Residual phosphate fertilizer compounds in soils

A variety of P compounds can accumulate in soils as residues of fertilizer and may influence soil test versus plant yield relationships. This work evaluates specific chemical extractants for their capacity to identify such Al, Fe and Ca phosphates in soils as a basis for increasing the precision of yield prediction. Aluminium phosphate, iron phosphate, calcium phosphate (apatite) and P sorbed onto gibbsite, goethite and calcite were added to four Western Australian lateritic soils. These soils were then subjected to sequential selective extraction using a modified Chang and Jackson procedure in order to evaluate the selectivity of these extractants for the different forms of P with the sequence of extraction: 1 M NH₄Cl, 0.5 M NH₄F, 0.1 M NaOH + 1 M NaCl, citrate-dithionite-bicarbonate (CDB), 1 M NaOH and 1 M HCl. The results show that the procedure is not sufficiently specific and thus might be of little value for estimating the forms and amounts of residues of phosphate rock fertilizers in soils.     

Seasonal patterns of phosphate uptake in relation to comparisons between fertilizers

Pot experiments, and field trials with temperate pastures, were used to compare pelleted Citraphos (ground calcined Christmas Island C rock phosphate) with soluble phosphates. The relative effectiveness of Citraphos increased markedly with time during a growing season. This increase occurred with residual Citraphos, topdressed twelve months previously and more, as well as with freshly applied Citraphos. It was not observed when Citraphos was compared with residual superphosphate.The apparent improvement of Citraphos thus derived very largely from the seasonal pattern of phosphorus uptake associated with soluble phosphate sources. As the availability of the latter decreased progressively after the early part of the season, the effectiveness of the more constant Citraphos improved by comparison. There is considerable arbitrariness in values of relative effectiveness assigned to slow acting phosphate fertilizers on the basis of single measurements. Direct assessments of the residual values of phosphatic fertilizers, by comparison with freshly applied soluble phosphates, appear to suffer similar limitations.The sharply decreasing availability of soluble phosphates has implications for the production of improved, long lasting phosphatic fertilizers.     

Mobilization of residual phosphate of different phosphate fertilizers in relation to pH in the rhizosphere of ryegrass

Only 10 to 20% of the P in fertilizers are utilized by crops in the year of application. The value of the remaining 90% to 80% for succeeding crops is uncertain. This paper is aimed at assessing the residual value of several P-fertilizers such as superphosphate (Super-P), Thomasphosphate (Thomas-P), Rhenaniaphosphate (Rhenania-P) and Hyperphos (Hyper-P) a ground rock phosphate. These fertilizers had been applied annually for ten years to supply a total of 520 kg P ha^–1 to a silt loam soil derived from loess. Fertilizer P accumulation compared to the unfertilized plots was 520 kg ha^–1 for Hyper-P and 410 kg ha^–1 for Super-P (4.2 and 3.3 mmol P kg^–1 soil). The residual value of this P was assessed by both conventional soil test procedures and P-depletion at the soil-root interface by ryegrass (Lolium perenne L.) supplied with either NO₃-N, NH₄-N or no N. The different N sources changed soil pH in the rhizosphere and thereby the solubility of P present in this region. To measure P depletion at the soil-root interface, ryegrass seedlings were grown on a soil block covered with a nylon screen, mesh 30µm. A dense root mat developed simulating a plane root surface. After 10 days of growth the soil block was sliced into 0.2 mm layers parallel to the root mat. These soil samples were analysed for P (4N HCl) giving P concentration as a function of distance from the root surface. Phosphorus depletion at the root surface, in mmol kg^–1, was 1.7 for the No-P and 3.2 for the Super-P treatment. Thomas-P and Rhenania-P were in between while for Hyper-P the depletion was only 1.0. This gave a residual effect of 47% for Super-P and a negative 15% for Hyper-P. Acidification of the rhizosphere due to NH₄-N supply had no effect on the residual effect of Super-P but increased that of Hyper-P to +18%. NH₄-N increased the residual effect of Thomas-P from 16 to 28% and of Rhenania-P from 9 to 37%. The supply of NO₃-N increased the rhizosphere pH and decreased the residual effect of Super-P but increased slightly the residual effect of the other P-fertilizers as compared to No-N application.The pattern of these changes of P depletion in the rhizosphere was similar for the No-P and the Super-P treatments suggesting the presence of similar P compounds in both cases. In contrast, Hyper-P apparently remained unchanged in soil, as Ca-P, if not mobilized by acidification. These results are supported by the soil tests. Reasons for the relatively low residual effect even of Super-P (only 47%) are discussed. It is concluded that, because of the low rate of P release from soil, the 10 days growth period of the plants was not long enough to include all P that could potentially be mobilized.     

