Phosphorus availability from partial acidulation of two phosphate rocks

Two phosphate rocks, one from Pesca (Colombia) and the other from Togo, were acidulated to various degrees with H₂SO₄ and H₃PO₄ for evaluation in varying granule size ranges. Products acidulated with H₂SO₄ were also prepared using different drying temperatures. Phosphorus availability was measured by dry-matter yield and P uptake in greenhouse experiments with maize.It was observed that partial acidulation with H₂SO₄ was effective in increasing the water-soluble P level of phosphate rock when the drying temperature of the product was not excessive. Crop response and P uptake were both highly correlated to the water solubility of the product. The relative agronomic effectiveness (RAE) of Togo rock increased from 3% when unacidulated to 33%, 47%, and 52% when 20%, 30%, and 40%, respectively, of the H₂SO₄ required to make SSP was added. Similar results were obtained with Pesca rock. No consistent effect due to granule size was observed.Twenty percent acidulation of Pesca rock with H₃PO₄ was 53–76% as effective as TSP with a single crop and 79–90% as effective over three cropping periods, showing a potential for high residual value.     

Availability of banded triple superphosphate with urea and phosphorus use efficiency by corn

Phosphorus fixation results in low P use efficiency in acid soils. Increase in soil pH through urea hydrolysis may improve P availability and use efficiency. Growth chamber and field experiments were conducted to evaluate effects of urea on triple superphosphate (TSP) transformation and P use efficiency. A Ste. Rosalie clay (Typic Humaquept), an Ormstown silty clay loam (Typic Humaquept) and a Chicot sandy clay loam (Typic Hapludalf) were used in the growth chamber experiment with three rates of N (0, 200 and 400 mg N kg-1), two N sources, either urea or NH4 NO3, based on 87 mg P kg-1 soil. In the field, three rates of urea (0, 60 and 120 kg N ha-1) and two rates of TSP (26 and 52 kg P ha-1) were compared on a Ste. Rosalie clay and an Ormstown silty clay loam. Compacted or blended mixtures of urea-TSP with different ratios of N:P were used in the field experiment. In the growth chamber experiment, soil pH and dissolved organic carbon (DOC) concentration was increased by added urea, and Mehlich (3) and water extractable P thus increased with increased urea. Soil pH, DOC and available P levels were not significantly affected by added NH4 NO3. Phosphorus uptake increased with added N, either urea or NH4 NO3, but P concentration increased only with addition of urea. In the field, soil Mehlich (3)-P at day 20, P uptake and use efficiency, corn yields increased when urea was applied with TSP. Compacted mixtures of urea-TSP increased P uptake and use efficiency, corn yields in comparison with blended mixtures. The beneficial effects of banded urea with TSP on P availability and P use efficiency were primarily attributed to urea hydrolysis, subsequent pH increase and organic matter dissolution as well as synergistic effect of N and P. These results indicate that compaction of urea plus TSP may offer a significant advantage over blended mixtures.     

Interaction of urea with triple superphosphate in a simulated fertilizer band

Fertilizer nutrient diffusion from fertilizer bands and transformations in soil can affect fertilizer nutrient availability to crops and knowledge of the transformations is necessary for proper management. The interaction of urea and triple superphosphate (TSP) on urea hydrolysis and P transformations during diffusion processes from a fertilizer band was evaluated in a laboratory incubation experiment with two eastern Canadian soils (Ste Rosalie clay, Modifiers Typic Humaquept, pH 5.0; Ormstown silty clay loam, Modifiers Typic Humaquept, pH 6.0). Two fertilizer sources (urea and TSP) and three N and P rates (0, 100 and 200 kg ha^–1) were combined in a factorial arrangement. Fertilizer combinations were placed on segmented soil columns, incubated and segments were analyzed for N and P content. Acidification from dissolution of TSP retarded urea hydrolysis, and curtailed the rise in soil pH surrounding the fertilizer band. Urea hydrolysis caused dissolution of organic matter in soils, which might inhibit precipitation of insoluble phosphates. Banding urea with TSP increased 1M KCl extractable soil P, soil solution P, sorbed P concentration and total P diffused away from the band. Urea decreased 0.01M CaCl₂ extractable P, indicating probable precipitation of calcium phosphates with CaCl₂ extraction. Banding urea with TSP could benefit P diffusion to plant roots in low Ca soils and increase fertilizer P availability.     

