Modeling time-dependent phosphate buffering capacity in different soils as affected by bicarbonate and silicate ions

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1 CSIRO PUBLISHING Australian Journal of Soil Research, 28, 46, Modeling time-dependent phosphate buffering capacity in different soils as affected by bicarbonate and silicate ions Nirmal De A,B and Samar Chandra Datta A,C A Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 1112, India. B Present address: Department of Soil Science & Agricultural Chemistry, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi 2215, UP, India. C Corresponding author. samar1953@yahoo.com Abstract. The aim of this paper was to establish a relationship between phosphate buffering capacity and time in the presence of specifically sorbed anions like bicarbonate and silicate. P sorption isotherms were obtained at different times of equilibration for 3 surface soil samples, namely, Typic Haplustept, Calcic Chromustert, and Ultic Paleustalf in 3 different systems namely, bicarbonate (.1 M), silicate (.1 M), and a control system without any bicarbonate or silicate and having a common concentration of.1 M NaCl. Phosphate sorption data at different times could be fitted very closely to a modified Freundlich equation of the form: X/m = KC n t p, where X/m is the amount of phosphate sorbed at solution phosphate concentration C and time t in hours. The values of n and p were positive fractions (mostly) and found to vary with soils and ionic medium. The silicate system was more effective in decreasing P sorption. Phosphate buffering capacity, defined as the first-order partial derivative of X/m with respect to C, Knt p C n 1, was calculated at a particular concentration of.3 mg/l (usual P concentration of soil solution) at different times from the optimised value of K, n, and p. Phosphate buffering capacity was maximum in the control system and found to increase with time. Bicarbonate and, particularly, silicate ion decreased buffering capacity drastically and also the rate of change of buffering capacity with time. The practical implications of this decrease in buffering capacity by bicarbonate and silicate ions is discussed. Additional keywords: P sorption isotherm, time, silicate, bicarbonate, Inceptisol, Vertisol, Alfisol. Introduction Phosphate sorption isotherms are frequently used in modelling P dynamics in soil as well as to recommend phosphate fertiliser dose for optimum P uptake and yield. Several reports (Fox and Kamprath 197; Memon et al. 1991; Samadi 23) use phosphate sorption isotherms to determine the quantity of fertiliser required to bring solution P concentration to an optimum value, say.2.4 mg/l (Samadi 23). This quantity of fertiliser increases with the buffering capacity of the soil (Moody 27). In fact, buffering capacity of a soil is a key factor in making differences in P diffusive flux in different soils. Thus, the diffusive flux, F, is given by: F ¼ðD 1 :f:q=bþðdc=dxþ where, D 1 is the diffusion coefficient of phosphate ion in pure water, f is impedance factor, q is volumetric moisture content, b is buffering capacity, and dc/dx is the concentration gradient of diffusible or labile P in soil. Thus, F is inversely proportional to buffering capacity b and so a decrease in buffering capacity increases diffusive flux. A very common mathematical formulation used to describe P sorption is the Langmuir isotherm. In some cases the Langmuir isotherm fails to describe P sorption data, particularly at higher concentrations of solution P, where the curve does not flatten as it should in the case of surface of a constant sorption maximum. This behaviour was attributed to creation of fresh sorption sites by rupturing Al O Si bonds (Rajan et al. 197) or multiple-layer sorption followed by precipitation (Datta and Aggarwal 1998; Datta 22). In cases of variable sorption capacity where the Langmuir isotherm fails, the Freundlich isotherm often could be fitted well because of its flexibility. But neither the Langmuir nor Freundlich isotherm involves time explicitly in its expression, which is an important parameter of P sorption because real equilibrium is hardly attained with soil solution. As described by