Phosphorus bioavailability of fertilizers: A predictive laboratory method for its evaluation

A laboratory method and a laboratory index is proposed to estimate the phosphorus taken up by plants that is derived from fertilizers (Pdff). Pdff values were measured using greenhouse experiments and³²P labelling technics. The laboratory index estimates the proportion of PO₄-ions derived from the fertilizer in the soil solution and is measured by means of an isotopic exchange of³²PO₄-ion procedure. This indicator was named JCF. Two typical soil-fertilizer conditions were studied. One concerned measurement of Pdff and JCF values for freshly-applied phosphorus as diammonium phosphate (DAP) at levels of 15, 30, 45, 60 and 90 mg P kg^–1 soil. The other concerned measures of Pdff and JCF values for two types of P residues previously applied in soils as concentrated superphosphate (CSP) or Gafsa rock phosphate (GRP) applied at 0 and 43.7 kg.ha^–1 each year over a 15 yr period.For freshly-applied DAP a linear relationship between Pdff and JCF values was obtained over the range of 0 to 90 mg P (kg soil)^–1 levels of application: JCF = 1.16 Pdff + 1.78, (r ² = 0.98). For the P residues, JCF and Pdff values were not significantly different for a given residual treatment. However JCF and Pdff pair data for CSP treatments (56.0, 65.9) were about tenfold superior to those for GRP treatments (5.3, 4.6)). Consequently the nearly 1:1 ratio between JCF and Pdff values that was obtained for the two different soil-fertilizer conditions suggests that the proposed laboratory method can be used to predict availability of P fertilizers to plants. Thus it deserves to be considered in helping to estimate P fertilizer applications.     

Effect of soil pH on the requirement for water-soluble phosphorus in triple superphosphate fertilizers

A greenhouse study was conducted to determine if soil pH affects the requirement for water-soluble P and the tolerance of water-insoluble impurities in TSP fertilizers. Two commercial TSP fertilizers were selected to represent a range in phosphate rock sources and impurities. Phosphate fertilizer impurities were isolated as the water-washed fraction by washing whole fertilizers with deionized water. TSP fertilizers with various quantities of water-soluble P (1.2 to 99% water-soluble P) were simulated by mixing the water-washed fertilizer fractions or dicalcium phosphate (DCP) with reagent-grade monocalcium phosphate (MCP). The fertilizers were applied to supply 40 mg AOAC available P kg^–1 to a Mountview silt loam (fine-silty, siliceous, thermic Typic Paleudults). Wheat (Triticum aestivum (L.)) was harvested at 49 and 84 days after planting. Soil pH values at the final forage harvest were 5.4±0.16 and 6.4±0.15. At a soil pH of 5.4, the TSP fertilizers required only 37% water-soluble P to reach maximum yields while at pH 6.4 the fertilizers required 63% water-soluble P. Results of this study show that higher levels of water -insoluble P can be tolerated in TSP fertilizers when applied to acid soils. Phosphorus uptake was not affected by soil pH, but for the mixtures containing the fertilizer residues the source having the lowest level of Fe and Al had a higher relative agronomic effectiveness.     

Field Evaluation of the Liming Value of Two Phosphate Rocks and their Partially Acidulated Products After 16 Years of Annual Application to Grazed Pasture

Measuring pH of soil samples (at four to five depths down to 300 mm) collected three times from a long-term (16 years) field trial involving annual application of six forms of phosphate fertilizers at the rate of 30 kg P ha⁻¹ yr⁻¹ showed that soil acidity in all treatments, including the untreated control, increased with time. The rates of acidification (pH unit yr⁻¹ during the first 10 years) in the topsoil (0–75 mm depth) were in the order, diammonium phosphate (0.038)>control, single superphosphate>Jordan partially acidulated phosphate rock (JPAPR)>North Carolina partially acidulated phosphate rock (NCPAPR), Jordan phosphate rock (JPR)>North Carolina phosphate rock (NCPR) (0.010). Of the 480 kg P ha⁻¹ applied over the 16 year period, 71 and 57% of P from NCPR and JPR dissolved. The theoretical liming values derived from the dissolution of NCPR and JPR were 1698 and 1303 kg CaCO₃ ha⁻¹ respectively. Liming values of the two PRs calculated from the increase in soil pH over control treatment (ΔpH) down to 300 mm soil depth were 640 and 414 kg CaCO₃ ha⁻¹ for NCPR and JPR, respectively. The lower liming values estimated from the ΔpH method is probably due to proton transfer resulting from the secondary reactions of dissolved fertilizer phosphate with soil constituents, the unaccounted liming effect of the PRs below 300 mm soil depth and the lower soil pH buffering capacities measured from a short-term pH titration method used in the estimation of the liming values. The results of this long-term field study showed that continuous use of certain phosphate rocks (PRs) can significantly slow down the rate of acidification in pastoral soils.     