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.     

Induced phosphorus fixation and the effectiveness of phosphate fertilizers derived from Sukulu Hills phosphate rock

A greenhouse study was conducted with two surface, acidic soils (a Hiwassee loam and a Marvyn loamy sand) to measure the effect of increasing P-fixation capacity, on the relative agronomic effectiveness (RAE) of phosphate fertilizers derived from Sukulu Hills phosphate rock (PR) from Uganda. Prior to fertilizer application, Fe-gel was added to increase P-fixation capacity from 4.4 to 14.3% for the Marvyn soil and from 37.0 to 61.5% for the Hiwassee soil. Phosphate materials included compacted Sukulu Hills concentrate PR + Triple superphosphate (CTSP) at a total P ratio of PR:TSP = 50:50; 50% partially acidulated PR (CPAPR) from Sukulu Hills concentrate PR made with H₂SO₄; and Sukulu Hills concentrate PR (PRC) made by magnetically removing iron oxide from raw PR ore. Triple superphosphate (TSP) was used as a reference fertilizer. After adjusting soil pH to approximately 6, P sources were applied at rates of 0, 50, 150, and 300 mg total P kg^–1 soil. Two successive crops of 5 week old corn seedlings (Zea mays L.) were grown. The results show that the RAE of the phosphate materials measured using dry-matter yield or P uptake generally decreased as P-fixation capacity was increased for both soils. CTSP was more effective in increasing dry-matter yield and P uptake than CPAPR. PRC alone was an ineffective P source. Soil chemical analysis showed that Bray 1 and Mehlich 1 extractants were ineffective on the high P-fixation capacity Fe-gel amended Hiwassee soil. Mehlich 1 was unsuitable for soils treated with PRC since it apparently solubilizes unreactive PR. When all of the soils and P sources were considered together, Pi paper was the most reliable test for estimating plant available P.     

Comparison of the agronomic effectiveness of a phosphate rock and triple superphosphate as phosphate fertilisers for tea (Camellia sinensis L.) on a strongly acidic Ultisol

Phosphorus deficiency is a major problem affecting tea production in the highly weathered acid soils of humid and sub-humid tropics which are known to have high P fixing capacities. As many of these soils are strongly acidic and receive high rainfall, low-cost phosphate rock (PR) may effectively supply the plant P needs and limited preliminary experiments suggest this is so. A long-term glasshouse trial was conducted on 8-month old tea seedlings to compare the agronomic effectiveness of a locally available PR (Eppawala phosphate rock, EPR) with triple superphosphate (TSP) applied to a strongly acidic (pH water 4.55) marginally P deficient Rhodustult from Sri Lanka at six rates ranging from 10 to 60 kg P ha⁻¹. The results showed that TSP or EPR fertiliser at a rate as low as 10 or 20 kg P ha⁻¹ was sufficient to obtain maximun tea yield. The agronomic effectiveness of EPR was equal to that of TSP at the 5- and 10-month samplings. The concentration of soil P extracted by a cation–anion exchange resin membrane (resin-P) was higher in the TSP treated soil at 5 months due to its greater solubility but at 10 months, the EPR produced higher resin-P due to its increased dissolution over time. In the presence of tea plants, 52% of P from the EPR applied at the rate of 10 kg ha⁻¹, was dissolved at 5 months compared to 75% of dissolution at the 10-month sampling. In the absence of plants, the corresponding dissolution figures were 40% at 5 months and 55% at 10 months. The concentration of inorganic P extracted by 0.1 M NaOH (NaOH-P_i) (loosely characterising Fe + Al bound P) was significantly higher in the TSP treated soil and concentration of P extracted by 0.5 M H₂SO₄ (Ca bound P) was higher in the EPR treated soil. The results suggest that the low-cost, locally available EPR may be used profitably as a maintenace P fertiliser for tea plantations in moderately P deficient soils, which need to be confirmed by field studies. This revised version was published online in June 2006 with corrections to the Cover Date.     

