Effect of phosphate on the sorption,desorption and plant-availability of selenium in soil

Phosphate, applied at 5µg P cm^–3, decreased selenite sorption by from 30–70% in three soils studied. Both maximum sorption (X_m) and the binding-energy of sorption as indicated by the binding-energy related constant (k) or the molar free energy (G) of the sorption reaction derived from the Langmuir equation were considerably decreased. On the other hand, phosphate sorption was decreased by increasing concentration of selenite from 0.2µg Se cm^–3 to 1.0µg Se cm^–3 in the initial solution. The competitive sorption of phosphate with selenite was likely the main mechanism involved in the P-Se interactions. The competitively sorbed selenite exhibited much larger desorption in 0.01M CaCl₂ solution, more readily extractable to 0.5M NaHCO₃ and significantly higher isotopic exchangeability compared to that sorbed without the competing anion. Results from pot trial using ryegrass indicated that phosphate application increased more efficiently the plant-availability of applied fertilizer Se than that of indegeneous Se in soil.     

Phosphorus sorption in relation to soil properties in some cultivated Swedish soils

Phosphorus (P) sorption properties are poorly documented for Swedish soils. In this study, P sorption capacity and its relation to soil properties were determined and evaluated in 10 representative Swedish topsoils depleted in available P. P sorption indices were estimated from sorption isotherms using Langmuir and Freundlich equations (X_m and a_F, respectively) and P buffering capacity (PBC). X_m ranged from 6.0 to 12.2 mmol kg^–1. All indices obtained from sorption isotherms were significantly correlated with each other (r=0.96^*** to r=0.99^***). Two single-point sorption indices (PSI₁ and PSI₂) were also determined, with additions of 19.4 and 50 mmol P kg^–1 soil, respectively. Both PSI indices were well correlated with X_m (r0.98^***), with PSI₁ giving the highest correlation. As isotherms for determining P sorption capacities involve laborious analytical operations, PSI₁ would be preferable for routine analyses. X_m was significantly correlated with Fe extracted by sodium pyrophosphate and ammonium oxalate, to Al extracted by ammonium oxalate and dithionite-citrate-bicarbonate and to organic c. X_m was also significantly correlated with the sum of Fe and Al extracted by ammonium oxalate. The best prediction of X_m through multiple regression was obtained when Fe extracted in ammonium oxalate and Al extracted in dithionite-citrate-bicarbonate were used. Based on the results obtained, both PSI₁ and oxalate-extractable Fe plus Al can be used for predicting P sorption capacity in Swedish soils.     

Iron chlorosis in olive in relation to soil properties

Chlorosis due to iron (Fe) deficiency is becoming severe in olive (Olea europaea L.) trees growing on some highly calcareous soils in southern Spain. We investigated the relationships between the incidence of Fe chlorosis in three olive varieties (`Hojiblanco', `Manzanillo' and `Picual') and soil properties. Leaf chlorophyll content, estimated by the mean value of three SPAD measurements during the growing season, was poorly correlated with soil carbonate content and reactivity. In contrast, it was significantly correlated with the clay content and with the amounts of Fe extracted with oxalate (Fe_ox), citrate/ascorbate (Fe_ca), and diethylendiaminepentaacetid acid (Fe_DTPA). This suggests that the content and reactivity of poorly crystalline Fe compounds play an important role in Fe nutrition. The three olive varieties did not differ significantly in their susceptibility to Fe chlorosis. Soil test critical levels separating chlorotic and non-chlorotic trees were 300 g clay kg^–1 soil and 0.35 g Fe_ox kg^–1 soil.     