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.     

Comparison of greenhouse and 32P isotopic laboratory methods for evaluating the agronomic effectiveness of natural and modified rock phosphates in some acid soils of Ghana

Phosphorus deficiency is one of the major constraints for normal plant growth and crop yields in the acid soils of Ghana and therefore addition of P inputs is required for sustainable crop production. This is often difficult, if not impossible for small-scale farmers due to the high cost of mineral P fertilizers and limited access to fertilizer supplies. Direct application of finely ground phosphate rocks (PRs) and their modified forms have been recommended as alternatives for P fertilization. The direct application of the natural and modified PRs to these acid soils implies the need to predict their agronomic effectiveness of the PRs in the simplest and most cost-effective manner. In this study the classical greenhouse pot experiment was compared to the ³²P isotopic kinetics laboratory method for evaluating the agronomic effectiveness of natural and modified Togo PR in six highly weathered Oxisols from southwest Ghana. In the ³²P isotopic kinetics laboratory experiment the six soil samples were each fertilised at the rate of 50 mg P kg^–1 soil in the form of triple superphosphate (TSP), Togo PAPR-50%, and Togo PR, respectively. Controls without P amendment were also included. Isotopic exchange kinetics experiments were carried out on two sets of samples, immediately after P fertilizer additions (without incubation) and after 6 weeks of incubation under wet conditions and at a room temperature of 25 °C. In the greenhouse pot experiment, P fertilizers in the form of Togo PR, Togo PAPR, Mali PR and TSP were each applied to the six soils at rates equivalent to 0, 30, 60, and 120 kg P ha^–1, respectively. The P fertilizers were mixed with the soils and maize (Zea mays L.) variety Obatanpa was grown for 42 days before harvest. The isotopic kinetics data of the control samples indicated that 5 of the studied soils had very low P fertility status as reflected by their low P concentrations in solution (C_P<0.02 mg P l^–1) and low exchangeable P (E₁min < 5 mg P kg^–1). The capacity factor and the fixation index of the soils were variable. Application of water-soluble P as TSP increased both the C_P and E₁ values of all the soils above the critical levels. Togo PR was least effective among the fertilizers tested for all soil soils, except in Boi soil. Acidulation of Togo PR (Togo PAPR-50%) was an effective means to increase its agronomic effectiveness. Direct application of natural Togo PR would be only feasible in the Boi soil series as reflected by its high Pdff% value in soil solution. Incubation with the P fertilizers caused an increase in the soil pH and a decline in the effectiveness of the applied P fertilizers, irrespective of the soil and the fertilizer utilized. Based upon the results of the greenhouse pot experiment, the relative crop response index (RCRI) in terms of increasing dry matter yield and P uptake followed the order of TSP > PAPR = Mali PR >Togo PR = Control. Both the laboratory index, Pdff% in soil solution derived from the isotopic method and the RCRI values obtained from the pot experiment produced similar results in ranking the P fertilizers tested according to their agronomic effectiveness. The isotopic kinetic method may be considered as an alternative to both greenhouse and field methods in the evaluation of agronomic effectiveness of P fertilizers in tropical acid soils when it offers comparative advantages in assessing the soil P status and its changes. But trained staff and adequate laboratory facilities are needed to perform this technique. Also the method can be used as a reference for comparison purposes as in this case. Further research is needed to assess the overall agronomic effectiveness (immediate and residual effects) of PR sources in predominant cropping systems of this region of Ghana.     