Agronomic evaluation of fertilizers with special reference to natural and modified phosphate rock

Most of the phosphate fertilizes currently used worldwide (16 million tonnes of P) are water soluble. Direct application of phosphate rock accounts for just 3.5%. Since most phosphate rock is used to produce water-soluble phosphate fertilizer, about 150 million tonnes of byproduct phosphogypsum are produced annually on a worldwide basis. This has given rise to concerns about its management and disposal in an environmentally acceptable manner. Despite the small amount of phosphate rock presently used for direct application, its use is becoming increasingly common in the developing countries, especially Latin America, due to the high cost of water-soluble phosphate fertilizers and to the variety of soils, crops and climatic conditions of the region. Agronomic effectiveness of phosphate rocks has been measured by the concept of Relative Agronomic Effectiveness where the yields are compared to those obtained with Triple Superphosphate (TSP) or Diammonium Phosphate (DAP). Phosphorus uptake as a response of the phosphate rock applied is measure by tissue analysis. This methodology is already standard for the member countries of the Latin America Phosphate Rock Network.     

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.     

Dissolution of phosphate rocks in soils. 2. Effect of pH on the dissolution and plant availability of phosphate rock in soil with pH dependent charge

The effect of soil pH on the dissolution of phosphate rocks (PRs) and the subsequent availability of the dissolved inorganic phosphorus (Pi) to plants was examined in a volcanic soil adjusted to different pH values. Potassium dihydrogen orthophosphate (KH₂PO₄) and three PRs, Nauru (NPR), Jordan (JPR) and North Carolina (NCPR) were incubated with the pH-amended soils at a rate of 800µg P g^–1 soil for 84 days. The extent of PR dissolution was determined by measuring the increases in the amount of 0.5 M NaOH extractable Pi (NaOH-P) in the PR treated soil over the control soil. The amount of plant available P was measured either by extracting with 0.5 M NaHCO₃ or by growing ryegrass in soil samples incubated with the phosphate sources.At each pH the order of the extent of PR dissolution followed NCPR > JPR > NPR, which was consistent with the decreasing order of their chemical reactivities. As the pH decreased from 6.5 to 3.9 the dissolution of PRs increased from 29.3% to 83.5%, from 18.2% to 78.9%, and from 12.5% to 60.3% for NCPR, JPR and NPR, respectively. In contrast, as the soil pH decreased from 6.5 to 3.9, the proportion of the dissolved P extracted by 0.5 M NAHCO₃ decreased from 38% to 5% and the proportion taken up by ryegrass plants decreased from 46% to 7%. This decrease in plant available P corresponded to an increase in the adsorption of inorganic P with a decrease in pH. However, the uptake of P from PR relative to that from KH₂PO₄ was higher at low pH than at high pH. Further, the amount of P taken up by plants was more closely related to the amount of NaHCO₃ extractable P than to the amount of dissolved P present in the soil.     

Effectiveness of different phosphatic fertilizers measured using labelled superphosphate and phosphorus taken up by plants

The agronomic effectiveness of three P fertilizers (diamonium phosphate, rock phosphate and compost) was studied in a greenhouse experiment using wheat. A radioisotopic method, using triple superphosphate labelled with³²P, was used to evaluate the P in dried tops that was derived from i) the soil, ii) labelled superphosphate and iii) the fertilizer being studied.The ratio between P uptake from each fertilizer and P uptake from the soil was used to compare the effectiveness of the different fertilizers. P derived from diammonium phosphate was greater than P derived from the soil, except in one soil. P derived from rock phosphate was always lower than P derived from the soil. The effectiveness of compost depended on soil type. Compost can produce two kind of effects: i) a direct P contribution and ii) an indirect effect improving P uptake from the soil. The radioisotopic method can be used to study the effectiveness of fertilizers even when there are no differences in yield.     

