Agronomical evaluation of phosphate fertilizer as a nutrient source of phosphorus for crops: Isotopic procedure

The agronomic effectiveness of P fertilizers, as sources of phosphorus for crops, was evaluated using the quantities, P_f, of phosphorus taken up byLolium perenne grown on 14 soils during greenhouse experiments in pot cultures. The P_f quantities were determined using³²P-labelled fertilizers. Data were analysed using a new concept: the Isotopic Relative Agronomic Effectiveness (IRAE). The IRAE value was defined as the ratio of the P_f quantity, taken up by a crop, of a tested fertilizer over the P_f quantity, taken up by a crop, of a fertilizer used as standard. In our experiments diammonium phosphate (DAP) was used as standard P fertilizer and two rock phosphates, the North Carolina rock phosphate (NCPR) and a calcium-iron-aluminium phosphate (Phospal), were tested. As a linear relationship between P_f(NCPR) quantities and P_f(DAP) quantities was obtained, with r² = 0.95, when the application rates increased from 15 mgP (kg soil)^–1 to 200 mgP (kg soil)^–1, it is conciuded that IRAE values for a given fertilizer, other than the standard fertilizer, could be determined with a single rate of application. As regards soil pH in the range 4.7 to 8.2 the IRAE_NCPR is related to soil pH by a curvilinear relationship: log IRAE_NCPR = –(0.44) pH + 4.05 with r² = 0.89. The average of IRAE_phospal values was 0.15 with a standard error = 7% irrespective of soil pH. Then a logarithmic relationship was obtained between IRAE values of the two tested fertilizers and their water P-solubility determined at the soil pH where they were applied.     

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.     

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.     

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

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

Fate of phosphorus applied in slurry and mineral fertilizer: accumulation in soil and release into surface runoff water

Phosphorus (P) accumulation on the soil surface and its effect on the concentration of dissolved orthophosphate P (PO₄-P) in surface runoff water were studied after three years of surface application of slurry and mineral fertilizer to grass ley on a sandy soil, poor in P. The total amount of P applied was 107–143 kg ha^–1>, of which 72–119 kg ha^–1> was applied on the soil surface during two or three years without incorporation or mixing. The addition of slurry and mineral fertilizer resulted in an increase in inorganic P in the 0–5 cm but not the 5–25 cm soil layer, but organic P was not affected. The measured changes in inorganic P deviated only by 4–6 kg ha^–1> from the values derived from inputs and outputs of P (crop uptake + losses in surface runoff and drainage water). The increase in inorganic-P was accompanied by increases in the degree of P saturation (DPS) and in P extracted with acid am monium acetate (P_Ac ), sodium bicarbonate (P_Olsen) and anion-exchange resin (P_Resin). In surface runoff, 10–18 months after the last surface application of P, the mean flow-weighted concentration of PO₄-P was linearly increased with the values of DPS, P_Ac, P_Olson and P_Resin in the 0–5 cm soil layer. PO₄-P was lowest (0.033 mg l^–1> ) in the control plots and highest (0.62 mg l^–1>) in the plot where 143 kg ha^–1> P had been applied in slurry and fertilizer. On that plot, the corresponding values of DPS, P_Ac, P_Olson and P_Resin were 16%, 13 mg kg^–1>, 85 mg kg^–1> and 71 mg kg^–1 , even within a few years, and multiply the P loading to surface runoff from the site. A very shallow soil sampling (< 5 cm) is needed to assess P loading potential in a soil where P has been surface-applied.     

Human urine - Chemical composition and fertilizer use efficiency

Stored human urine had pH values of 8.9 and was composed of eight main ionic species (> 0.1 meq L^–1), the cations Na, K, NH₄, Ca and the anions, Cl, SO₄, PO₄ and HCO₃. Nitrogen was mainly (> 90%) present as ammoniacal N, with ammonium bicarbonate being the dominant compound. Urea and urate decomposed during storage. Heavy metal concentrations in urine samples were low compared with other organic fertilizers, but copper, mercury, nickel and zinc were 10–500 times higher in urine than in precipitation and surface waters. In a pot experiment with¹⁵N labelled human urine, higher gaseous losses and lower crop uptake (barley) of urine N than of labelled ammonium nitrate were found. Phosphorus present in urine was utilized at a higher rate than soluble phosphate, showing that urine P is at least as available to crops as soluble P fertilizers.     