Sorption of ruthenium by dipropylmethyl-2-(N,N-diisobutyl) acetamidoammonium iodide impregnated Amberlite XAD-4 resin from nitric acid medium

Dipropylmethyl-2-(N,N-diisobutyl)acetamidoammonium iodide has been impregnated on Amberlite XAD-4 resin and investigated for sorption of Ru from nitric acid medium. Equilibrium sorption data for Ru uptake were represented well by the Langmuir isotherm equation (R² = 0.98) compared to Freundlich isotherm equation (R² = 0.86). The maximum monolayer coverage (Q₀) value of 6.25 mg/g as obtained from Langmuir isotherm was close to the experimental value (5.63 mg/g). The heterogeneity parameter (1/n) = 0.37 obtained from the slope of Freundlich isotherm indicates slight heterogeneity in sorption process. Aqueous solutions of 5% ammonia or 10% sodium hydroxide were found suitable for desorption. The method can be applied for separation of Ru from acidic waste solutions.     

A model of S,P and N uptake by a perennial pasture

Phosphate sorption curves give useful information about the management of the P fertility in soils. This study was conducted to examine how P sorption characteristics were influenced, and could be predicted from the properties of Mediterranean soils of Spain. The 114 soils studied differed widely in origin and properties and were grouped as calcareous (43) and noncalcareous (71). Citrate-bicarbonate-dithionite extractable Fe (Fe_d) and Al (Al_d) and clay were the properties best correlated with the P sorbed and the P buffer capacity at an equilibrium concentration of 0.2 mg P l^–1. This suggests that Fe oxides and silicate clays play a significant role in P sorption. Stepwise regression analysis showed, however, that clay was not a significant variable in the calcareous group, but active CaCO₃ was. Prediction of the P sorption parameters was better for the noncalcareous than for the calcareous group. This reduced ability to predict P sorption in calcareous soils may be due to the inability of total or active CaCO₃ to adequately measure the reactivity of carbonates towards P sorption. From 53 to 75% of the variation in P sorption parameters could be explained by regression equations including all or several of the following routinely-determined soil properties: clay, pH, Fe_d, and active CaCO₃. These equations could, therefore, provide a simple and rapid estimation of P sorption in soils of Mediterranean regions.     

Ammonia sorption by peat and N fractionation in some peat-ammonia systems

Ammonia sorption is an important peat property for making composts and peat-mineral fertilizers. In this study, we investigated the influence of moisture content of fibric, hemic and sapric peat materials on ammonia sorption capacity, and we determined the N forms and the amount of solubilized humic substances following peat ammoniation. Ammonia sorption capacity increased curvilinearly with degree of decomposition from the fibric to the sapric peats, and increased with peat moisture content up to 600 g H₂O [kg wet peat]^–1 in the range of 400 to 700 g H₂O [kg wet peat]^–1. The amount of chemically sorbed ammonia was highest in the sapric peat containing [600 g H₂O] [kg wet peat]^–1. On a dry weight basis, maximum sorption capacity was 24 g NH₃-N kg^–1 for the fibric peat, 29 g NH₃-N kg^–1 for the hemic peat and 49 g NH₃-N kg^–1 for the sapric peat. Ammonia loss of physically retained ammonia was greatest at the highest moisture content and for the least decomposed peat. At ammoniation rates exceeding 30 g NH₃ [kg dry peat]^–1, exchangeable ammonium and acid-hydrolyzable N were the dominant N fractions in ammoniated peats. Peat ammoniation increased significantly (p < 0.001) the amounts of water-soluble carbon in all peat materials, of fulvic acid in the fibric peat and of both humic and fulvic acids in the hemic and the sapric peats. At maximum ammonia sorption capacity, the total amount of water-soluble and alkali-extractable organic matter reached 338 g kg^–1 in fibric peat, 683 g kg^–1 in the hemic peat and 848 g kg^–1 in the sapric peat. The hemic and sapric peats appeared more appropriate than the fibric peat for making peat-base fertilizers.     

Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe3+ Hydrolysis and Fe2+ Oxidation: Kinetics,Isotherms and Adsorption Mechanism

Good sorption properties and simple synthesis route make schwertmannite an increasingly popular adsorbent. In this work, the adsorption properties of synthetic schwertmannite towards Cr(VI) were investigated. This study aimed to compare the properties and sorption performance of adsorbents obtained by two methods: Fe³⁺ hydrolysis (SCH_A) and Fe²⁺ oxidation (SCH_B). To characterise the sorbents before and after Cr(VI) adsorption, specific surface area, particle size distribution, density, and zeta potential were determined. Additionally, optical micrographs, SEM, and FTIR analyses were performed. Adsorption experiments were performed in varying process conditions: pH, adsorbent dosage, contact time, and initial concentration. Adsorption isotherms were fitted by Freundlich, Langmuir, and Temkin models. Pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models were used to fit the kinetics data. Linear regression was used to estimate the parameters of isotherm and kinetic models. The maximum adsorption capacity resulting from the fitted Langmuir isotherm is 42.97 and 17.54 mg·g⁻¹ for SCH_A and SCH_B. Results show that the adsorption kinetics follows the pseudo-second-order kinetic model. Both iron-based adsorbents are suitable for removing Cr(VI) ions from aqueous solutions. Characterisation of the adsorbents after adsorption suggests that Cr(VI) adsorption can be mainly attributed to ion exchange with SO₄²⁻ groups.     

Fertility characterization of soils at six research sites in NW Cameroon

Fertility capability of surface (0–20 cm) soils was evaluated at six sites in the North-West Cameroon highlands. Two main soil groups, designated as Classes A and B, were identified based on elevation. The Class A soils from low elevations (600–1178 m) had higher Ca, Mg, K, pH, sorbed less P and were lower in organic carbon and sesquioxides than the highland (> 1200 m) soils. Soil acidity (Al saturation > 30%) and high P sorption appeared to be the most limiting factors to crop production especially on the Class B soils where the Standard P Requirement exceeded 500 mg kg^–1. Phosphorus sorption data were best described by the Freundlich equation. Amorphous aluminium was the most important determinant of solution P concentration (r = 0.85,p < 0.001) followed by soil organic carbon, (r = 0.80,p < 0.001) at high P rates. Nitrogen deficiency symptoms of maize were pronounced on the Class B soils. Consequently, crop growth and yield were lower on Class B than on Class A soils despite the high organic carbon in B. We hypothesize that the supply of high quality organic material (high in N and low in lignin and polyphenols) at site B through agroforestry and related cropping systems, would improve the fertility of the soil and crop yield.This article is a contribution from the IITA-IRA-NCRE, USAID-supported National Cereals Research and Extension Project in Cameroon.     

Synthesis of Fe3O4/polyaniline nanocomposite and its application for nitrate removal from aqueous solutions

The aim of this work was to investigate the sorption characteristics of Fe₃O₄ coated on polyaniline (Fe₃O₄/PAn) for the removal of NO₃^? ions from aqueous solutions. The sorption of NO₃^? ions by the batch method was carried out. The optimum conditions of sorption were found to be a Fe₃O₄/PAn dose of 0.4 g in 100 mL of NO₃^? solution, a contact time of 10 min, pH and temperature 7 and 40°C, respectively. Temperature had a negative effect on the removal efficiency. Three equations, i.e., Morris–Weber, Lagergren, and pseudo‐second‐order, were tested to track the kinetics of the removal process. The Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models were subjected to sorption data in order to estimate sorption capacity, intensity, and energy. The thermodynamic parameters ΔH, ΔS, and ΔG were evaluated. They showed that the adsorption of NO₃^? onto Fe₃O₄/PAn was feasible, spontaneous, and exothermic under the studied conditions. It can be concluded that Fe₃O₄/PAn has potential to remove NO₃^? ions from aqueous solutions at different concentrations. The system Fe₃O₄/Pan was successfully tested for a high removal efficiency of NO₃^? from urban wastewater. J. VINYL ADDIT. TECHNOL., 19:147–156, 2013. © 2013 Society of Plastics Engineers     