The comparative agronomic effectiveness of rock phosphate and superphosphate for banana

The agronomic effectiveness of three rock phosphates (Idaho, Florida and North Carolina) as influenced by mycorrhizal inoculation withGlomus aggregatum was evaluated using small banana (Musa paradisiaca L.) corms as planting material. The treatments included superphosphate and a no-P control. The soil was fumigated to eliminate mycorrhizal propagules. The amount of P added was based on the quantity of material needed as superphosphate to establish 0.2 mg P L^–1 in solution. Plants were grown in an Oxisol in 9-liter pots for 3 months after growth commenced. Plant dry weight, P percentage in the 3rd leaf, and total P uptake were increased when plants fertilized with insoluble rock phosphates were inoculated with mycorrhiza-producing fungi. Phosphorus uptake by plants fertilized with Idaho, Florida, and North Carolina rock phosphates was 0.18, 0.42, and 0.97 as much as by plants fertilized with superphosphate. The beneficial effect of mycorrhiza on phosphate uptake was 136, 30, 2 and 24% for plants fertilized with Idaho, Florida and North Carolina rock phosphate, and superphosphate, respectively.     

The determination of phosphate availability for some less conventional phosphate fertilisers

More than in the case of the other major fertiliser elements the performance of phosphorus in fertilisers in dependent on the state of chemical combination of the element. As distinct from typical American practice where fully acidulated fertilisers are emphasised, some other countries have moved in part to production of materials of less than full acidulation where some reliance is placed on the reactivity of the original phosphate rock. Solutions of various aliphatic acids and their salts have been used as tests for the availability of phosphorus in fully acidulated fertilisers and, to a degree, for phosphate rocks themselves. Less than fully acidulated products are at present assessed in the same way as fully acidulated materials, but in this and other evaluations problems and anomalies arise which are discussed within the framework of a number of fertiliser systems. Analytical data are adduced which illustrate the effects of some extraneous cations on the results achieved which conflict with some previous opinions. Research towards a fuller understanding of the subject is suggested.     

Greenhouse evaluation of the effect of topsoil moisture and simulated rainfall on the volatilization of nitrogen from surfaceapplied urea,diammonium phosphate,and potassium nitrate

The effect of topsoil moisture content at the time of nitrogen fertilization and distribution of precipitation following N fertilization on volatile loss of surfaceapplied fertilizer N was studied in two greenhouse experiments using¹⁵N-labeled fertilizers. Loss of applied NO ₃ ^- -N was small compared with loss of urea-N and diammonium phosphate (DAP)-N; this suggests that NH₃ volatilization was the major pathway of N loss for urea and DAP. Loss of applied NO ₃ ^- -N averaged less than 6% of that applied regardless of initial topsoil moisture or amount of precipitation. Increased initial topsoil moisture content increased losses of urea-N greatly but losses of DAP-N only slightly. Increasing depths of precipitation, added five days after N fertilization, greatly decreased loss of urea-N but had no effect on the loss of DAP-N. Variations in moisture and precipitation treatments caused losses of urea-N to vary from 40 to 6% of that applied in a slightly acidic silty loam and from 26 to 11% in a calcareous clay. Moisture and precipitation treatments caused volatilization of DAP-N to vary from 20 to 10% in the silty loam and from 40 to 27% in the calcareous clay. In a second experiment, moisture and precipitation conditions affected N loss from urea as in the previous experiment. Addition of phenylphosphorodiamidate (PPDA), a known urease inhibitor, to urea at 20 g kg^–1 resulted in only a small reduction of N loss in the calcareous clay soil used.It was concluded that soil moisture at the time of N fertilization and precipitation following N fertilization can greatly affect volatile loss of fertilizer N. Since the effect of moisture on N loss is not the same for all N sources, moisture parameters are expected to affect the ranking of N sources by their susceptibility to N loss and their uptake by plants in field experiments. Results obtained suggest some management practices by which fertilizer N might be conserved. The great effect of moisture and precipitation on N loss in these studies underscores the need for detailed meteorological records for field sites of N trials.     