Barrow(198), the reaction between soil and P in solution takes place in 2 steps a rapid step in which some of the phosphate is adsorbedonthesoilsurface,andaslowerstepinwhichsomeofthe phosphate is converted into a more firmly held form. Because of thiscontinuing slowreaction, phosphate-bufferingcapacity ofsoil increases with time. Barrow and Shah (1975) developed one expression by incorporating the level of added P, time, and temperature to describe the long-term reaction of phosphate; P concentration in soil solution after time t is given by: P s ¼ b ðp b 1 a =tb 2 Þexpðb3 =TÞ where P a is the level of P addition; T is temperature in degrees K; and b, b 1, b 2, and b 3 are constants. Kuo and Lotse (1974) Ó CSIRO /SR /8/4315

2 316 Australian Journal of Soil Research N. De and S. C. Datta derived an equation to describe the rate of disappearance of P from solution as: q ¼ kc t 1=m where q is mg of P adsorbed per g of solid, c o is the initial P concentration in solution, t is reaction time, and k and m are constants. Both of these equations involve time but they lack the relationship between adsorbed and solution phosphate concentration in a form from which buffering capacity can be calculated. Goldberg and Sposito (1984) modelled effectively the phosphate adsorption envelope by application of a constant capacitance model (Stumm et al. 198) involving surface complexation. This involves chemical reaction of a surface hydroxyl group with phosphate anion species. But the limitations (Zachara and Westall 1999) of the model for application to a heterogeneous system such as soil are due to difficulties in characterising: (1) the surface reaction and identity of the adsorbed species, (2) the concentration of reactive sites, and (3) the distribution of these sites between sorbing phases. The presence of other weak acid anions which may be present in soil solution in appreciable quantities may further complicate the process. For example, bicarbonate and silicate ions, which may be present in appreciable quantities in soil solution during active decomposition of straw, may compete with phosphate ion species for the same sites, which is expected to increase P concentration in soil solution. Incorporation of organic residues, particularly paddy straw (rich in silicate), in soil increases bicarbonate and silicate ion concentration in solution to as high as.1 mm. Hence, P sorption studies in the presence of these specifically adsorbed ions will simulate the real situation. The objective of the present investigation is to predict or model mathematically P sorption and, hence, P buffering capacity as bivariate function of both time and P concentration in soil solution in the presence of competing anions such as bicarbonate and silicate. We propose the following modified Freundlich kinetic equation of P sorption in relation to solution concentration and time: X =m¼kc n t p ð1þ where X/m and C are the amount of phosphate sorbed per unit mass of soil and phosphate concentration in solution, respectively, after time t; K, n, and p are constants. This equation will be tested for its validity describing the longterm effect of P sorption by soil in the presence of competitive bicarbonate and silicate ions. Once Eqn 1 is validated, buffering capacity will be obtained by partial differentiation of X/m with respect to C, i.e.: Buffering capacity ¼ qx =m qc ¼ KtP nc n 1 Materials and methods Soils Three surface soil samples, namely Typic Haplustept, Calcic Chromustert, and Ultic Paleustalf, were collected, respectively, from IARI farm (New Delhi), Nagpur (Maharashtra), and Midnapur (West Bengal) in India, The initial properties of the soils and the methodology followed are listed in Table 1. X-ray diffraction analysis of the 3 soil clays were done according to Jackson (1956). Table 1. Initial properties of the soils CEC, AEC: Cation, anion exchange capacity Properties Vertisol Inceptisol Alfisol Methodology followed Taxonomic classification Calcic Chromustert Typic Haplustept Ultic Paleustalf ph (soil : water 1 : 2.5) Jackson (1967) CEC (cmol c /kg) Koenigs et al. (198); AEC (cmol c /kg) MgSO 4 as impregnating salt; Mg estimated by titration and SO 4 by turbidimetry Organic C (g/kg) Walkely and Black (1934) Clay (%) Jackson (1956) Silt (%) Sand (%) Free CaCO 3 (%) Nil Piper (195) Available P 2 O 5 (kg/ha) Jackson (1967) P-fractions (mg/kg): Non occluded Al- and Fe-P Olsen and Sommers (1982) Occluded P w/i Fe-oxides and hydroxides Ca-P Amorphous material derived (%): SiO Hashimoto and Jackson (196) Al 2 O Fe 2 O Total

