graphic procedure for each of the P soil testing methods. The Bray P₁ method appeared to be the most reliable procedure and was used to study the effect of accumulated total or total water + citrate-soluble P rates on final P availability. A single quadratic function was able to describe this effect when the P rates were expressed as water + citrate-soluble P for both PAPRs and water-soluble fertilizers independently of the P fertilizer source.     

Solubility and agronomic effectiveness of partially acidulated phosphate rocks as influenced by their iron and aluminium oxide content

Partially acidulated phosphate rock (PAPR) has been shown to be an effective source of P for plants grown on acid soils. Less information in available, however, regarding the effect of the phosphate rock (PR) source on the solubility and agronomic effectiveness of PAPR.The effect of Fe₂O₃ + Al₂O₃ content in PR on the quality of PAPR produced was investigated in this study. Nine sources of PR from Africa, Latin America, and the United States, representing a range of Fe₂O₃ + Al₂O₃ from 0.7% to 12.4%, were used. In a single-step process, the finely ground PRs were partially acidulated with H₂SO₄ at the 30% or 50% acidulation level and granulated (–3.35 + 1.18 mm or –6 + 14 mesh). It was found that the water-soluble P content in PAPR decreased with increasing Fe₂O₃ + Al₂O content in the PR used. Apparently, the presence of Fe₂O₃ + Al₂O₃ resulted in a reversion of some of the water-soluble P to citrate-soluble P and sometimes even to citrate-insoluble P.A short-term (6 weeks) greenhouse study was conducted to evaluate crop response to PAPRs and single superphosphate (SSP); maize, the test crop, was grown on an acid soil (pH 4.5)—Hartsells silt loam (Typic Hapludults). The agronomic effectiveness of PAPRs with respect to SSP (in terms of dry-matter yield of maize) decreased with increasing Fe₂O₃ + Al₂O₃ content in PRs. Phosphorus uptake by maize from PAPRs was found to correlate well with water solubility but not with citrate solubility. The results obtained in this study show that the detrimental effect of Fe₂O₃ + Al₂O₃ content on the solubility and P availability of PAPR should be considered when selecting a PR for PAPR production.     

Single superphosphate-reactive phosphate rock mixtures. 2. The effect of phosphate rock type and denning time on the amounts of acidulated and extractable phosphate

Ground samples of Nauru (N), Christmas Island A (X), Jordan (J) or North Carolina (NC) phosphate rocks (PRs) were acidulated with³²P spiked sulphuric acid to produce single superphosphate (SSP) reaction mixtures. Subsequently, single superphosphatereactive phosphate rock (SSP-RPR) mixtures were manufactured by adding reactive phosphate rock (RPR) as either ground or unground NCPR or ground JPR to SSP reaction mixtures that had been denned for either 22 or 47 minutes after acid addition. The solubility of P in the final SSP-RPR products was assessed either by extraction with water, 2% citric acid, 2% formic acid or 1M neutral ammonium citrate (NAC), or by calculation of the exchangeable P content of the fertilizer by isotopic dilution techniques. The measurement of exchangeable P allowed calculation of the amounts of acidulated P in the ex-den SSP and the amount of RPR P acidulated on addition to ex-den SSP containing free phosphoric acid.Among the PRs used for SSP manufacture, the highest degree of acidulation at the ex-den stage was obtained for NCPR (92%) and the lowest was obtained for XPR (75%). As a consequence, the presence of XPR in the SSP reaction mixture decreased the amount of exchangeable P in the SSP-RPR mixtures. Whereas initially the conversion of PR P increased with time of acidulation at 22 minutes and 47 minutes (i.e. the time of addition of RPR) the differences in the degree of acidulation of PR in the ex-den SSP were not large and hence had no significant effect on the extractability of P in the SSP-RPR mixtures.The nature of the RPR added to the ex-den SSP reaction mixture had a significant effect on the solubility of P in the SSP-RPR mixtures. SSP-RPR mixtures with added unground NCPR or ground JPR had lower P solubility than when ground NCPR was added. RPR P constituted between 38 and 46% of the total P in the SSP-RPR mixtures and at acid/PR (A/R) ratios of 0.60 to 0.70, between 28 to 49% of the RPR P was acidulated by the free acid in the SSP reaction mixture during manufacture.The results also indicate that RPR mixtures made using ex-den SSP made from unreactive PRs will always contain more unreactive PR residue than those made with mature SSP. However, given the practical difficulties of producing the SSP-RPR mixtures with mature SSP, denning times should be extended for as long as practicably possible.     