Assessment of the relative agronomic effectiveness of phosphate rocks under glasshouse conditions

Six phosphate rocks (PRs) of varying reactivities were compared with monocalcium phosphate (MCP) in a glasshouse experiment growing perennial ryegrass (Lolium perenne cv. Nui) as the test plant on four soils of contrasting P sorption capacity and exchangeable Ca. The cumulative dry matter yield over 10 harvests showed a significant response to P application in all soils. Based on relative yield and P uptake, MCP was the most effective P fertilizer followed by the reactive phosphate rocks, which were superior to the unreactive rocks in all soils. The relative agronomic effectiveness (RAE) and substitution ratio (SR) of individual PR fertilizers, calculated with respect to MCP using the methods of vertical and horizontal comparison, respectively, were similar over a range of fertilizer rate. There was a decline or slight increase in the performance of PRs with time in the low P sorption soils but a consistent increase in the high P sorption soils. Some initial influence of exchangeable Ca content of the soils on the relative performance of PRs was also observed. Generally the PRs performed better in high P sorption soils than low P sorption soils and in low exchangeable Ca soils than high exchangeable Ca soils.     

Use of phosphate rocks in the tropics

The use of finely ground phosphate rocks (PRs) as directly applied P fertilizers in tropical farming systems is a cheaper alternative to acidulated, water-soluble P products. However, the effectiveness of PRs in tropical environments depends on the extent to which the required P uptake rate of the crop plant can be maintained by the rate of PR dissolution in that soil. That extent that this outcome is achieved depends on the properties of the PR, the soil, climate, plant factors, and on management practice. Environmental conditions in the surface layers of highly weathered soils in the humid tropics are generally conducive of the attainment of satisfactory rates of PR dissolution, especially as the reactivity of the PR increases. In soils with very high P sorption capacities, however, the agronomic effectiveness of PRs is reduced as the acquisition of dissolved P by plant roots is restricted by competition from P sorption processes in the soil.In determining the required reactivity of PRs for use in tropical regions one must consider the rate of P demand by the crop or pasture and the suitability of the soil environment for PR dissolution. The use of some water-soluble P in combination with the PR might enable PRs of low reactivity to also be used, where alone they would be relatively ineffective. The ability of PRs to provide Ca, in addition to P, needs further study because subsoil Ca deficiency is becoming more widely recognized as a production constraint in highly weathered tropical soils. The future use of PRs in tropical agriculture is expected to expand for plantation crops and pastures and especially for landlocked countries with local deposits of PR. Increased use of PRs will also occur where more reactive PRs can effectively be used to increase the yield of annual food crop.     

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.     

湿法磷酸制备磷酸脲的研究进展

磷酸脲是一种高浓度、高效氮磷复合水溶肥。基于湿法磷酸生产能耗低、环境污染小、成本低廉的优点,采用湿法磷酸制备磷酸脲近几年得到了国内学者的广泛关注。综述了磷酸脲的性质、农业行业与国家标准、制备原理与方法及近年来湿法磷酸制备磷酸脲的研究进展,展望了磷酸脲制备的发展方向,并指出磷酸脲合成及结晶过程中杂质离子传质现象及理论是实现并优化湿法磷酸制备磷酸脲的基础。     

The initial and residual value for subterranean clover of phosphorus from crandallite rock phosphates,apatite rock phosphates and superphosphate

The residual value of phosphorus from superphosphate, crandallite rock phosphate (Christmas Island C-grade ore), 500°C calcined crandallite rock phosphate (Calciphos) and apatite rock phosphate from Queensland, Australia, was measured in a 6 year field experiment sited on lateritic soil in south-western Australia. Different amounts of each fertilizer were applied at the commencement of the experiment, and either left on the soil surface or mixed through the soil by cultivating to a depth of about 10 cm. Dry matter production of subterranean clover measured in spring (August) and bicarbonate-extractable phosphorus determined from soil samples collected in summer (January–February) were used as indicators of fertilizer effectiveness.The effectiveness values calculated for each fertilizer each year were similar for the treatments that were left on the soil surface and those which were mixed through the soil. The effectiveness of both ordinary and triple superphosphate were similar each year. They were the most effective fertilizers for the duration of the experiment. Using pasture yield as an indicator, the effectiveness of the superphosphates decreased by about 50% from year 1 to year 2, and by a further 10% over the remaining 4 years. Using bicarbonate-extracted soil phosphorus the effectiveness of both superphosphates decreased in a more uniform fashion by about 60% from year 2 to year 6. The effectiveness of all the rock phosphate fertilizers was approximately constant through time. As calculated from yield and bicarbonate-soluble phosphorus values, C-grade ore, Calciphos and the Queensland apatite were respectively 5%, 20% and 7% as effective as freshly applied superphosphate.The proportion of the total phosphorus content present in the rock phosphates which was initially soluble in neutral ammonium citrate was a poor predictor of the effectiveness of the phosphorus from these fertilizers determined using herbage yield or the amount of bicarbonate — soluble phosphorus extracted from the soil.The bicarbonate soil test did not predict the same future production for superphosphate and some of the rock phosphates in years 2 and 3 of the experiment, indicating that different soil test calibration curves are needed for the different fertilizers.     

