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.     

Influence of pH,time and rate of application on phosphate rock dissolution and availability to pastures

Soil Samples were collected from a field experiment conducted to evaluate the agronomic effectiveness of a reactive phosphate rock (PR), Sechura sand, relative to that of monocalcium phosphate (MCP) at different soil pHs and rates of application. The samples were analysed for P soluble in the soil solution and bicarbonate extractable P. The rate of dissolution of PR was calculated from the data on the fractionation of inorganic P. In MCP plots P in the soil solution decreased sharply with time especially at low pHs and high rates of fertiliser application. In PR plots the concentration remained with time at the same as or a slightly higher level than that was found one month after application. Solution concentration of P was lower at very high rates of PR application than at intermediate rates. In both MCP and PR plots bicarbonate extractable P decreased with increasing pH. Bicarbonate extractable P was linearly related to MCP but not to PR applied. The rate of dissolution and the proportion of PR dissolved decreased with increasing rates of PR application but the amount dissolved increased. Phosphate dissolved at high level of PR application did not seem to enhance proportionately either the concentration of P in soil solution or bicarbonate extractable P.     

利用磷矿石干法制磷(复)肥的研究

本文介绍了磷矿石的资源概况、结构性质,简要慨述了我国磷矿直接开发的工艺,详细地讨论和分析了干法磷复肥的研究工艺,探讨了干法中存在的问题,对今后的研究方向提出了建议.     

中低品位磷矿浮选尾矿制备磷镁肥的实验研究

研究开发一种以中低品位磷矿浮选尾矿为原料,采用硫磷混酸直接分解浮选尾矿制备磷镁肥的方法。阐述了浮选尾矿的矿物特性,硫磷混酸分解浮选尾矿制备磷镁肥的基本原理、工艺条件、产品质量,论证了中低品位磷矿浮选尾矿制备磷镁肥的可行性。     

Characterizing the dissolution of phosphate rock materials by electro-ultrafiltration

Electro-ultrafiltration (EUF) was evaluated as a potential technique for characterizing the dissolution and assessing the chemical reactivity of contrasting phosphate rock (PR) materials. The types of rock used were: three reactive rocks, Sechura phosphate rock (SPR), North Carolina phosphate rock (NCPR), and Chatham Rise phosphorite (CRP), which contains significant amounts of calcium carbonate; one unreactive rock, Tennessee phosphate rock (TPR); and one iron and aluminium phosphate, calcined Christmas Island C grade phosphate rock (Calciphos). Dissolution of SPR increased as the solution:solid ratio increased to 250:1, the voltage was increased from 0 to 400 V, and the ionic strength of the extracting solution was increased. The neutralizing effect of any CaCO₃ present in PR materials, which is a major limitation with single chemical extraction procedures, does not appear to be a problem with EUF. A limitation of using de-ionized water as the extracting solution with EUF is the small amounts (1 to 6%) of total of P extracted. Addition of NaCl to the extracting solution increased the dissolution of all PR materials, although this varied with the PR. With both de-ionized water and NaCl as the extracting solution, EUF was inferior to 2% formic acid for assessing agronomic effectiveness of the PR materials. EUF appears to be of limited value in assessing the chemical reactivity of PR materials.     

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.     

A comparison of sequential extraction procedures for measuring phosphate rock residues in soils

Laboratory studies have been undertaken to evaluate sequential extraction procedures to measure phosphate rock (PR) residues in a range of temperate (New Zealand) and tropical (Indonesian) soils. The amounts of soil and PR-P were measured by using a sequential procedure which involved pre-extraction with either 1M NaCl or 0.5M NaCl/TEA (pH 7), followed by 1M NaOH and mineral acids. The acids included varying concentrations of HCl or H₂SO₄ (0.5–4M) and a tri-acid mixture of HCl, HNO₃ and HClO₄ (5:5:7). Amounts of PR residues in the soil were estimated from increases in amounts of the acid-extractable P (acid-P) or Ca (acid-Ca) in the PR-treated over untreated soil.Amounts of PR-P residues (measured by HCl-P) in New Zealand soils varying in pH and Ca levels were completely recovered (±5%) when the soils were pre-extracted with 0.5M NaCl/TEA, rather than 1M NaCl (or no pre-extraction). Pre-extraction of acid soil/PR mixtures with 1M NaCl caused dissolution of PR during the extraction. Although the HCl-Ca also estimated the amount of PR residues well, particularly after a 0.5M NaCl/TEA pre-extraction, this method was less sensitive than the HCl-P method, particularly in the soils with high native Ca saturation.In contrast to the high recovery obtained from the New Zealand soils, the recovery of the PR-P residues in the Indonesian soils in the HCl extracts following the 0.5M NaCl/TEA and 1M NaOH extractions were low ranging from 62 to 86%. Although increasing the concentration of HCl (upto 4M) did not cause any improvement in the recovery of PR-P, extraction with H₂SO₄ achieved complete recovery of PR-P. For all soils, a sequential extraction involving 0.5M NaCl/TEA-1M NaOH-0.5M H₂SO₄ is recommended for measuring PR residues in pairs of PR-treated and untreated field or laboratory incubated soils.     