Soil testing for available phosphorus in soils where phosphate rock-based fertilizers are used

Traditional soil tests for phosphorus (P) were developed to arrive at fertilizer recommendations when water-soluble P fertilizers have been used. When slowly water-soluble fertilizers such as phosphate rock (PR) have been used, soil tests using acidic extractants overestimate bioavailability of P, whereas alkaline extractants underestimate it. Therefore, separate calibration curves are needed for soluble and PR-based fertilizers. There are two soil tests that show promise as suitable tests in soils fertilized with soluble as well as PR-based fertilizers. These are the iron oxide impregnated paper (P _i ) test and the ion exchange resin paper test. In both cases, the strips act as a sink for P mobilized in a soil solution, and P measured depends only on the concentration of P mobilized in the solution and not on the source of P or properties of the soil. Both tests somewhat simulate the sorption of P by plant roots without disturbing the chemical equilibrium, unlike other tests that extract P by the destructive dissolution of specific soil P compounds. In both cases, P measured from soils fertilized with PR-based fertilizers has shown very good correlation with plant response. Field calibration with crops under different pedological and agroecological regimes is needed for using these soil tests in developing fertilizer recommendations.     

Natural radioactivity in phosphate fertilizers

Radioactive materials, like uranium and radium, are normal constituents of the earth's crust. The radioactivity of phosphate rock is enhanced by geological processes. Exposure of workers and the public to radiation from phosphate rock and fertilizer is therefore not unlikely. The European Commission has issued a draft proposal for revision of the Basic Safety Standards for the protection of workers and the general public against the dangers of ionizing radiation. In this proposal the exposure to natural radiation sources is also regulated. The radioactivity present in some of the phosphate ores is such that the production and use of phosphate fertilizer should not be allowed without prior notification, and it is essential that a system of control be set up. The radiation dose to which workers in a fertilizer plant are subjected is not negligible, although depending on the inhalation of dust. Stocks of fertilizer form a serious radiation problem. At the retrieval of phosphate from the rock, gypsum is formed whose release causes environmental problems. The contamination of land and food with radioactivity is compared with current legislation in the Netherlands.     

Effects of organic manure on solubility and mobility of different phosphate fertilizers in two paddy soils

The solubility and mobility of three phosphate fertilizers [superphosphate (SP), Ca-Mg phosphate (Ca-Mg-P) and diammonium phosphate (DAP)] with or without combination of farmyard manure (FYM) and/or farmyard manure juice (FYMJ) in two paddy soils (Ultisol and Entisol) from the Zhejiang Province of China were determined. Incubation experiments were conducted under controlled greenhouse conditions in the Institute of Agricultural Chemistry, Bonn University, Germany. The results showed that water soluble P and Olsen P in both the application band and the adjacent zones (10 and 20 mm from the application band) was highest with DAP, and lowest with Ca-Mg-P. Both FYM and FYMJ increased water soluble P (100–300%) and Olsen P (80–300%) for the application band in both soils. Moreover, the combination of FYM significantly increased water soluble P and Olsen P for the adjacent zones (20 mm for the Ultisol and 10 mm for the Entisol), but such effects were not significant in combination with FYMJ. Consequently, the main effect of FYM is due to organic substances from decomposition rather than organic compounds in the FYMJ. Also, the combination of FYMJ with SP and DAP significantly increased the pH value in the application band. The pH in the adjacent zone (10 mm) was slightly increased by the combination of FYM with each of three P fertilizers. The solubility and mobility of phosphate in soils differed greatly among phosphate fertilizers. The combinations of organic manure with mineral phosphate fertilizers increased solubility and mobility of phosphate in both paddy soils, especially in the Ultisol.This paper is dedicated to Professor Xi Sun.     

Developing alternatives to phosphate fertilizers of high water solubility

Superphosphate applied to annual pastures growing on the acidic, sandy-surfaced soils of the coastal plain areas of south-western Australia is the major cause of eutrophication of waterbodies. Watersoluble phosphate, both from freshly applied and previously applied fertilizer, is leached from the profile from deep sandy soils or moved by lateral water flow on shallow sands. As part of an integrated catchment management program for the eutrophic waterbodies, an intensive research program was conducted to develop alternatives to highly water soluble phosphatic fertilizers which were argonomically and economically effective. Field, glasshouse and laboratory work identified several sources of phosphate with low initial water solubility which were at least as effective as superphosphate, and which could be formulated and commercially produced as a practical alternative to superphosphate. On the deep sandy soils, these slow release sources - the most useful of which was found to be sulphurfortified, partially-acidulated rock phosphates - had lower phosphorus leaching losses associated with their use than superphosphate on the basis of equal production of dry matter. The experimental work also investigated some of the factors controlling the agronomic effectiveness of these fertilizers, to enable identification of specific soils on which they are likely to be effective substitutes for superphosphate.     