Synthesis,characterization, and application of amberlite XAD‐2‐ salicylic acid‐ iminodiacetic acid for lead removal from human plasma and environmental samples

A new chelating resin is prepared by coup‐ling Amberlite XAD‐2 with salicylic acid (SAL) through an azo spacer. Then the polymer support was coupled with iminodiacetic acid (IDA). The resulting sorbent has been characterized by FT‐IR, elemental analysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) and studied for the preconcentration and determination of trace Pb (II) ion from human biological fluid and environmental water samples. The optimum pH value for sorption of the metal ion was 5. The sorption capacity of functionalized resin is 67 mg g⁻¹. The chelating sorbent can be reused for 20 cycles of sorption–desorption without any significant change in sorption capacity. A recovery of 95% was obtained for the metal ion with 0.5M nitric acid as eluting agent. The profile of lead uptake on this sorbent reflects good accessibility of the chelating sites in the Amberlite XAD‐2‐SAL/IDA. Scatchard analysis revealed that the homogeneous binding sites were formed in the polymers. The equilibrium adsorption data of Pb (II) on modified resin were analyzed by Langmuir, Freundlich, Temkin, and Redlich‐Peterson models. Based on equili‐brium adsorption data the Langmuir, Freundlich, and Temkin constants were determined 0.428, 20.99, and 7 × 10⁻¹² at pH 5 and 20°C. The method was successfully applied for determination of lead ions in human plasma and sea water sample. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Sorption/release of diclofenac sodium in/from free‐standing poly(acrylic acid)/poly(ethyleneimine) multilayer films

Polyelectrolyte multilayer films deposited onto various substrates have been used extensively as drug delivery systems. However, little attention has been paid to the release of drugs from free‐standing polymeric films. Herein, we report the construction of thermal crosslinked free‐standing poly(acrylic acid) (PAA)/branched poly(ethyleneimine) (PEI) multilayer films composed of 25 double layers [(PAA/PEI)₂₅] and their use in sorption/release of diclofenac sodium (DS). The (PAA/PEI)₂₅ multilayer films were characterized by scanning electron microscopy, potentiometric titrations and Fourier transform infrared spectroscopy, while the sorption/release of DS was monitored by UV – Vis spectroscopy. The DS sorption equilibrium data were fitted with five isotherm models (Langmuir, Freundlich, Sips, Dubinin–Radushkevich, and Temkin). The maximum equilibrium sorption capacity, q_m, given by the Langmuir model was 32.42 mg DS/g. The Korsmeyer–Peppas semiempirical equation showed that the release of DS from the free‐standing (PAA/PEI)₂₅ films proceeded by pseudo‐Fickian diffusion, irrespective of the releasing media. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43752.     

Preconcentration and determination of chromium in water with flame atomic absorption spectrometry by thiourea-formaldehyde as chelating resin

Thiourea-formaldehyde chelating resin is synthesized simply and rapidly from thiourea and formaldehyde by condensation polymerization and characterized by IR spectra and studied for the preconcentration and determination of trace Cr(III) ion from solution samples. The optimum pH value for sorption of the metal ion was 6.5. The sorption capacity of resin for Cr(III) was determined. The chelating resin can be reused for 20 cycles of sorption-desorption without any significant change in sorption capacity. A recovery of 96% was obtained for the metal ion with 0.5M HNO₃ as eluting agent. The equilibrium adsorption data of Cr(III) on modified resin were analyzed by Langmuir, Freundlich and Temkin models. Based on equilibrium adsorption data the Langmuir, Freundlich and Temkin constants were determined as 0.016, 0.040 and 0.074 at pH 6.5 and 20°C. The method was applied for chromium ion determination from river water sample.     