3 P buffering affected by bicarbonate and silicate ions Australian Journal of Soil Research 317 Analytical technique The method consists of shaking 2-g soil samples with 4-mL solutions having different P concentrations (, 2., 5., 1., 15., and 2. mg/l) prepared in 3 different media: (1).1 M NaCl (blank system), (2).1 M NaCl þ.1 M NaHCO 3 (bicarbonate system), (3).1 M NaCl þ.1 M Na 2 SiO 3 (silicate system). The.1 M NaCl was common for all 3 media to maintain a constant ionic strength. The shaking was done in a thermostatic water bath (258C) for different periods from.5 to 168 h. At the end of each period of shaking, duplicate samples were centrifuged at 2g for 1 min and filtered through Whatman no. 1 filter paper. Phosphorus content of the clear supernatant solution was determined spectrophotometrically at 66 nm after developing colour by a chlorostannous-reduced molybdophosphoric blue colour method, in sulfuric acid system, which provides non-interference of Si in solution up to 2 mg/l (Jackson 1967). The amount of phosphate sorbed was calculated from the changes in solution P concentration after shaking. Amount of sorbed P at different times were plotted against solution P concentrations for all the three different media and different curves were obtained. Curves for the Vertisols only have been shown in Figs 1 3. Predicted X/m (mg/kg) y =.9844x R 2 =.9365 s.e. = 2.6 K = 59.5 n =.53 p = (c) y =.993x R 2 =.9597 s.e. = 69.4 y =.9971x R 2 =.9869 s.e. = K = 65.9 n =.51 p =.2 K = 2.69 n =.71 p =.79 Fitting of data to modified Freundlich equation Equation 1 gives the modified Freundlich equation: X =m ¼ KC n t p where X/m is the amount of P adsorbed per unit of soil; C is phosphate ion concentration at equilibrium; t is time of shaking; and K, n, and p are empirical constants. Data were fitted to the Eqn 1 by optimising K, n, and p by a non-linear regression technique. This was done by minimising sum of square of deviations between experimental and calculated values of X/m from different experimental values of C at different time t. Initially, we make a guess of the values of K, n, and p and calculate the value of X/m with known values of t and C (from experiment) and then obtain the deviation of the calculated value from the corresponding experimental value of X/m. Thus, for a series of deviations we calculate sum of square of deviations or residual. The next step is to minimise the sum of squares. This minimisation can be done by several numerical techniques; one of them is the Newton Raphson technique. During this minimisation process, values of the parameters K, n, and p change automatically, and when there is no further change in the value of residual, the corresponding values of the parameters are accepted as optimised values. This optimisation was done by using a solver program (Microsoft Excel 2, Microsoft Corporation USA) with level of convergence as.1. To check the closeness of fit, regression analysis has been done between calculated and observed values of X/m, and standard error of regression has been calculated. The optimised values of parameters K, n, and p are shown in Table 2 for different soils and ionic media Observed X/m (mg/kg) Fig. 1. Regression between observed P sorbed and predicted P sorbed estimated by using equation X/m = KC n t p for a Vertisol in control, bicarbonate, and (c) silicate system. Optimised value of the parameters K, n, and p are shown inside the figures. Results and discussions Properties of the soils According to the mechanical composition of the soils (Table 1), the Vertisol belonged to clay loam textural class, while the Inceptisol and Alfisol belonged to sandy loam textural class. With respect to organic carbon content (g/kg), the Vertisol contained the greatest amount of organic carbon (6.7), followed by Inceptisol (4.7) and Alfisol (4.1). The CEC [cmol(+)/kg] of the Vertisol was highest (57.6), followed by the Alfisol (9.8) and Inceptisol (7.9), whereas the AEC (cmol( )/kg) was highest in the Vertisol (1.4), followed by Inceptisol (1.1) and Alfisol (.6). The ph of the Inceptisol was slightly basic (7.8), the Vertisol was neutral (7.2), and the Alfisol was acidic (5.4). Among the different P fractions, Ca-bound P (mg/kg) was the dominant fraction in the Vertisol (381.6) and Inceptisol (315.8). The nextdominant fraction was non-occluded Al- and Fe-bound P, which was highest in the Inceptisol (57.9), followed by Alfisol (34.2) and Vertisol (1.5). P occluded within Fe-oxides and hydrous oxides fractions was highest in the Alfisol (52.6), followed by Inceptisol (1) and Vertisol (4). Based on high value of free CaCO 3 content, the Vertisol was found to be a calcareous soil. Vertisol clay was dominant in smectite (smectite 83%, illite 7%, kaolinite 6%, chlorite 4%). Inceptisol clay was dominant in illite (illite 45%, kaolinite 26%, chlorite 18% smectite 11%), whereas

4 318 Australian Journal of Soil Research N. De and S. C. Datta y =.985x R 2 =.9233 s.e. = K = 57.8 n =.6 p = y =.972x R 2 =.883 s.e. = 22.8 K = 34.4 n =.56 p = Predicted X/m (mg/kg) y =.9951x R 2 =.9799 s.e. = K = 5.99 n =.43 p = Predicted X/m (mg/kg) 2 1 y =.978x R 2 =.846 s.e. = K = n =.46 p = (c) y =.9685x R 2 =.894 s.e. = K =.6 n = 1.78 p = (c) y =.9944x R 2 =.9676 s.e. = K = n =.36 p = Observed X/m (mg/kg) Observed X/m (mg/kg) Fig. 2. Regression between observed P sorbed and predicted P sorbed estimated by using equation X/m = KC n t p for Inceptisol in control, bicarbonate, and (c) silicate system. Optimised value of the parameters K, n, and p are shown inside the figures. Fig. 3. Regression between observed P sorbed and predicted P sorbed estimated by using equation X/m = KC n t p for Alfisol in control, bicarbonate, and (c) silicate system. Optimised value of the parameters K, n, and p are shown inside the figures. Alfisol clay was dominant in kaolinit (kaolinite 57%, illite 43%). Amorphous material derived SiO 2% þ Al 2 O 3% þ Fe 2 O 3% was maximum in the Inceptisol (21.7%), followed by Alfisol (2.5%) and Vertisol (16.7%) Closeness of data fitting Table 2 shows the optimised values of phosphate sorption parameters, i.e. K, n, and p, for all the soils in control, bicarbonate, and silicate systems. The optimisation has been done by minimising the sum of squares of deviation of the experimental value of P sorbed (X/m) from that of the predicted values according to the equation X/m = KC n t p. Figures 1, 2 and 3 show the regression between observed and predicted values of P sorbed for 3 soils each in 3 different media. Data have been fitted to a straight line passing through the origin. From the equation of the regression line, it is evident that in general there has been close agreement between observed and Table 2. Optimised values of phosphate sorption parameters K, n,p Medium Soil K n p Control Vertisol Inceptisol Alfisol Bicarbonate Vertisol Inceptisol Alfisol Silicate Vertisol Inceptisol Alfisol predicted values as indicated by high R 2 values, which range from.87 to.98, and low value of standard error of the predicted Y-value for each X in the regression. The slope of the regression equation is very close to 1., which indicates an

5 P buffering affected by bicarbonate and silicate ions Australian Journal of Soil Research 319 approximate 1 : 1 line. This shows that our hypothesis of expressing kinetic P sorption data through the modified Freundlich equation X/m = KC n t p is statistically valid. Sorption parameters Affinity parameter, K The constant, K, indicates general affinity of the soil for P sorption. It may be defined as the amount of P sorbed at unit time and unit concentration. In the control system (Table 2), the Vertisol had the maximum value of K (65.9), followed by the Inceptisol (57.8) and Alfisol (34.). The highest K value of the Vertisol was due to maximum clay content (56%) and maximum CaCO 3 content (11.5%) (Table 1). Presence of bicarbonate decreased the K value of the Vertisol from 65.9 to 59.5 and that of the Inceptisol from 57.8 to 5.99; in the case of the Alfisol the decrease was from 34 to Thus, the decrease was maximum in the case of the Inceptisol