Effect of compost and soil properties on the availability of compost phosphate for white clover (Trifolium repens L.)

Wide variation in results exists in the literature on the effectiveness of composts to sustain the phosphorus (P) nutrition of crops. The aim of this work was to assess the importance of some soil and composts properties on the utilization of compost-P by white clover (Trifolium repens L.). This study was carried out with samples collected from four composts made from solid kitchen and garden wastes, and with two soil samples taken from the A horizon of a P-rich sandy acidic Dystrochrept and of a P-limited clayey calcareous Eutrochrept. Changes in the amount of inorganic P (Pi) isotopically exchangeable within 1 min (E_1min) were measured during 32 weeks in incubated soil-composts or soil-KH₂PO₄mixtures where P sources had been added at the rate of 50 mg P kg^–1 soil. Uptake of compost-P or KH₂PO₄-P by white clover was measured on the same amended soils during 16 weeks. In both soils, the application of composts resulted after 32 weeks of incubation in E_1min values ranging between those observed in the control without P and those observed in the KH₂PO₄treatment, i.e., in values ranging between 4.2 and 5.9 mg P kg^–1 in the sandy acidic soil and between from 1.6 to 4.3 mg P kg^–1 in the clayey calcareous soil. The total coefficient of utilization of compost-P (CU-P) by white clover reached values in both soils for the four composts ranging between 6.5% and 11.6% of the added P while in the presence of KH₂PO₄ the CU-P reached values ranging between 14.5% in the clayey calcareous soil and 18.5% in the sandy acidic soil. Results obtained in the sandy acidic soil suggest, that white clover initially used a fraction of the rapidly exchangeable compost P, while at a latter stage plant roots enhanced the mineralisation of compost organic P and took up a fraction of the mineralized P. These relations were not observed in the clayey calcareous soil probably because of its high sorbing capacity for P. In the sandy acidic soil, composts application increased the uptake of soil P by the plant from 31.4 mg P kg^–1 soil in the control without P to values ranging between 37.9 to 42.7 mg P kg^–1 soil in the presence of composts. This indirect effect was related to a general improvement of plant growth conditions in this soil induced by compost addition (from 9.9 g DM kg^–1 soil in the control without P to values ranging between 14.0 to 16.1 g DM kg^–1 soil in the presence of composts) and/or to the release of Al- or Fe bound soil P to the solution due to soil pH increase following compost application. Finally the total coefficient of utilization of P (CU-P) derived from KH₂PO₄ and composts was related to the total amount of N exported by white clover in the P-limited clayey calcareous soil but not in the P-rich sandy acidic soil. This suggests that in a soil where N₂ biological fixation is limited by low P availability, the CU-P of a compost by white clover is not only related to the forms of P present in the compost but also to its effect on N nutrition. However, it is not clear whether this improved N nutrition was due to compost mineralisation, or to an indirect compost effect on the N₂ biological fixation.     

Characterization of soil-fertilizer P reaction products and their evaluation as sources of P for gram (Cicer arietinum L.)

Laboratory studies on the characterization of soil-fertilizer P reaction products were carried out by reacting three-soils occurring in a toposequence in the plateau region of Bihar (India) with saturated solutions of diammonium orthophosphate (DAP), triple superphosphate (TSP) and ammonium polyphosphate (APP) for 1 hour and 24 hours. The reaction products (precipitates) formed in the solutions after 120 days of incubation were isolated and identified through X-ray diffraction technique.Results indicate the formation of Brushite [CaHPO₄ · 2H₂O, Strengite (FePO₄ · 2H₂O), Variscite (AIPO₄ · 2H₂O) and Fe₄(P₂O₇)₃ as major soil-fertilizer P reaction products in these soils with ortho-and polyphosphates as source of phosphorus.Pot cultures were used to evaluate the relative efficiency of reaction products (Struvite, Brushite, Variscite and Strengite), orthophosphates (DAP and SSP) and polyphosphate (APP) as sources of P for gram (Cicer arietinum L.) in a typical acid soil. Results indicate significant response of gram to different sources and level of added P. The dry weight and P uptake at 0, 6 and 12 mg P kg^-1 soil were 0.406, 0.519 and 0.609 (g pot^-1); and 0.289, 0.428 and 0.575 (mg P pot^-1), respectively. Among the sources , struvite proved to be superior or equally effective as APP, DAP or SSP as sources of P for gram. Uptake of P also varied significantly with different P sources and levels of P application. Strengite was least effective in enhancing yield and P uptake by the crop.     

