Lemire, E., Taillon, K. M. and Hendershot, W. H. 2006. Using pH-dependent CEC to determine lime requirement. Can. J. Soil Sci. 86: 133-139. Controlling soil pH is important to ensure good crop yield. This study was conducted to determine whether the accuracy of the existing Shoemaker-McLean-Pratt (SMP) pH-buffer method could be improved by using the pH-dependent cation exchange capacity curve (CECpd). Soil pH, SMP and CECpd measurements were performed on 18 acid surface horizon soil samples, with textures from sandy loam to clay loam. These soils were incubated with three levels of calcium carbonate for 12 wk, after which the soil pH and the effective cation exchange capacity (CECe) were measured. The correlation coefficient (R 2 ) for the CECpd and CECe curves was 0.96. The main factor affecting the slope of the curves is the soil organic matter content. The increase of CECe in the soil was also found to be directly proportional to the amount of lime applied, regardless of the type of soil. By using the slope of the Qv versus pH curve for each soil and the relationship between CECe and lime application, we were able to determine the lime required to raise the soil pH in water to 6.5. As an alternative to the current practice of using the SMP buffer, we propose that it should be possible to estimate the pH-dependent CEC curve from measurable soil properties (e.g., organic matter) and to estimate the lime requirement as the difference in CECpd between the existing and desired pH values. Once the slope of the Qv/pH relationship has been determined or estimated for a soil, the only measurement necessary for calculating lime requirement in subsequent years would be the soil pH. The proposed method would provide lime requirement estimates while decreasing the annual cost of soil analysis. En comparant la pente du (Qv) de chaque sol à celle du pH et en utilisant le lien entre la CECe et l'application de chaux, les auteurs ont réussi à établir la quantité de chaux nécessaire pour relever le pH du sol dans l'eau à 6,5. Au lieu de la pratique habituelle qui consiste à utiliser les solutions tamponnées SMP, les auteurs proposent qu'on estime la courbe de la CEC dépendant du pH à partir des propriétés quantifiables du sol (à savoir, la concentration de matière organique), la quantité estimative de chaux requise correspondant à la différence entre le pH existant et la valeur souhaitée sur la CECpd. Une fois qu'on a établi ou estimé le rapport Qv/pH d'un sol, la seule mesure nécessaire pour déterminer la quantité de chaux requise les années subséquentes serait donc le pH du sol. Pareille méthode permettrait d'estimer les besoins en chaux tout en réduisant le coût annuel de l'analyse du sol.
Taillon, K. M. and Hendershot, W. H. 2008. Measurement and modeling of surface charge and cation binding in agricultural soil. Can. J. Soil Sci. 88: 749Á759. Models of metal adsorption have typically been developed and tested for soil components rather than whole soils and at cation concentrations higher than usual environmental conditions. This study investigates whether the non-ideal consistent competitive adsorption (NICA) model can be applied to ion binding in whole soils at low total metal concentrations. Surface charge was measured for 18 agricultural soils from southern Quebec over the pH range 3.5 to 8. The adsorption of Ca, Cd, Cu, Pb and Zn was also measured for three total metal concentrations of 2, 5 and 10 mg L(1 at pH 6 in a 0.005 M Ca(ClO 4 ) 2 solution. NICA model parameters were solved for each soil using the surface charge and adsorption data and a non-linear least squares fitting routine. Two types of binding sites were identified: the first type had a pK a near 4 while the second type had a pK a near 7.5. The first type of binding site contributed the greater proportion of the variable charge over most of the pH range, and the second site was unimportant at the lower pHs. The surface charge was accurately described by the NICA model with a mean R 2 of 0.995. A mean of the surface charge parameters describing H ' binding accounted for more than 95% of the variable charge on the soils. For individual soils, the NICA model gave a fit to the experimental data with mean R 2 values for Ca, Cd, Cu, Pb and Zn of 1.00, 0.906, 0.879 (two concentrations only), 0.825 and 0.918 respectively. When mean adsorption parameters, instead of values determined for each soil individually, are used, the model gave mean R 2 s for Ca, Cd, Cu, Pb and Zn of 0.938, 0.941, 0.998 (two concentrations only), 0.978 and 0.935. It seems that the NICA model can be used to describe the surface charge and adsorption of cations by whole soils. The mean adsorption parameters appear to describe the adsorption behavior of the soil nearly as well as the individually fitted parameters. This implies that mean parameters for these agricultural soils may provide satisfactory predictions for the adsorption behavior of similar soils. En ge´ne´ral, les mode`les d'adsorption des me´taux ont e´te´cre´e´s et teste´s pour les composantes du sol plutoˆt que pour le sol complet, et a`une concentration de cations supe´rieure a`celle habituellement observe´e dans l'environnement. La pre´sente e´tude devait e´tablir si on peut appliquer le mode`le d'adsorption stable concurrentielle en conditions non ide´ales (NICA) a`la liaison des ions dans le sol entier, quand la concentration totale de me´taux est faible. Pour cela, les auteurs ont mesure´la charge superficielle de 18 sols agricoles du sud du Que´bec dont le pH variait de 3,5 a`8. Ils ont aussi mesure´l'adsorption du Ca, du Cd, du Cu, du Pb et du Zn a`trois concentrations totales de me´taux [2, 5 et 10 mg par litre a`pH 6 dans une solution a`0,005M de Ca(ClO 4 ) 2 ]. Ils ont de´termine´les parame...
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