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.     

Influence of manufacturing variables on characteristics and the agronomic value of partially acidulated phosphate fertilizers

Partially acidulated phosphate fertilizers are manufactured either by direct partial acidulation of phosphate rocks (PRs) with sulphuric and/or phosphoric acid (directly acidulated PAPR) or indirectly by mixing reactive phosphate rocks (RPRs) with single superphosphate (SSP-RPR mixture). This form of low cost fertilizer manufacture is suitable for improving the agronomic value of unreactive PRs or production of high analysis fertilizers that can have agronomic values similar to fully acidulated phosphate fertilizers.The solubility characteristics of the directly acidulated PAPRs are affected by the type, composition and concentration of the acid used for acidulation, degree of acidulation, nature and fineness of PR and the method of manufacture. In general, partial acidulation with phosphoric acids which contain minimum amounts of metallic impurities acidulates more PR and results in more soluble P in the product. In the case of SSP-RPR mixtures made by adding RPR to immature SSP, the nature of PR used for SSP manufacture and the time of addition of RPR to ex-den SSP mixture affects the quality of the product. In order to minimize the selective reaction of the RPR with residual acid present in the ex-den SSP reaction mixture, RPR should not be added until PR acidulation (used for SSP) is essentially complete.The agronomic value of partially acidulated phosphate fertilizers is affected by the amount of water soluble P and the solubility of residual PR. None of the single extraction tests such as 2% citric acid, 2% formic acid and neutral ammonium citrate appear to be appropriate as indicators of plant available P in these fertilizers. Double extraction procedures which remove both the soluble P and the residual P have been investigated, but need to be correlated with agronomic data before they can be adopted as quality tests.     

Agronomic evaluation of fertilizer products derived from Sukulu Hills phosphate rock

Partial acidulation of phosphate rock (PR) or compaction of PR with soluble P fertilizers can improve the usefulness of unreactive PR for use as P fertilizer. A greenhouse study was conducted to evaluate nonconventional phosphate fertilizers derived from a low reactive Sukulu Hills PR from Uganda. Raw PR (which contained 341.0 g kg^–1 Fe₂O₃), beneficiated or concentrate PR, partially acidulated PR (PAPR) and PR compacted with triple superphosphate (TSP) were evaluated. Compacted materials had a P ratio of PR:TSP = 50:50. PAPR materials were made by 50% acidulation with H₂SO₄. TSP was used as a reference fertilizer. Fertilizers were applied to an acidic (pH = 5.4) Hiwassee loam (clayey, kaolinitic, thermic Rhodic Kanhapludults) at rates of 0, 50, 100, 200, 300 and 400 mg P kg^–1 soil. Two successive corn (Zea mays L.) crops were grown for 6 weeks. Compacted concentrate PR + TSP and raw PR + TSP were 94.4 and 89.7% as effective as TSP, respectively, in increasing dry-matter yields for the first corn crop. PAPR from the concentrate was 54.8% as effective as TSP. Raw PR, concentrate PR and the PAPR from the raw PR were ineffective in increasing dry-matter yields. The same trends were obtained when P uptake was used to compare effectiveness. Ineffectiveness of the raw PR and its corresponding PAPR was attributed to a high Fe₂O₃ content in the raw PR. Bray I and Pi paper were found to be nearly equally suitable at estimating available P in the soils treated with responsive fertilizer materials. Mehlich 1 overestimated available P in soil treated with raw PR, concentrate PR or the PAPR from the raw PR.     