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.     

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.     

Fate of Fertilizer15N applied as urea and ammonium sulphate in opium poppy (Papaver somniferum L.) grown under greenhouse conditions

Laboratory incubation and greenhouse experiments were conducted to investigate the comparative effectiveness of urea and ammonium sulphate in opium poppy (Papaver somniferum L.) using¹⁵N dilution techniques. Fertilizer treatments were control (no N), 600 mg N pot^–1 and 1200 mg N pot^–1 (12 kg oven dry soil) applied as aqueous solution of urea or ammonium sulphate. Fertilizer rates, under laboratory incubation study were similar to that under greenhouse conditions. A fertilizer¹⁵N balance sheet reveals that N recovery by plants was 28–39% with urea and 35–45% with ammonium sulphate. Total recovery of¹⁵N in soil-plant system was 77–82% in urea. The corresponding estimates for ammonium sulphate were 89–91%. Consequently the unaccounted fertilizer N was higher under urea (18–23%) as compared to that in ammonium sulphate (9–11%). The soil pH increased from 8.2 to 9.4 with urea whereas in ammonium sulphate treated soil pH decreased to 7.3 during 30 days after fertilizer application. The rate of NH₃ volatilization, measured under laboratory conditions, was higher with urea as compared to the same level of ammonium sulphate. The changes in pH of soil followed the identical trend both under laboratory and greenhouse conditions.     

Mussoorrie rock phosphate-pyrite mixture as phosphate fertilizer

Mussoorrie rock phosphate (MRP), MRP + pyrite (25% by weight), diammonium phosphate (DAP), ammonium polyphosphate (APP) and nitrophosphate (NP) were compared in a field experiment as fertilizers for wheat. At 20 kg P ha^–1, MRP was only 6 per cent as effective as DAP. However, when it was mixed with pyrite, the efficiency of MRP increased to 64 per cent at 20 kg P ha^–1 compared with 97 per cent at 40 kg P ha^–1. The P requirement for a targeted yield for 4.5 t ha^–1 decreased from 39.4 kg P ha^–1 as MRP to 23.7 kg P ha^–1 as MRP + pyrite. Of the other P fertilizers studied, NP was as effective as DAP, whereas APP was 9 to 37 per cent more effective than DAP. However, the P requirement as DAP, NP and APP for a targeted yield of 4.5 t ha^–1 was similar (11 ± 0.5 kg P ha^–1).     

Effect of NPK fertilization on the mineral nutrition and yield of three coconut genotypes

Annual application of NPK fertilizers over a 18 year period to coconut on red sandy loam soils resulted in a minimal increase in mineralisable N, but in a marked increase in available P and K. Plant N levels, however, reflected the improved N nutrition but did not reach sufficiency levels found elsewhere. An available P status of 15 ppm in the control plots kept leaf P at sufficiency levels. P fertilizers did not increase the P content of leaves. K fertilizers raised the K leaf content to sufficiency levels. Doubling the M₁ fertilizer rates of 500 g N, 220 g P and 830 g K per palm per year had no effect on N, P and K levels in the palm leaves.Changes in K levels of the leaves had antagonistic effects on leaf Mg (r = – 0.68^**) and leaf Na (r = – 0.87^**). As this effect brings leaf Mg close to deficiency values palms receiving K might need additional Mg as well.The findings and interpretation of soil and leaf analysis data were confirmed by large yield responses to application of NPK fertilizers. Genetic differences between palms in their response to levels of nutrient supplies were apparent. The CDO × WCT hybrid outyielded the high yielding WCT variety especially when NPK was given at the M₁ level. The response in yield to applied fertilizers was linear for WCT and curvilinear for the hybrids CDO × WCT and WCT × CDO.     