Iminodiacetic acid‐containing polymer brushes grafted onto silica gel for preconcentration and determination of copper(II) in environmental samples

Silica gel has been modified by silylation with 3‐mercaptopropyltrimethoxysilane followed by graft polymerization of dimethylacrylamide and (N,N‐bis‐carboxymethyl)amino‐3‐allylglycerol‐co‐dimethylacrylamide, synthesized via the reaction of allyl glycidyl ether with iminodiacetic acid. The sorbent, poly(AGE/IDA‐co‐DMAA)‐grafted silica gel, has been characterized by FTIR, elemental analysis, thermogravimetric analysis (TGA), FT‐Raman, and scanning electron microscopy and studied for the preconcentration and determination of trace amounts of Cu(II) ion in environmental water samples. The optimum pH value for quantitative sorption of Cu(II) in batch mode was 5.5 and desorption was achieved, using 0.5 mol L^?1 nitric acid. The sorption capacity of functionalized sorbent is 32.3 mg g^?1. The chelating sorbent was reused for 15 sorption–desorption cycles without any significant change in sorption capacity. The profile of copper uptake by the sorbent reflected good accessibility of the chelating sites in the poly(AGE/IDA‐co‐DMAA)‐grafted silica gel. Scatchard analysis demonstrated homogeneous nature of binding sites. The equilibrium adsorption data of Cu(II) on modified sorbent were analyzed by Langmuir, Freundlich, Temkin, and Redlich–Peterson models. Based on equilibrium adsorption data, the Langmuir, Freundlich, and Temkin constants were determined as 0.0665, 4.26, and 8.34, respectively, at pH 5.5 and 20°C. Adsorption isotherms were analyzed at different temperatures to obtain free energy, enthalpy, and entropy of adsorption. The method was applied for Cu(II) determination in sea water samples. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The distribution of phosphorus fractions and desorption characteristics of some soils in the moist savanna zone of West Africa

The fractionation of soil P into various organic and inorganic pools with differing levels of bioavailability, coupled with knowledge of the P adsorption and desorption characteristics of the soils, provides insights into management strategies that enhance P availability to crops. Sequential soil P fractionation was conducted on samples from 11 soil profiles and different experimental fields selected from the derived savanna (DS) and northern Guinea savanna (NGS) zones of the West African moist savanna to assess the influence of soil characteristics and management on soil P pools. Phosphorus adsorption and desorption studies were conducted on samples from the surface horizon of the soil profiles. The total P content varied within and among the soil profiles and tended generally to decrease as depth increased. The total P content in topsoil varied from 90 to 198 mg kg^–1 of which about 30% was organically bound P. The resin P fraction was generally low (mean = 5 mg kg^–1, topsoil) and decreased with depth. These low resin P levels indicate low P availability. Within the DS, where the organic resource (OM) was Senna siamea residues, the effects on soil P fractions of OM and soluble P fertilizer (PF), whether sole or in combination, were site-specific. While resin P was significantly increased by OM in some sites, no significant differences were observed in others. In the NGS fields, farmyard manure (organic resource, OM) combined with PF and PF applied alone increased the inorganic P (Pi) fractions extractable with resin, bicarbonate, and NaOH by about 400% but had no significant effect on the organic P (Po) pools and the more stable Pi forms. The P sorption capacities were low, with the adsorption maximum deduced from the Langmuir equation ranging from 36 to 230 mg kg^–1. The amount of P sorbed to maintain 0.2 mg l^–1 in solution ranged between 0.6 and 16 mg kg^–1. Phosphorus desorption with anion exchange resin differed among the soils, with the recovery of added P ranging from 17 to 66% after 96 h. On average, more of the applied P was recovered in the DS soils than in the NGS soils. Because of the relatively low sorption capacity and the relatively high percentage recovery, small additions of P to most of the soils studied might be adequate for crop growth. In essence, quantities of P fertilizer needed in these soils might be estimated based on considerations of P uptake by crops rather than on sorption characteristics.     