Preparation,forms and properties of controlled-release phosphate fertilizers

Controlled-release phosphate fertilizers include phosphate rocks (PRs) for direct application, partially acidulated phosphate rocks (PAPRs) and thermal phosphates. Phosphate rocks contain apatite as the main P containing mineral, the composition and the chemical nature of which vary between PRs. Based on the solubility in chemical extractants PRs are broadly grouped into ‘reactive’ and ‘unreactive’. The ‘reactivity’ of PRs is influenced strongly by the extent of carbonate substitution for phosphate in the apatite minerals. Under certain soil and climatic conditions reactive PRs (RPRs) can be used as a source of P for direct application. Partially acidulated phosphate rocks (PAPRs) are produced either by direct partial acidulation of PRs with mineral acids or by mixing PRs with fully acidulated superphosphate reaction mixtures. Partial acidulation of PRs with H₃PO₄ generally results in higher water soluble P contents than those acidulated with H₂SO₄. Mixing of RPRs with superphosphate reaction mixtures sometimes results in the preferential consumption of free acid and thereby increases the amounts of residual unreacted PRs. Thermal phosphates are produced by either heating PRs below melting point both in the presence and the absence of silica (calcined phosphates) or heating PRs with silica above melting point (fused phosphate). These phosphates are alkaline in nature and hence suitable for acidic soils.     

Factors affecting the agronomic effectiveness of phosphate rock for direct application

Phosphorus (P) is critically needed to improve soil fertility for sustainable crop production in large areas of developing countries. In recent years, phosphate rock (PR) for direct application has been tested in tropical acid soils as a potential alternative to conventional water-soluble P fertilizers like single superphosphate (SSP) and triple superphosphate (TSP). Some developing countries have PR deposits which, if used to supplement other imported P fertilizers, would allow a saving of much needed foreign exchange. Solubility of P fertilizers is not the only criterion in selection of the most suitable P fertilizer. This paper discusses the results of experiments to compare the relative agronomic effectiveness (RAE) of various PR sources with respect to SSP or TSP as influenced by four important factors: PR sources, soil properties, management practices, and crop species. Under certain conditions, PRs can be agronomically effective.     

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.     

Field evaluation of partially acidulated phosphate rocks in a Ferralsol from Cuba

Phosphorus (P) is needed in large areas of developing countries toimprove soil fertility for crop production. The use of phosphate rock (PR) isan alternative to costly soluble P fertilizers, but it is ineffective usuallyin non-acid soils unless it is modified i.e. partially acidulated (PAPR). Alaboratory incubation study using the isotopic exchange kinetic method of³²P and field experiments were undertaken on a neutral Ferralsol ofCuba to evaluate the effectiveness of PAPRs as fertilizers for common bean(Phaseolus vulgaris, L.). Sulfuric-acid based PAPR using40%, 50% and 60% of the acid required to produce singlesuperphosphate were studied. In the laboratory experiment Trinidad de GuedesPAPR was effective in providing P to the soil, through increases inisotopicallyexchangeable P and the percentage of P derived from fertilizer (%Pdff). In the three field experiments carried out to compare the P sources,yields of common bean were increased by PAPR, though the response was less thanwith triple superphosphate (TSP). The relative agronomic effectiveness (RAE) ofPAPR was greater than that of unacidulated PR. Taking into account the RAEvalues and the current cost of the P sources, the choice of Trinidad de GuedesPAPR instead of TSP could be economic, although the RAE value for PAPR waslowerthan that of TSP. This result indicates that PAPR could be used in thesoil understudy to obtain the best economic return. DM yield, P uptake and grain yield ofcommon bean were significantly increased by applying P as 50% PAPR. Lowcost improvement of the agronomic value of PR can be achieved by partialacidulation, so this modification of the phosphate rock show promise forutilization of PR reserves indigenous to developing countries.     

Single superphosphate-reactive phosphate rock mixtures. 1. Factors affecting chemical composition

The effect of additon of reactive phosphate rock (RPR — North Carolina) on the degree of acidulation of unreactive phosphate rocks (PRs — Nauru and Christmas Island A) during the manufacture of single superphosphate (SSP) was examined using³²P in isotopic dilution studies. Acidulation of unreactive PR during SSP manufacture continued through denning, granulation and drying. Even after 3 hours drying, between 20 and 30% of the total P remained as free phosphoric acid in the reaction mixture. The addition of North Carolina phosphate rock (NCPR) to ex-den SSP reaction mixture (3:7 NCPR:SSP reaction mixture) preferentially consumed the free phosphoric acid remaining in the reaction mixture. This resulted in reduced acidulation of the unreactive PR in the reaction mixture and partial acidulation (10–23%) of the RPR. Hence the SSP-RPR mixture contains more residual, unreactive PR than is present in SSP.The extent of partial acidulation of the RPR when mixed with SSP was determined by the nature of free acid remaining in the SSP reaction mixture, which in turn is affected by the type of unreactive PR used for SSP manufacture. The free acid in the Christmas Island A reaction mixture contained approximately 8 and 12 times as much Fe and Al respectively as that in the Nauru reaction mixture, and was only half as effective at converting the P in RPR to soluble P. Unless made with extended denning times and carefully chosen PR, SSP-RPR mixtures can contain (a) undesirable amounts of unreactive PR residues, and (b) low quality partially acidulated RPR, both of which have low agronomic value.     