Chemical retardation of phosphate diffusion in an acid soil as affected by liming

The retention of phosphate (P) in soil limits its mobility through bothdiffusion and mass flow. Soil P retention is a complex reaction with aluminum(Al) and iron (Fe) playing major roles in P retention in acid soil. This studyexamined the one-dimensional diffusion of applied phosphate andwater-extractable Al in a gibbsite-rich acid soil as affected by liming. Thesoil used was a loamy sand, a Typic Hapludox (US Taxonomy), obtained from asite150 km south of Perth, Australia. Soil was limed at 5 gkg^–1 to raise the soil pH from 5.5 to 7.1.KH₂PO₄ was applied to the surface of unlimed and limedsoil columns, which were then incubated in a constant humidity chamber for 3,7,14 and 28 d. Water-extractable P (WE-P) concentration in the surface ofunlimed soil (0 to 0.2 cm) increased from 0 in the original soilto22.3 mol g^–1, 3 d after the Papplication. However, the WE-P decreased with incubation period to a value of9.7 mol g^–1 on day 28. Similar WE-Pconcentration patterns were observed with the limed soil, but the WE-Pconcentration near the surface decreased to a much lower value under limed (7.6mol g^–1) than unlimed (9.7 molg^–1) conditions. Liming also reduced P diffusion. After28 d of incubation P diffused to a depth of 2.1 cmin the unlimed soil, but to only 1.5 cm under the limedsoil.Acid-extractable P (AE-P) also increased from 0 to 50 molg^–1 in the surface soil layer for both unlimed and limedsoil as a result of P fertilizer application. There was little change in theAE-P for both treatments from day 3 of incubation to day 28. The pH and Al andFe concentrations in the soil solution were elevated near the soil surfacewhereK and P ions interacted with soil matrices. Liming significantly reduced thetotal amount of WE-P and P diffusive movement in the soil, but reduced Altoxicity since Al ions shifted to less toxic species with increases in pH. Pfertilizer application also increased soil pH under both unlimed and limedconditions. This would reduce Al toxicity, benefiting crop production.     

Chemical characterization and agronomic effectiveness of phosphorus applied as a polyphosphate-chitosan complex

This work was undertaken to characterize a chitosan-polyphosphate complex (CH-PP) and to evaluate its agronomic effectiveness as a source of phosphorus for ryegrass (Lolium perenne) grown on loamy and clayey soils. High resolution solid state³¹P and¹³C nuclear magnetic resonance were used to characterize this complex and to monitor the structural changes occurring to it during an 8-week incubation period in a loamy soil. A pot experiment was conducted on the two soils after labelling the available P with³²PO₄ ions. This experiment allowed for the determination of the agronomic effectiveness of the chitosan-polyphosphate complex compared to polyphosphate and to monocalcium phosphate. Results showed that chitosan immobilized up to 147 mg P kg^–1 as pyrophosphate and hexametaphosphate. This reaction did not involve major structural changes in the pyrophosphate or hexametaphosphate groups nor in the chitosan. The chitosan-polyphosphate complex was as efficient as the polyphosphate alone to sustain the P nutrition of ryegrass. The relative agronomic effectiveness of these P sources was slightly lower compared to that of monocalcium phosphate. The high P fertilizing value of the chitosan-polyphosphate complex was attributed to its gradual hydrolysis in the soil. The potential interest of chitosan to remove polyphosphates from waste waters while preserving the high P fertilizing value of polyphosphates was addressed.     