Use of electroultrafiltration (EUF) method for investigating the behaviour of phosphate fertilizers in tropical soils

The solubility of superphosphate and rock phosphate was studied in six soils differing widely in pH, clay content, and type of clay minerals. Soils were incubated with increasing amounts of phosphate fertilizer for 1, 4, 12, and 24 weeks at 60% maximum water capacity and then extracted by means of electroultrafiltration (EUF). In the acid soils (pH-H₂ O: 4.3 to 5.6) the phosphate quantities extracted did not differ much between fertilizer types. In the neutral soils (pH-H₂ O: 6.6 to 6.9) 2 to 10 times as much was extracted from the superphosphate treatments as from the rock phosphate treatments. A fractionated EUF extraction (change of extraction time and voltage) showed that in the acid soils, rock phosphate was more soluble than superphosphate, but in neutral soils the opposite was true. In five of the six soils investigated the solubility of superphosphate declined during an incubation period of 24 weeks, whereas the solubility of rock phosphate showed no clear trend in relation to the time of incubation.

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Zusammenfassung Die Lölichkeit von Superphosphat und Rohphosphat wurde in 6 Böden untersucht, die sich beachtlich in ihrem pH-Wert, in ihrem Tongehalt und in ihren Tonmineraltypen unterschieden. Die Böden wurden mit steigenden Phosphatmengen für die Dauer von 1, 4, 12, und 24 Wochen bei 60% der maximalen Wasserkapazität inkubiert und dann mittels Elektroultrafiltration (EUF) extrahiert. In den sauren Böden (pH-H₂O): 4.3 bis 5.6) unterschieden sich die Phosphatmengen nicht wesentlich, die von den beiden Phosphatdüngemitteln extrahiert wurden. In den neutralen Böden (pH-H₂ O: 6.6 bis 6.9) wurde von den Superphosphatvarianten 2 bis 10 fach mehr Phosphat extrahiert als von den Rohphosphatvarianten. Eine fractionierte EUF-Extraktion (Änderung der Desorptionszeit und der Spannung) zeigte, daß in den sauren Böden das Rohphosphat besser löslich war als das Superphosphat; in den neutralen Böden wurde die umgekehrte Beziehung gefunden. In 5 von den 6 untersuchten Böden nahm die Löslichkeit des Superphosphates im Verlaufe der 24 wöchigen Inkubationsdauer ab. Die Löslichkeit des Rohphosphates zeigte keinen klaren Trend in Abhängigkeit von der Inkubationsdauer.

     

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.     

Ammonia volatilization from fertilizers applied to irrigated wheat soils

A series of experiments using flow chambers was undertaken in the field to investigate the effects of stubble and fertilizer management, soil moisture and precipitation on ammonia volatilization following nitrogen application on chromic luvisols. In the first factorial experiment, urea at 100 kg N ha^–1 was applied to the soil surface one, three and six days following irrigation; there were four rice stubble management systems comprising stubble burnt, stubble burnt then rotary hoed, stubble rotary hoed into the soil and stubble retained on the surface. Cultivation almost halved ammonia loss. The higher loss from uncultivated plots was ascribed to an alkaline ash bed on burnt plots, and to higher soil moisture and some retention of urea prills in the crop residue above the soil surface of the stubble retention plots. Average volatilization over a 12 day period following urea application from plots fertilizer one, three or six days after irrigation was 16, 15 and 4 kg N ha^–1, respectively. Daily application of up to 1.7 mm of water did not reduce volatilization and 35 kg N ha^–1 was lost within five days of fertilization. Daily precipitation of 6.8 mm reduced loss to 14 kg N ha^–1. This quantity of rain is uncommon in the region and it was concluded that showery conditions are unlikely to reduce volatilization. The third experiment demonstrated that the quantity of stubble on the soil surface had no effect on volatilization, and all plots lost 25% of applied nitrogen. In the fourth experiment, 100 kg N ha^–1 as urea or ammonium nitrate was either broadcast onto the surface or stubble retention plots, or placed, and partly covered to simulate topdressing with a disc implement. Partial burial of urea reduced ammonia volatilization from 36 to 7 kg N ha^–1, while partial burial of ammonium nitrate reduced loss from 4 to 0 kg N ha^–1.     