Effect of phosphate fertiliser type on the accumulation and plant availability of cadmium in grassland soils

Cadmium (Cd), a potentially toxic heavy metal for humans and animals, accumulates in the liver and kidneys of older animals grazing New Zealand and Australian pastoral soils. Phosphorus (P) fertiliser is the major input of Cd into these farming systems. A study was conducted to evaluate the effects, over 10 years, of annual application (30 kg P ha^–1 yr^–1) of four forms of P fertilisers having different solubilities and Cd contents [41, 32, 10 and 5 g Cd g^–1 for North Carolina phosphate rock (NCPR), single superphosphate (SSP), diammonium phosphate (DAP) made from low Cd phosphate rocks and Jordan phosphate rock (JPR) respectively] on soil and herbage Cd concentrations. Ten years of fertiliser application caused a marked increase in surface soil Cd concentrations. Total soil Cd was significantly higher in SSP and NCPR treatments compared to control (no P fertiliser), JPR and DAP treatments in the 0–30 and 30–75 mm soil depths. Plant-available Cd (0.01 M CaCl₂ extractable Cd) was higher in SSP treatments than in control and other fertiliser treatments. Chemical analysis of herbage samples showed that there was no significant difference in Cd concentration in pasture grasses between treatments in the second year of the trial but in the eighth and tenth year, plots fertilised with SSP and NCPR had significantly higher Cd in pasture grasses in most of the seasonal cuts compared to control, JPR and DAP. Cadmium recovery by both grasses and clover was less than 5% of Cd applied in fertiliser. Clover Cd concentration and yield were much lower than those for grass and therefore its contribution to pasture Cd uptake was very low (< 7%). A strong seasonal effect on grass Cd concentration, which is inversely related to pasture growth rate, was observed in all three sampling years — Cd concentration was highest during autumn and lowest in spring. Total Cd contents of the fertilisers and their rate of dissolution rather than soil pH [pH (H₂O) at 30–75 mm depth of 5.39, 5.20, 5.11 and 5.36 for NCPR, SSP, DAP and JPR treatments respectively]influenced soil and herbage Cd. These results showed that the use of P fertilisers with low Cd contents will reduce herbage Cd levels and has the potential of reducing Cd levels in grazing animals and their products.     

Solubilization of soil organic N by alkaline-hydrolysing N fertilizers

Interactions between¹⁵N-labelled fertilizers applied at concentrations representative of the fertilizer microsite and the solubility of the nitrogenous component of soil organic matter were investigated in laboratory experiments. Soil organic N was solubilized in a-irradiated soil due to addition of NH_3(aq), and the fertilizer-induced loss of unlabelled total N in the extracted soil (TU_s) increased with increasing N fertilizer concentration and soil pH. TU_s was linearly correlated with ammoniacal-N concentration and the pH of the fertilized soil within the range of 7.5-10 (r = 0.94).Total organic N in the soil extract (OT_e) increased rapidly up to day 14 following addition of 2000 mg urea-N kg^–1 soil, but was then stable up to day 28. OT_e of a range of soils increased from between 5 and 148 to between 15 and 368 mg N kg^–1 soil after application of 1045 mg NH₃-N kg^–1 soil. While up to 25% of the organic N was solubilized by the fertilizer in nine soils, the change in total organic N in the extracts (OT_e) of three soils was not significant. The highest OT_e of 399 mg N kg^–1 soil (35.4% of soil organic N) was measured after application of 2000 mg NH₃-N kg^–1 soil.pH and OT_e decreased in the order of NH_3(aq) > urea > di-ammonium phosphate > ammonium sulphate at equivalent rates of N addition. A negative OT_e was measured following application of ammonium sulphate. OT_e was correlated with the pH of the fertilized soil but not ammoniacal-N concentration for different N fertilizer sources.     

Comparison of P- availability from monocalcium and diammonium phosphates using a mechanistic nutrient uptake model

Calcium and ammonium phosphates are the most commonly used phosphate fertilizers. Since they differ in some chemical aspects it is important to compare their ability for supplying P to plant roots in different soils. The objective of this research was to compare the predicted effectiveness of monocalcium phosphate (MCP) and diammonium phosphate (DAP) for supply of P to maize in 13 soils. Phosphorus was applied at rates varying with soil from 50 to 400 mg kg^–1. Thirty days later P, Ca, pH, and Al were measured in the soil solution and in the solid phase. We calculated buffer power (b) and effective diffusion coefficient (D_e) for P, and used them, together with solution P (C_li), in the Barber-Cushman mechanistic nutrient model to predict P uptake. Monocalcium phosphate and DAP were similarly effective in supplying P to plant roots. Predicted P uptake differed between fertilizers in only three soils, and maximum differences between fertilizers in C_li or resin-exchangeable P (C_si) in any one soil were always less than 30%. The determinations most highly correlated with predicted P uptake were D_e (r = 0.93^**) and C_li (r = 0.60^*). Resin-exchangeable P was not significantly correlated with C_li, D_e, b or P uptake. Calcium, Al, and pH varied with source of P and soil: soils treated with DAP had lower extractable Al, lower Al in solution, and higher soil pH than soils where MCP was applied. Monocalcium phosphate increased extractable Ca whereas DAP did not affect it.Contribution of Purdue Univ. Agric. Exp. Stn. Purdue Journal Paper No. 12094. Received 0000.     