Isotopic assessment of soil phosphorus fertility and evaluation of rock phosphates as phosphorus sources for plants in subtropical China

Soil phosphorus (P) deficiency is a major factor limiting crop productivity in many tropical and subtropical soils. Due to the acidic nature of these soils, rock phosphate (RP)-based P fertilizers that are cheaper than manufactured water-soluble P fertilizers can be an attractive alternative under certain conditions. Assessment of the efficacy of these alternative P fertilizers and a rational management of local P resources for sustainable agricultural production require an understanding of the dynamics of P in the soil–plant system and the interactions of various P sources in soils and monitoring of soil available P levels. The present work was conducted to test the applicability of the ³²P isotopic kinetic method to assess the soil P fertility status and evaluate the agronomic effectiveness of local rock phosphates in subtropical China. A series of experiments was carried out in the laboratory, greenhouse and field conditions with the following specific objectives: (a) to evaluate the suitability of this isotopic kinetic method in evaluating soil P fertility in 32 soil samples collected across southern China, (b) to test and further develop chemical extraction methods for routine soil P testing, (c) to monitor the dissolution kinetics of local low to medium grade rock phosphate sources and their effect on soil properties and (d) to evaluate their agronomic effectiveness in greenhouse and field experiments. Since most of the studied soils had very low concentrations of soluble P and high P-fixing capacities, the isotopic kinetic method was found unsuitable for evaluating soil P fertility and to predict plant P uptake. In contrast, the proposed chemical extraction method (NaHCO₃-NH₄F) predicted very well plant P uptake, suggesting that this extraction method can be routinely used to evaluate soil bioavailable P in similar soils in subtropical China. From the incubation study, it was found that although the local low to medium grade RPs were inferior to the reactive NCPR in increasing soil available P levels, they have the potential to improve soil chemical properties. Field experiments indeed demonstrated that the medium grade Jinxiang RP significantly increased crop yield, suggesting that local low to medium grade RPs could be used as P sources to provide P to plants and also to improve soil chemical properties. Overall, these results provide important information for a rational management of P resources for sustainable agriculture in subtropical China.     

Agronomic effectiveness of partially acidulated rock phosphate and fused calcium-magnesium phosphate compared with superphosphate

The agronomic effectiveness of two partially acidulated rock phosphate (PARP) fertilizers, made from either North Carolina or Moroccan apatite rock phosphate, and a fused calcium-magnesium phosphate (thermal phosphate or TP), was compared with the effectiveness of superphosphate in two glasshouse experiments. A different lateritic soil from Western Australia was used for each experiment. Oats (Avena sativa) were grown in one experiment and triticale (×Triticosecale) in the other. Fertilizer effectiveness was measured using (i) yield of dried tops, (ii) P content (P concentration in tissue multiplied by yield) of dried tops, and (iii) bicarbonate-extractable soil P (soil test value).The following relationships differed for the different fertilizers: (i) yield of dried tops and P content in the dried tops; (ii) yield and soil test values. Consequently the fertilizer effectiveness values calculated using yield data differed from those calculated using P content or soil test data. Freshly-applied superphosphate was always the most effective fertilizer regardless of the method used to calculate fertilizer effectiveness values. For one of the soils, as calculated using yield data, relative to freshly-applied superphosphate, the PARP and TP fertilizers were 15 to 30% as effective for the first crop, and 20 to 50% as effective for the second crop. The second soil was more acidic, and for the first crop the PARP and TP fertilizers were 80 to 90% as effective as freshly-applied superphosphate, but all fertilizers were only 5 to 15% as effective for the second crop. For each soil, the two PARP fertilizers had similar fertilizer effectiveness values. Generally the TP fertilizer was more effective than the PARP fertilizers.     

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 commercial triple superphosphate fertilizers

Field studies were conducted for three years (1987–1989) at two locations to evaluate 4 commercial triple superphosphate (TSP) fertilizers containing various levels of water-soluble P. The fertilizers had been produced from phosphate rock deposits located in Florida, North Carolina and Morocco. AOAC available P was 81 to 94% water-soluble. Water-soluble P was inversely related to the level of Fe and Al in the fertilizers. Phosphorus from each source was applied to a Malbis soil (Plinthic Paleudults) and a Hartsells soil (Typic Hapludults) at rates of 0, 25, 49 and 99 kg ha^–1. Potato (Solanum tuberosum L.) yields were increased by the application of P, except for the Malbis soil in 1988. Yields were not affected by the source of added P on either soil during the three years of the study. Fertilizer performance was not affected by the level of water-soluble P or the content of Fe and Al when band applied to potatoes under field conditions in the Southeastern United States.