HEAVY METALS REMOVAL FROM AQUEOUS SOLUTIONS USING TiO2, MgO,AND Al2O3 NANOPARTICLES

This study investigated the removal of Cd²⁺, Cu²⁺, Ni²⁺, and Pb²⁺ from aqueous solutions using nanoparticle sorbents (TiO₂, MgO, and Al₂O₃) with a range of experimental approaches. The maximum uptake values (sum of four metals) with multiple component solutions were 594.9, 114.6, and 49.4 mg g^?1, for MgO, Al₂O₃, and TiO₂, respectively. The sorption equilibrium isotherms were described using the Freundlich and Langmuir models. The best interpretation for experiment data was given by the Freundlich model for Cd²⁺, Cu²⁺, and Ni²⁺ in single- and multiple-component solutions. A first-order kinetic model adequately described the experimental data using MgO, Al₂O₃, and TiO₂. SEM-EDX both before and after metal sorption and soil solution saturation indices (SI) in MgO nanoparticles indicated that the main sorption mechanism for heavy metals was attributable to adsorption and precipitation, whereas heavy metal sorption by TiO₂ and Al₂O₃ adsorbents was due to adsorption. These nanoparticles may potentially be used as efficient sorbents for heavy metal removal from aqueous solutions. MgO nanoparticles were the most promising sorbents because of their high metal uptake.     

Sorption studies for Cd(II) sequestration from aqueous solution on chemically modified Albizia lebbeck

In the present work, a new sorbent was successfully prepared by chemically modifying pods of Albizia lebbeck (AL) by Fenton’s reagent, followed by sodium silicate. Sorption studies were carried out by batch process. The optimum pH was found to be 6. Equilibrium isotherm data were analysed by non-linear curve fitting analysis, to fit Langmuir, Freundlich and Temkin isotherm models. Based on the Langmuir isotherm model, maximum monolayer sorption capacity (qm) was found to be 21.22 mg.g^?1 at 50°C. Breakthrough and exhaustive capacities were found to be 10 and 50 mg.g^?1, respectively. Desorption study showed 95% recovery of Cd(II) ions.     

Phosphorus fractions and adsorption characteristics in grassland soils of varied soil phosphorus status

Characterization of phosphorus (P) in soils is important both agronomically and environmentally, although the outcome may depend on the technique applied. Consequently, we evaluated fractionation and adsorption, individually and jointly, and relevant ancillary soil attributes, to determine the dominant functional characteristics of soil P in 32 fertilized temperate grassland Inceptisols classified by eight soil series, and by two soil-P index and parent material groups. Residual P was low (30.7%) and organic P (Po) prominent, 42.0% vs. 17.5% for equivalent soils in unfertilized natural ecosystems. Labile fractions comprised 6.8% inorganic P (Pi) and 9.1% Po. The proportional increase in high vs. low index soils (Morgan P > 6.0 mg l⁻¹ vs. ≤ 6.0 mg l⁻¹) was higher for Pi, and highest for labile and moderately labile fractions. Only moderately labile Pi and Po differed significantly between soils of limestone and non-limestone origin. Oxalate extractable Fe (Fe_ox) and buffering (EBC) were higher in the latter. The equilibrium P concentration (EPC) was substantially higher in the high index group, and EBC and binding energy (k) substantially lower, with no significant difference in sorption maximum (Pmax). EBC equated with weak to strong buffering in different soil series, and conformed better than k to ancillary attributes. Pmax correlated in order Al_ox > clay > OC > Fe_ox, and more broadly reflected sorption attributes than oxalate-based sorption capacity (PSC). Principal component (PC) analysis showed consistent differentiation of P fractions, mostly labile and moderately labile, in PC 1 vs. adsorption and ancillary attributes in PC 2. However, scatterplots of PC scores showed that adsorption characteristics provided better functional differentiation than P fractions for distinguishing individual soil series, which may have implications in selection and interpretation of extractants not only for environmental but also for agronomic soil-tests.     