because of its highest ph (7.8). This decrease was due to the specific adsorption of HCO 3 and CO 2 3 on the same sites as that for P sorption. Specifically adsorbed ions such as silicate and bicarbonate had a decreasing effect on the affinity parameters for all the soils, but there was an interaction effect of medium and soil. For the Vertisol and Inceptisol the effect of medium on K varied as follows: control > bicarbonate > silicate; whereas, in case of Alfisol the sequence was control > silicate > bicarbonate. The effect of silicate on decreasing the affinity parameter, K, of the Alfisol was less than that of the Vertisol and Inceptisol. This was due to the difference in ph of the soils. Maximum sorption of a weak acid anion takes place when the ph is approximately equal to the pk value of that weak acid (Hingston et al. 1968). In the case of silicate, maximum sorption takes place at ph 9.2 (1973). Thus, the competitive effect of silicate will be more effective when the ph is closer to 9.2. The maximum effect of specifically adsorbed ions on P sorption was observed in the case of the Inceptisol of ph 7.8, whereas the silicate ion had least effect on the Alfisol of ph 5.4. On the other hand, carbonic acid had 2 dissociation constants, of which pk 1 is equal to 6.37 and pk 2 is equal to Thus, HCO 3 will be more effective for decreasing the K value in case of the Alfisol than the silicate ion. Effect of P concentration, parameter n The coefficient n indicates the effect of solution concentration on P sorption. A high value of n indicates greater effect of increasing solution P concentration on P sorption. These values are shown in Table 2 for all 3 soils and for all the solution media. Usually, the values of n were found to be in fractions ranging from.36 to.71, indicating the shape of the curve as concave downwards. The high n value of 1.78 in the case of the Inceptisol silicate system indicated concentration recovery of P sorption. In this case, the curve was found to be concave upward. In the low concentration range, P sorption was low due to the competitive effect of silicate, but as P concentration increased the P sorption increased at an increasing rate and recovered from the competitive effect. Changes of P sorption isotherm with time The coefficient p indicates effect of time on P sorption. A high value of t indicates greater effect of increasing time on P sorption. These values are shown in Table 2 for all 3 soils and for all the solution media. Values of p were always found to be fractions varying from as low as.79 in the case of the Vertisol silicate system to as high as.81 in the Inceptisol silicate system. Other values are clustered around.2. The lowest value of p for the Vertisol silicate system indicates that after initial P sorption, there has been little change with time, due to the competitive effect of silicate. On the other hand the very high value of.81 in the Inceptisol silicate system indicates time recovery of P sorption. Initially, there was very little sorption as indicated by the very low value of K of.6, due to the competitive effect of the silicate ion, but it recovered with time. Figures 4, 5, and 6 show P sorption isotherms of the Vertisol in control, bicarbonate, and silicate systems, respectively. For other soils (not shown), the trend was the same. These figures show how these isotherms changed with time. The curves became steeper with time, i.e. slope of the curves increased with time. This was due to the fact that more phosphate gets transformed slowly to the non-labile form. Initially, the changes were rapid, but with time the changes were slower and attained a limiting position. After a long time, there would be virtually no change in the isotherm, but the slope would decrease to its initial value after fresh addition of fertiliser P and would increase with time again. Predicted isotherms (Figs 4b, 5b and 6b) closely follow the experimental isotherms except for the shorter period of shaking. With the shorter period of shaking the observed isotherms were X/m (mg/kg) h 1h 2h 4h 8h 24 h 72 h 12 h 168 h Equilibrium conc. (mg/l) Fig. 4. Effect of time on P sorption isotherm of Vertisol: observed, predicted in control system.