Phosphorus supplying power of some thermally promoted reaction products of phosphate rocks

The P availability in soil and agronomic efficiency of the products of non-premium grade, unreactive Purulia phosphate rock (PPR) heated alone or with Na₂CO₃ or KCl at different temperatures were investigated in two P deficient soils. The heated products of PPR alone did not improve the P availability in soil or P utilisation by rice over the original PPR. The products of PPR-KCl mixtures heated at 300-900°C were not effective at all. Out of several products of PPR with Na₂CO₃, the product prepared from PPR and Na₂CO₃ mixture in the weight ratio 2:1 heated at 900°C was comparable to superphosphate (SP) with respect to P availability in soil, straw and grain yield and P uptake by rice. The effectiveness of the products of PPR-Na₂CO₃ mixtures heated at 700°C though inferior to SP were superior to that of the original PPR in the highly acidic P deficient soil from Choudwar. However, products of another phosphate rock (PR) from Jordan and NA₂CO₃ mixtures heated at 900°C were less effective in comparison to SP. The amount of inherent silica present in Jordan PR was inadequate to promote the apatite-NA₂CO₃-SiO₂ reaction towards completion thus leading to an inferior product. On the other hand, similar products of non-premium grade Kasipatnam and Mussoorie PRs which are not suitable for direct application were comparable to SP in their effectiveness when these PRs were fused with Na₂CO₃ in the weight ratio 2:1 at 900°C. X-ray diffraction studies indicated presence of water and citrate soluble phosphate phases viz., Na₃PO₄, NaCaPO₄ and possibly Ca₇ (PO₄)₂ (S10₄)₂ in these products of PR-Na₂CO₃ mixture heated at 900°C. The water and citrate soluble phases of these products could release adequate P for absorption by crop.     

The residual value of superphosphate and rock phosphates for lateritic soils and its evaluation using three soil phosphate tests

The residual value of superphosphate and several rock phosphates was measured in three field experiments in Western Australia. The rock phosphates were Christmas Island C-grade ore, calcined C-grade ore (Calciphos) and apatite rock phosphates. The predictive capacity of the Colwell, Olsen and Bray 1 soil tests for phosphate were also evaluated.As measured by yields of variously wheat, oats, barley or clover, the effectiveness of an initial application of superphosphate decreased to about 50% of that of newly applied superphosphate between years 1 and 2, and further decreased to about 20% over subsequent years. At low levels of application, all the rock phosphates were between 10–20% as effective as superphosphate in the year of application for all experiments. Relative to newly applied superphosphate their effectiveness remained approximately constant in subsequent years for two experiments and doubled for the other experiment.The Colwell soil test predicted that the effectiveness of superphosphate decreased to about 45% between years 2 and 3, followed by a more gradual decrease to approximately 15%. At low levels of application, the effectiveness of the rock phosphates as predicted by the Colwell soil test values was initially very low relative to superphosphate (2–30%), and remained low in subsequent years (2–20%). For superphosphate treated soil, the proportion of the added phosphorus extracted generally increased as the level of application increased. By contrast, for rock phosphate treated soil, the proportion of added phosphorus extracted decreased as the level of application increased.For all three experiments there were highly significant positive correlations between amounts of P extracted by the three soil tests. Consequently all soil tests were equally predictive of yield but usually for each soil test separate calibrations between yield and soil test values were required for the different fertilizers and for each combination of fertilizer and plant species and for each year.