Modification and characterization of poly (ethylene terephthalate)‐grafted‐acrylic acid/acryl amide fiber for removal of lead from human plasma and environmental samples

A new fibrous adsorbent was prepared by grafting acrylic acid/acryl amide (AA/AAm) comonomers onto poly (ethylene terephthalate) (PET) fibers. The resulting sorbent has been characterized by Fourier transform infrared (FT‐IR), elemental analysis, thermogravimetric analysis (TGA), FT‐Raman, and scanning electron microscopy (SEM) and studied for the preconcentration and determination of trace Pb (II) ion from human biological fluid and environmental water samples. The optimum pH value for sorption of the metal ion was 8. The sorption capacity of functionalized resin is 44.1 mg g^?1. The chelating sorbent can be reused for 20 cycles of sorption–desorption without any significant change in sorption capacity. A recovery of 100.2% was obtained for the metal ion with 0.5M nitric acid as eluting agent. Effect of grafting yield, shaking time, shape of sorbent, and pH of the medium on adsorption of the metal ion were investigated. Scatchard analysis revealed that the homogeneous binding sites were formed in the polymers. The equilibrium adsorption data of Pb (II) on modified fiber were analyzed by Langmuir, Freundlich, Temkin, and Redlich‐Peterson models. Based on equilibrium adsorption data, the Langmuir, Freundlich, and Temkin constants were determined as 0.236, 10.544, and 9.497 at pH 8 and 20°C, respectively. The method was applied for lead ions determination from human plasma and sea water sample. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Comparison of resin beads and resin membranes for extracting soil phosphate

Six Portuguese soils of varying P sorption capacity were incubated aerobically at 30° C without and with added P in order to give 0.1.mg P L^–1 in the soil solution. Two methods of measuring extractable P were compared: (i) mixed-bed cation-anion-resin beads in bags and (ii) a simpler method with anion-resin membrane only. The bag method extracted about twice and 1.5 times as much as the strip method, respectively, without and with added P. The relationships were much closer after one extraction for 2 hours (r = 0.982, p < 0.01) instead of the cumulative extraction of 24 hours (r = 0.635,p > 0.05.). P recovery after incubation was inversely related to some soil properties as organic matter, buffer capacity, selective dissolution Al forms (Al_ox and Al_d) and P sorption. It is suggested that the simpler resin membrane method is more adequate to assess P for many studies of P reaction with soil. A simpler incubation method was tried, consisting of incubation as a soil suspension in water at a high temperature (50° C). The results suggested that this method gave similar results to aerobic incubation, with the advantage that there was no need to measure the required and final water contents of incubated soil.     

Sorption of labelled phosphate by a Vertisol and an Alfisol of the semi-arid zone of India

Information on phosphate sorption properties of Vertisols is scarce, but can help to explain the different responses of crops to fertilizer P on Vertisols, as compared with Alfisols.Adsorption isotherms for total adsorbed phosphate and isotopically exchangeable phosphate were measured for typical examples of a Vertisol and an Alfisol, occurring in close proximity at the ICRISAT centre. For each soil, the relationships of exchangeable P and total adsorbed P with phosphate solution concentration were described well by the Freundlich isotherm. Neither of the soils adsorbed significant amounts of P in a non-exchangeable form. The Vertisol had a higher capacity and buffer power for phosphate sorption, implying a lower response to fertilizer P. However, all adsorbed P remained in forms labile to³²P, equilibrated for 22 h, so that for equal amounts of CaCl₂ extractable P there was more labile P in the Vertisol. In the absence of added P, the data suggested that the Vertisol maintained a greater level of dissolved and labile P. These observations are in accord with the results of field experiments, where larger applications of P may be required in Vertisols, compared with Alfisols, to achieve the same yield response, but that P is more freely available to crops grown in Vertisols than is suggested by chemical extraction methods for available P.