Mono- and diammonium phosphates and triple superphosphate as sources of P in a calcareous soil

Recovery of phosphorus from monoammonium phosphate (MAP), diammonium phosphate (DAP) and triple superphosphate (TSP), at rates of 0, 15, 30, or 45 mg P kg^–1 was determined in a pot experiment on a Calcaric Lithosol soil (21% CaCO₃). At the 15 mg P kg^–1 rate DAP was as effective as MAP and more effective than TSP in supplying P, but it was less effective than MAP and TSP at the higher rates of 30 and 45 mg P kg^–1. At the two higher P rates residual bicarbonate extractable P was also significantly lower with DAP. Yield dry matter was not affected by the source of P.     

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.     

Evaluation of compacted urea fertilizers prepared with acid and non-acid producing chemical additives in three soils varying in pH and cation exchange capacity; I. NH3 volatilization

Efficacy of different acid-producing chemical additives was evaluated in terms of pH, urea hydrolysis, NH₄-N dynamics and NH₃ volatilization in an Alfisol, a Vertisol and an Inceptisol. Compacting phosphogypsum (PG), diammonium phosphate (DAP), ZnSO₄ and KCl separately with urea slowed down urea hydrolysis and reduced NH₃ volatilization loss. Peak volatilization loss of NH₃ occurred between 2 to 4 days of fertilizer application in Vertisol and Alfisol, but between 4 to 6 days in Inceptisol. Cation exchange capacity (CEC) of soil influenced more in reducing NH₃ loss than native soil pH, as lower amount of NH₃ was lost from Vertisol (pH=8.0, CEC=43.92 cmol_c kg^-1) than from Alfisol (pH=5.8, CEC=13.82 cmol_c kg^-1). The loss from Inceptisol was in between the above two soils.     

Cowpea varietal differences in uptake of phosphorus from Gafsa phosphate rock in a low-P Ultisol

A greenhouse experiment was conducted to evaluate varietal differences in the uptake and availability of P from Gafsa phosphate rock (PR) to five cowpea cultivars grown in a low-P Paleustult soil from Ghana, using the A value technique. The³²P radioisotope used as a tracer was³²P-labelled triple superphosphate (TSP). Each cultivar received sole or a combined application of the two fertilizers (TSP, PR). From these treatments it was possible to estimate for each cultivar, A_R + A_S, A_S and by difference A_R (A_R and A_S stand for A values for phosphate rock and soil, respectively). Using this approach we measured significant genotypic differences in P uptake from PR and A_R values. The ranking of the cultivars in P uptake from PR was the same as for A_R, i.e. Asontem > Vallenga > Soronko > IT81D-1137 > Amantin. Similarly, ranking for uptake from soil P was the same as A_S, i.e. Vallenga, Soronko, IT81D-1137, Asontem and Amantin. Thus although PR could not be labelled directly, using the A value approach it was possible to distinguish between P availability from PR and soil to the plant. The recoveries of applied TSP ranged from 8.0% to 9.4% and those of PR from 2.3% to 3%.The other advantage of the A value method is that it made it possible for the different genotypes tested to be compared directly in units of a standard fertilizer, TSP in this case. Thus for Vallenga in this soil 3.01 kg of P in Gafsa PR was capable of supplying the same amount of P that could be supplied by 1 kg P of TSP; whereas for Amantin a higher amount i.e. 3.5 kg P as Gafsa PR was needed. This information is useful for adjusting application rates to be recommended for different P fertilizer sources in field trials so as to achieve similar effects.     