6 32 Australian Journal of Soil Research N. De and S. C. Datta Predicted X/m (mg/kg) Observed X/m (mg/kg) h 1h 2h 4h 8h 24 h 72 h 12 h 168 h h 1h 2h 4h 8h 24 h 72 h 12 h 168 h Equilibrium conc. (mg/l) Fig. 5. Effect of time on P sorption isotherm of Vertisol: observed, predicted in bicarbonate system. concave upwards at higher concentration, which was an indication of lack of equilibrium in the lower concentration range. This might be due to a relaxation effect. P-Buffering capacity If the sorption isotherm is expressed by X/m = KC n t p, then buffering capacity BC at a particular time is expressed by: qx =m BC¼ ¼ Knt p C n 1 qc Thus, buffering capacity is a function of both C and t. The time t starts from the point where fresh addition of phosphatic fertiliser takes place. With time, buffering capacity increases, but with fresh addition of P, buffering capacity again decreases to its original value. This increase in buffering capacity was due to slow conversion of labile P into non-labile P. To study the changes in buffering capacity with time, we will fix the concentration at.3 mg/l, which is usually found in soil solution. Several reports (Fox and Kamprath 197; Memon et al. 1991; Samadi 23) use phosphate sorption isotherms to determine the quantity of fertiliser required to bring solution P concentration to an optimum value,.2.4 mg/l There are several reports of measuring phosphate-buffering capacity in the range.2.35 mg/l. Pena and Torrent (199) and Duffera and Robarge (1999) selected a solution P concentration of.2 mg/l following the suggestion Beckwith (1965) that this t X/m (mg/kg) h 1h 2h 4h 8h 24 h 72 h 96 h 12 h 168 h Equilibrium conc. (mg/l) Fig. 6. Effect of time on P sorption isotherm of Vertisol: observed, predicted in silicate system. concentration, if maintained continuously in soil solution, will provide adequate P to many plants. Even though the P concentration required by plants varies, the P sorption at a solution P concentration of.2 mg/l (standard P requirement) can be used as a standard for comparing the P requirement of different soils (Juo and Fox 1977; Loganathan et al. 1987). Bolland and Allen (23) estimated P-buffering capacity from well-defined P sorption curves when several levels of P were added to soil suspensions, and it was the amount of P sorbed when the concentration of P in the final solution was raised from.25 to.35 mg P/L. Figure 7 shows calculated values of buffering capacity at different time for all soils for the control and the other 2 media. The effect of silicate and bicarbonate ions in decreasing the buffering capacity of soil is evident in Fig. 7. Further, the increase in buffering capacity slows with time. This effect was more pronounced in the Inceptisol. Sui and Thompson (2) also observed a decrease in P-buffering capacity with addition of biosolid amendments and attributed that to the competitive sorption of organic anions released from biosolids on soil colloids by ligand exchange mechanism. The increase in P-buffering capacity resulted in a decrease in phosphate concentration in soil solution, and hence its uptake rate and availability to plant decreases. This can be prevented by silicate addition or by incorporating straw, particularly paddy straw, in soil. Paddy straw is a rich source of Si (Wickramasinghe and Rowell 26)

7 P buffering affected by bicarbonate and silicate ions Australian Journal of Soil Research 321 P buffering capacity (c) Control Bicarbonate Silicate Days Fig. 7. Predicted changes in P-buffering capacity at a fixed P concentration of.3 mg/l with time in control, bicarbonate, and silicate system of Vertisol, Inceptisol, and (c) Alfisol, using the equation: Buffering capacity, BC = Kt p C p 1. Conclusion Modified Freundlich equation X/m = KC n t p could successfully predict P sorption in soil and hence buffering capacity of a soil as Kt p nc n 1 in relation to solution concentration C and time t. PresenceofsilicateandbicarbonateioncoulddecreaseP-buffering capacity by decreasing the values of the parameters K, n, and p. References Barrow NJ (198) Evaluation and utilization of residual phosphorus in soils. In The role of phosphorus in agriculture. p. 34. (ASA-CSSA-SSSA: Madison, WI) Barrow NJ, Shah TC (1975) The slow reaction between soil and anion: 2. Effect of time and temperature on the decrease in phosphate