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.     

The role of soil pH in the dissolution of phosphate rock fertilizers

The influence of soil pH on the dissolution of phosphate rock fertilizers was investigated in laboratory experiments with reactive North Carolina phosphate rock (PR) in a lateritic soil adjusted to several pH values. Increased soil pH resulted in decreased dissolution as estimated by the increase in exchangeable calcium (Ca) method. The extent of PR dissolution was related to soil pH by an equation of the form Log Ca = a–b pH, and it increased with contact period and rate of PR application. Increased plant available P, as estimated by NaHCO₃ soluble-P (BicP) was about one third of the P dissolved from PR. BicP was related to soil pH by an equation of the form Log Bic P = c–d pH. Dissolution of PR in soil can be considered as a simple chemical reaction between apatite and hydrogen ions supplied by soil constituents.     

Phosphorus availability from phosphate rock and sewage sludge as influenced by the addition of water soluble phosphate fertilizer

Phosphorus (P) inputs are required for sustainable agricultural production in most acid soils of the tropics and subtropics. Phosphate rocks (PR) and organic materials have been suggested as alternative P sources in these soils. Quantitative information on the P availability from sewage sludge (SL) is scanty. Methods to improve the effectiveness of PR such as partial acidulation and compaction with water-soluble P sources have been recommended. The objective of this greenhouse study was to evaluate the relative agronomic effectiveness (RAE) of Florida PR and sewage sludges (irradiated and non-irradiated) applied alone and in mixture with a water-soluble source (triple superphosphate, TSP) at two rates (50 and 150 mg P kg^–1 soil). The ³²P isotope dilution technique was utilised to determine the proportion of P in the plant taken up from the P fertilizer treatments. Wheat was grown on an acid loamy sand Dystric Eutrocrepts and harvested 6 weeks after planting. Results on total P uptake and the RAE of the P fertilizer sources tested indicated that the addition of 50 mg P kg^–1 soil as TSP was adequate in supplying P to the 6-week-old wheat plants as compared to PR and sewage sludge. Intermediate values were obtained for the mixtures. Similar responses were observed for the high P rate. For a given P rate, phosphorus uptake from PR and SL in presence of TSP was higher than P uptake from these sources alone, indicating an enhancement effect of TSP on the effectiveness of these non-readily available sources. With respect to P uptake from PR applied alone, the relative increases in P uptake from PR due to TSP influence were 52 and 67% for the low and high P rates, respectively. The relative increases in P uptake from SL due to TSP when compared to P uptake from SL alone were 102 and 59% for the low and high P rates of application. Application of a water-soluble P fertilizer together with a non-readily available P source shows an enhancement on the P uptake from the non-readily available P source by the wheat plants. In this experiment the estimated enhancement effects are very likely underestimated.     

超微细磷矿粉的特性及其肥效机理研究

采用室内物理化学分析与生物试验相结合的方法研究了超微细磷矿粉的理化性质及其肥效,主要研究结果如下:a.超微细磷矿粉的粒径分别为7.26 μm、4.06 μm;b.超微细磷矿粉能明显促进玉米幼苗生长及提高植株的磷利用率,与普通磷矿粉相比达到显著水平,生物有效性显著提高c.超微细磷矿粉与普通磷矿粉相比显著增加了土壤中有效磷含量.     