Assessment of methods for studying the dissolution of phosphate fertilizers of differing solubility in soil

Relationships between plant response and rates of dissolution of ground (< 150µm) North Carolina phosphate rock (NCPR), NCPR 30% acidulated with phosphoric acid (NCPAPR) and monocalcium phosphate (MCP) were assessed in pot experiments. The three fertilizers were incubated for 1, 50 and 111 days, at the rates of 75, 150 and 750µg P g^–1 soil, using two soils with different P-retention capacity. After each period of incubation, four pots were set up from each treatment, and perennial ryegrass (Lolium perenne) was grown in a growth chamber for about six weeks to assess the agronomic effectiveness of the fertilizers. Results in dry matter yield and P uptake showed that immediately following application (1 day incubation), the MCP (solution) was supplying more P to plants than either the NCPR or the NCPAPR applied at the same rate. After 50 and 111 days of incubation, the NCPR and NCPAPR were just as effective in the lower P-retention Tekapo soil. The relative agronomic effectiveness (RAE) of the NCPR and NCPAPR compared with MCP was generally poorer in the higher P-retention Craigieburn soil than in the Tekapo soil shortly after application, but improved with time of incubation. Ryegrass responses to the application of the three fertilizers corresponded to the changing trends of exchangeable P in the soils, measured by the isotopic method.Regressions were made between plant P uptake and indices describing the intensity factor (water extractable P), quantity factor (Bray I P, Olsen P, 0.5M NaOH extractable P and isotopic exchangeable P) and the kinetic factor (F_in) of soil P supply to plants in the Tekapo soil. The percentage of variation in plant P uptake explained by individual indices was generally less than 80%, no matter which of the three single variable models, the Mitscherlich, the quadratic or the power function was fitted. However, more than 96% of the variation in plant P uptake in the Tekapo soil could be explained by the power function models involving two variables. The rate of P dissolution (F_in) determined by the isotopic dilution method was included in all the two variable models. The results suggest that assessment of soil P supply to plants should consider the kinetic factor in addition to the intensity and quantity factors, particularly where P fertilizers with differing solubility are applied.     

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.     

Dissolution of phosphate rock during the composting of poultry manure: an incubation experiment

Dissolution of phosphate rocks (PRs) during composting with poultry manure was examined using a radioactive³²P labelled synthetic francolite and North Carolina phosphate rock (NCPR) through laboratory incubation experiments. Francolite or NCPR was mixed with different poultry manure composts at a rate equivalent to 5 mg P g^–1 and the dissolution was measured after 60 and 120 days incubation by a sequential phosphorus (P) fractionation procedure.The use of³²P labelled francolite showed that in manure systems, PR dissolution can be measured more accurately from the increases in NaOH extractable P (NaOH-P) than from the decreases in HCl extractable P (HCl-P) in the PR treated manure over the control. The dissolution measurements showed that approximately 8 to 20% of francolite and 27% of NCPR dissolved during incubation with poultry manure composts in the presence of various amendments. Addition of elemental sulphur (S°) to the compost enhanced the dissolution of PRs. The results provide no evidence for the beneficial effect of protons (H⁺), produced during the nitrification of NH ₄ ⁺ in manure composts, on PR dissolution. The low level of dissolution of PR in poultry manure composts was attributed mainly to the high concentration (4.8 × 10^–2 mol L^–1) of calcium (Ca²⁺) in manure solution.     

Phosphate availability in calcareous Vertisols and Inceptisols in relation to fertilizer type and soil properties

The availability to plants of fertilizer phosphorus (P) applied to soil, as measured by chemical extraction, is used to estimate P fertilizer needs. We studied the availability of P, applied as monocalcium phosphate (MCP) powder, ordinary superphosphate (OSP) granules and diammonium phosphate (DAP) granules in 24 calcareous Vertisols and Inceptisols of Andalusia, Spain, by using laboratory incubation techniques. The soils differed widely in their P adsorption- and Ca-phosphate precipitation-related properties. For MCP, availability (defined as the proportion of added P that is recovered by extraction with NaHCO₃ or is isotopically exchangeable) decreased markedly with incubation time and increasing addition rate. The mean recoveries after 180 d of incubation at field capacity at a rate of 246 mg P kg^–1 soil were 17% for Olsen P, 38% for Colwell P, and 16% for isotopically exchangeable P (IEP). Increasing the application rate to 2460 mg kg^–1 resulted in recoveries of 6% for Olsen P, 25% for Colwell P, and 4% for IEP. While IEP-based recovery was not significantly correlated to any soil property, that based on Olsen P (and, to a lesser extent, Colwell P) decreased sharply with increase in the ratio of clay (or Fe oxides) to total (or active) calcium carbonate equivalent. Accordingly, Olsen P might overestimate P availability in those soils relatively rich in carbonate and poor in clay and Fe oxides. On the other hand, recovery of applied P from soils containing more clay and Fe oxides, by a sequential extraction (with H₂O, two 0.5M NaHCO₃ treatments, 0.5M HCl), was lower than 100%, thereby suggesting phosphate occlusion by Fe oxides or clay.Availability of the fertilizers tested 90 d after application was found to decrease in the following order: MCP powder (rate, 246 mg kg^–1) > DAP granules (rate, 547 mg kg^–1) > MCP powder (rate, 738 mg kg^–1) > OSP granules (rate, 308 mg kg^–1). Differences between fertilizers tended to increase with increasing carbonate content in the soil. This may have been due to precipitation of Ca phosphates caused by the presence of Ca in the fertilizer and the high Ca- supplying capacity of the more calcareous soils.     