复合(混)肥中磷的形态及其在土壤中的作用

介绍磷酸盐的特性。提出在复合（混）肥生产中应合理配料,并控制好生产工艺,减少水溶磷在复合（混）肥中的退化。磷肥在不同类型土壤中的退化机理不同,大部分磷以金属磷酸盐和羟基磷灰石的形式储存在土壤中。磷在土壤中退化得很快,在一定的土壤和气候条件下,施用水溶性磷肥并非经济。在复合（混）肥生产中控制适当的水溶磷比例,既可满足作物生长的需要,又可减少土壤对磷的固定,提高施肥效益。     

缓释复合肥料制备工艺研究

研究了以改性淀粉与聚乙烯醇(PVA)为原料制备缓释复合肥料包膜料液的合成工艺,探索出其最佳配方:该料液w(固体)≈10%,m(交联剂):m(包膜料液)=6%,m(溶剂):m(包膜料液)=4%。用m(包膜料液):m(磷酸二铵)=6%包涂磷酸二铵制备的缓释复合肥料缓释性能良好,生产成本低廉,包膜材料可生物降解,环境污染小。     

Prediction of cadmium and zinc concentration in wheat grain from soils affected by the application of phosphate fertilizers varying in Cd concentration

Cadmium is an undesirable contaminant in phosphate fertilizer, and may represent a threat to food safety given its tendency to be taken up by plants and translocated into the edible parts. In this context, predicting wheat grain cadmium concentration from preliminary data would help to prevent exceeding the threshold values. Our study compared different approaches to estimate the concentrations of cadmium and zinc in wheat grains based on either soil solution chemistry of these elements, their quantities added to the soil and various soil parameters. Whereas the predictions based on soil solution chemistry show positive correlations between predicted and measured values of cadmium for some experimental sites, it was more difficult to predict grain cadmium concentrations in other sites. Reverse-wise, predictions based on applied cadmium and some soil parameters yielded systematically good correlations between predicted and measured values. The prediction of the concentration of zinc in wheat grains could not be achieved as its content was neither related to the measured soil solution chemistry nor to the quantities of zinc applied to the soil. We suspect that zinc interacts with the phosphate fertilizer thus obscuring the regressions for plant uptake. The prediction of grain cadmium in wheat cultures is best achieved through empirical modeling from soil parameters and soil inputs rather than through estimates of the bioavailable fractions in the soil solution.     

不同品种磷肥在土壤中的退化研究

对钙镁磷肥（FMP）、过磷酸钙（SSP）和部分酸化磷肥（PAPR）在河南郑州西岗沙土和河南西华县潮土中的退化情况进行了研究。     

Effects of different phosphate fertilizers on yield of barley and rape seed on reddish brown soils of the Ethiopian highlands

The effectiveness of six phosphorus sources at 4 rates were tested for two seasons on reddish brown soil at Holetta, Ethiopia, using barley (Hordeum vulgare L.) and rape (Brassica napus L.) as test crops. The fertilizer sources include: basic slag (BS), bone meal (BM), Ethiopian rock phosphate (ERP), Gafsa rock phosphate (GRP), triple superphosphate (TSP) and mixture of TSP and GRP in the ratio 1:4 (MIX). Yield, P uptake by both crops as well as available soil P showed a marked response to the application of the various P sources. On continuously cropped field, grain yield increase over the unfertilized plot ranged from 2.5 to 16.4 dt ha^–1 for barley and rape respectively. On newly cleared field no significant effects of the different P sources on barley were observed. On the other hand for rape, a grain yield increase over the unfertilized plot ranging from 10.6 to 17.8 dt ha^–1 was recorded. The highest agronomic effectiveness relative to TSP (RAE) for both crops was obtained with BS. Rape was found to utilize P not only from the reactive rock phosphate (RP) but also from the unreactive one, which had a total P content of only 3% and 0.4% ammonium citrate soluble P. Barley, on the contrary, could not utilize P from this magmatic rock phosphate and failed to grow.     

Evaluation of soil phosphate status where phosphate rock based fertilizers have been used

Soil phosphorus (P) tests have usually been calibrated using regression relationships between test values and crop yields for soils with a history of soluble P fertilizer use. However, the regression relationships have frequently been found to be different where phosphate rock (PR) based fertilizers have been used. Consequently, the traditional soil P tests often give incorrect estimates of soil P status of PR fertilized soils where calibrations were derived using soils treated with soluble P fertilizers. Alkaline soil tests (e.g., Olsen, Colwell) usually underestimate, while acid tests (e.g., Truog, Bray 2) usually overestimate, the soil P status of PR fertilized soils where normal calibrations are used. Several ways of overcoming this problem are discussed. Separate calibrations can be used for soluble and PR based fertilizers. In practice, this could involve mathematical modification of test values obtained with PR fertilized soils to enable use of the normal calibrations. Soil and fertilizer P models are available which use fertilizer history to derive current fertilizer recommendations and/or predict consequences of different fertilizer strategies. These could be extended to include functions describing the dissolution of PR in soil. This requires more detailed information on PR dissolution rates in different soils. Two soil tests for use with both soluble P and PR fertilized soils have recently been developed. They are the iron-oxide impregnated paper and the mixed anion exchange membrane/cation exchange membrane tests. While more evaluation is required in field situations, evidence to date indicates that both tests show promise.     

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.     

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 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.