酸性水稻土施用磷矿粉的农学有效性和经济效应的评价

4个酸性水稻土施用不同磷源的试验结果显示 :(1)水稻土施用磷肥的增产效应 ,油菜比水稻大得多 :发育第四纪红色黏土红壤稻田 (有效磷含量 6 .3mg/ kg)施用磷肥增加油菜籽 13.7～ 2 2 .2倍 ,而发育于河流冲积物和红砂岩的水稻土 (有效磷含量 10 .8mg/ kg)施用磷肥增加油菜籽 2 .6 8～ 3.6 3倍 ;发育第四纪红色黏土和花岗岩的红壤稻田施用磷肥增加稻谷产量分别为 1.4 %～ 3.8%和 3.5 %～ 7.3% ;(2 )红壤水稻土施用 Gafsa磷矿粉农学相对有效性 (86 .2 %～ 10 4 .7% )与过磷酸钙相近 ,而大大高于昆阳磷矿粉 (47.2 %～ 6 4 .5 % ) ;(3)油菜施用磷肥增加的产值 (2 839～ 5 30 7元 / hm2 )显著高于水稻施用磷肥增加的产值 (2 2 4～ 74 2元 / hm2 ) ;(4)油菜和水稻施用磷肥的产投比昆阳磷矿粉 >Gafsa磷矿粉 >过磷酸钙 ;(5 )在施磷矿粉的基础上 ,施用硫肥可使油菜籽产量提高 8.4 %～ 19.5 % ,而稻谷产量仅提高 1.6 %。     

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.     

The influence of seasonal conditions,plant species and fertilizer type on the prediction of plant yield using the Colwell bicarbonate soil test for phosphate

Testing for soil phosphate (P) using the Colwell procedure is widely used in south-western Australia to estimate fertilizer applications required for crops and pastures. The relationship between plant yield, expressed as a percentage of the maximum yield, and soil test values is assumed to be constant in different years for the same soil type and plant species. Data from 11 long-term field experiments in south-western Australia show that regardless of whether percentage of maximum or absolute yield is used, the relationship between yield and soil test values is different (1)in different years, for the same site and where the same P fertilizer type has been used. This occurred irrespective of whether the same or different plant species were grown in different years; (2)where different types of P fertilizer had been used, for the same site, same year and same plant species; (3)for different plant species, for the same site, same year, and same type of P fertilizer. We conclude that considerable errors in the recommendation of fertilizer rates may result from the assumption that there is a constant relationship between soil test and yield.     

Application of the Kirk and Nye phosphate rock dissolution model

An incubation experiment was carried out under glasshouse conditions to determine the ability of the Kirk and Nye model to predict the dissolution of a reactive phosphate rock in an acidic soil. The model accurately predicted the extent of the dissolution over time. However, the predictions were found to be highly sensitive to soil pH, pH buffer capacity and soil moisture content. The procedures used to measure these parameters need to be closely examined when using the model. Soil pH buffer capacity should be defined over a restricted pH range and the soil moisture content should be closely monitored over time.     

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。     

Effects of plant residue quality, application rate, and placement method on phosphorus availability from Sokoto rock phosphate

Plant residues are being suggested as an amendment to enhance P release from rock phosphate, however, plant residue enhanced P release could depend on the residue quality, application rate and placement method. Effects of plant residue quality, application rate and placement method on solubility and P release from rock phosphate (PR) were studied in laboratory and field experiments. Leaves of ten woody and␣herbaceous species were incubated in a P-deficient soil with Sokoto PR under laboratory conditions to study the effects of residue quality on P release from PR. Effects of residue application rate and placement method were investigated in a field trial with five rates (0, 1, 2, 4 and 8 t DM ha⁻¹) of leaves of Dactyladenia barteri, two placement methods (incorporation and mulching), and two levels of Sokoto PR (0 and 60 kg P ha⁻¹) in a split-split plot design replicated three times. The plant residues were applied 4 weeks before the main season planting. Maize (main season) followed by cowpea (minor season) was used as test plant. In the laboratory incubation study, addition of plant residues increased the soil pH. Phosphorus released at 8 weeks was greater for plant residues with high C/N ratio, and low magnesium and potassium. In the field trial, soil pH was not affected by the addition of plant residues. Residue placement method showed little effects on P availability from PR. When residues were incorporated with PR, soil Olsen P was highest at lower rates of residues (1 and 2 t DM ha⁻¹) at maize planting. Maize P concentration and P uptake were highest at the incorporation of 4 t DM ha⁻¹ residues with PR. Incorporation at higher rate (8 t DM ha⁻¹) resulted in the greatest P uptake of the second crop, cowpea. The study shows the potential of plant residues in enhancing P release from PR. However, there could be initial immobilization of P, but this could be overcome within a short period if residues of high C/N ratio are used. A lower rate of residues (1–4 t DM ha⁻¹) is efficient at releasing PR–P for short-term effect especially if incorporated.     

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.     

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.     

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.