concentration in the soil solution. Soil Science 119, Beckwith RS (1965) Sorbed phosphate at standard supernatant concentration as an estimate of the phosphate needs of soils. Australian Journal of Experimental Agriculture and Animal Husbandry 5, doi: 1.171/EA96552 Bolland MDA, Allen DG (23) Phosphorus sorption by sandy soils from Western Australia: effect of previously sorbed P on P buffer capacity and single-point P sorption indices. Australian Journal of Soil Research 41, doi: 1.171/SR298 Datta SC (22) Threshold levels of release and fixation of phosphorus: Their nature and method of determination. Communications in Soil Science and Plant Analysis 33, doi: 1.181/CSS Datta SC, Aggarwal B (1998) Complex nature of phosphate desorption isotherm Development of a Model. Journal of the Indian Society of Soil Science 46, Duffera M, Robarge W (1999) Soil characteristics and management effects on phosphorus sorption by Highland Plateau soils of Ethiopia. Soil Science Society of America Journal 63, Fox RL, Kamprath EJ (197) Phosphate sorption isotherm for evaluating the phosphate requirements of soils. Soil Science Society of America Proceedings 34, Goldberg S, Sposito GA (1984) Chemical model of phosphate adsorption by soils. I. Reference oxide minerals. Soil Science Society of America Journal 48, Hashimoto I, Jackson ML (196) Rapid dissolution of allophane and kaolinite halloysite after dehydration. Clays and Clay Minerals 7, doi: /CCMN Hingston FJ, Atkinson RJ, Posner AM, Quirk JP (1968) Specific adsorption of anions on goethite. Transaction of 9th International Congress of Soil Science 1, Jackson ML (1956) Soil chemical analysis advanced course. (ML Jackson, University of Wisconsin: Madison, WI) Jackson ML (1967) Soil chemical analysis. (Prentice Hall of India Pvt. Ltd: New Delhi) Juo ASR, Fox RL (1977) Phosphate sorption characteristics of some benchmark soils of West Africa. Soil Science 124, doi: 1.197/ Koenigs FFR, Leffelaar PA, Breimer T, Vollenbroek FA (198) The cation and anion exchange characteristics of soils with a large sesquioxide surface area. Z. Pflanzenernaehr Bodenkunde 143, Kuo S, Lotse EG (1974) Kinetics of phosphate adsorption and desorption by lake sediments. Soil Science Society of America Proceedings 38,5 54. Loganathan P, Isirimah NO, Nwachuku DA (1987) Phosphorus sorption by Ultisols and Inceptisols of the Niger delta in southern Nigeria. Soil Science 144, doi: 1.197/ Memon KS, Puno HK, Fox RL (1991) Phosphate sorption approach for determining phosphorus requirement of wheat in calcareous soils. Fertilizer Research 28, doi: 1.17/BF Moody PW (27) Interpretation of a single-point P buffering index for adjusting critical levels of the Colwell soil P test. Australian Journal of Soil Research 45, doi: 1.171/SR656 Olsen SR, Sommers LE (1982) Phosphorus. In Methods of soil analysis, Part 2. (Eds Page et al.) pp (ASA-SSSA Publication: Madison, WI) Pena F, Torrent J (199) Predicting phosphate sorption in soils of Mediterranean regions. Nutrient Cycling in Agroecosystems 23, Piper CS 195 Soil and plant analysis. (Hans Publishers: Mumbai, India) Rajan SSS, Perptt KW, Saunders MH (197) Identification of phosphate reactive sites of hydrous alumina from proton consumption during phosphate adsorption at constant ph. Journal of the Social Sciences 25, Samadi A (23) Predicting phosphate fertilizer requirement using sorption isotherms in selected calcareous soils of Western Azarbaijan Province. Communications in Soil Science and Plant Analysis 34, doi: 1.181/CSS

8 322 Australian Journal of Soil Research N. De and S. C. Datta Stumm W, Kumert R, Sigg LA (198) Ligand exchange model for the adsorption of inorganic and organic ligands at hydrous oxide interfaces. Croatica Chemica Acta 53, Sui Y, Thompson M (2) Phosphorus sorption, desorption, and buffering capacity in a biosolids-amended Mollisol. Soil Science Society of America Journal 64, Walkely A, Black IA (1934) An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Science 37, Wickramasinghe DB, Rowell DL (26) The release of silicon from amorphous silica and rice straw in Sri Lankan soils. Biology and Fertility of Soils 42, doi: 1.17/s Zachara JH, Westall JC (1999) Chemical modeling of ion adsorption in soils. In Soil physical chemistry. 2nd edn (Ed. DL Spark) (CRC Press: New York) Manuscript received 25 May 27, accepted 2 March 28