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.     

Changes in P Bioavailability Induced by the Application of Liquid and Powder Sources of P, N and Zn Fertilizers in Alkaline Soils

Alkaline and/or calcareous soils provide significant challenges for management of crop P nutrition, due to rapid fixation of P into sparingly soluble forms not readily accessible by plants. Three powder products, mono- and diammonium phosphate (MAP, DAP) and triple superphosphate (TSP), were compared to three liquid products, liquid MAP, ammonium polyphosphate (APP) and phosphoric acid (H₃PO₄), as sources of P for wheat grown in four alkaline soils (grey and red calcareous soils, a Vertosol and a Sodosol) sampled in Eyre Peninsula (South Australia), Wimmera and Central Mallee areas (Victoria) of southern Australia. Soils were labelled with ³²P and the labile P pool and P derived from the fertilizer (P_dff) determined. Residual value of the fertilizers was determined after a 4 month wet/dry incubation. Liquid formulations outperformed powder products in the grey calcareous soil and in the Vertosol, as measured by wheat growth, P uptake, plant-labile P pools and P_dff. These increases in P efficiency were not related to differential acidification of the bulk soils. In the most calcareous soil, large proportions of the added DAP, MAP or TSP were rapidly converted (fixed) into non plant-labile pools, likely through precipitation of Ca–P compounds not accessible to plant roots, while conversely, liquid formulations minimized P fixation. Our results suggest that one of the most likely hypotheses to explain difference in efficiencies between powder and liquid forms of fertilizers was that in relatively dry conditions and in high P fixing soils, the dissolution and diffusion of P outwards from the powder is limited. This induces localized areas in the soil with high concentrations of P leading to precipitation of insoluble Ca–P solid phases. In the field, where powder fertilizers are applied as granules rather than as fine powder, differences between the two forms of fertilizer are likely to be larger. The residual value of liquid formulations was equal or superior to powder products. Liquid fertilizers injected into soil may therefore have potential to improve P nutrition in a wide range of calcareous soils under dryland agriculture throughout the world. Field trials have actually been performed in southern Australia to confirm this important issue.     

A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets ha^–1, 3 plants pocket^–1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha^–1 and 30 kg P₂O₅ ha^–1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha^–1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha^–1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha^–1 had average yields of 317 kg grain ha^–1 while those planting at densities higher than 6,500 pockets ha^–1 showed average yields of 977 grain ha^–1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha^–1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha^–1) or no economic benefit from fertilizer use will be realized.     

32P isotopic techniques for evaluating the agronomic effectiveness of rock phosphate materials

Tropical soils are often low in available P and therefore require inputs of P fertilizer for optimum plant growth and production of food and fiber. The cost of applying imported or locally produced, water-soluble, P fertilizers is often greater than utilizing indigenous phosphate rock. Therefore quantifying the P availability of soils amended with phosphate rock-based products in a variety of crop management and environmental conditions in developing countries is desirable for making recommendations on best type and rate of fertilizers to use to obtain maximum agronomic and economic benefits. One adequate approach for evaluating the agronomic effectiveness of rock phosphate materials is through the use of³²P/³³P isotopic tracers. The present paper describes the principles and assumptions of the³²P isotopic techniques commonly used in the field and greenhouse for the agronomic evaluation of rock phosphate materials. An overview of the applications of these techniques is also given.