Effects of partial acidulation on chemical and mineralogical characteristics of residual phosphate rocks

Original phosphate rocks (PR) and water insoluble residues (WIR) from mixtures of reactive PRs and single superphosphate, known commercially as longlife single superphosphate (LLSSP), and from partially acidulated PRs (PAPR), were compared in terms of their elemental content, chemical reactivity as indicated by the apatite unit cell a dimension and solubility. Phosphate rock reactivity is known to be inversely related to the a dimension. Partial acidulation (20%) with commercial grade phosphoric acid resulted in an increase in aluminium (Al), iron (Fe) and fluoride (F) concentrations in the WIRs. The apatite a dimensions of WIRs from LLSSPs were greater than those of the respective original North Carolina (NC), Khouribga (KR), Jordan (JR), Sechura (SE) and Arad (AR) PRs added to single superphosphate (SSP), made from Nauru PR (NR)) to produce the LLSSPs. This was attributed to the presence of the less reactive NR in the WIRs left-over from the SSP. Partial acidulation with phosphoric acid increased the apatite a dimensions of NC and ElHassa (EH) PRs. The increase in apatite a dimension of NC and EH was probably due to selective dissolution of a more reactive fraction of the PRs during partial acidulation. Changes in the apatite a dimension following partial acidulation with phosphoric acid were not significant for the other PRs studied, e.g. Gafsa (GF), KR and AR, although differential X-ray diffractograms (DXRD) indicated that the material dissolved during partial acidulation was more reactive than the WIRs and the original PRs. The apatite a dimension of NC PR was not affected by pretreatment with 2% or 4% citric acid (CTA). The contrasting response in a of NC PR to acidulation with phosphoric and citric acids may be related to differences in the strength of these acids, and/or to the differing environments under which the reactions took place.The 2% CTA and formic acid (FMA) solubilities of the WIRs from LLSSPs and PAPRs were markedly lower than those of the original PRs. This reduction in solubility of PRs following partial acidulation was probably related to changes in mineralogical and chemical composition of the WIRs as indicated by the increases in apatite a dimension of some residual PRs and shifts in peak positions in DXRD, to increases in the concentrations of Fe, Al and F compounds, and to coating effects of PR particles by Fe, Al and F compounds. This, in turn, may reduce the agronomic value of the residual PR component of PAPR and LLSSP fertilizers, particularly over the short-term.The solubility of residual PRs following pretreatment with 2% or 4% CTA was slightly lower than that of the original PRs. The pretreatment caused no significant change in the apatite a dimension of NC PR. The complexing effects of CTA and its lack of Fe and Al impurities may have prevented the formation of Fe, Al and F compounds. The effect of citric acid on PR reactivity is thus quite different from that of the mineral acids used to prepare LLSSPs and PAPRs.     

Partial acidulation of an ‘unground’ phosphate rock: I. Preparation and characteristics

Partially acidulated phosphate rocks were prepared from unground North Carolina phsophate rock and H₃PO₄ by (i) mixing phosphate rock with the requisite amount of H₃PO₄, (ii) mixing with a portion of the acid followed by adding the remaining acid during granulation and (iii) single-step acidulation and granulation. The degrees of acidulation were 20, 30, 40 and 50%. Only 20% and 30% acidulations were done by method (iii). The phosphate rock granulated readily on addition of H₃PO₄ either as in method (ii) or (iii) and the products did not need external drying before storage. The citric and water soluble P showed that from the viewpoint of acid-phosphate rock interaction all three methods of preparation were satisfactory. The granules were equally strong as or stronger than commercially available single or triple superphosphate samples tested. The percent degradation on abrasion was less than 4% compared to about 8% for superphosphate and 0.4% for triple superphosphate. A sand incubation study suggested an interaction in the partially acidulated phosphate rock between the monocalcium phosphate component and unreacted phosphate rock which initially increased the solubility of P.