Influence of soil solution Ca concentration on short-term K release and fixation of a loamy soil

The Ca concentration of the soil solution influences K plant nutrition by its influence on K concentration of the soil solution and on soil buffer power through ion exchange and K release or fixation. The effects of the imposed solution Ca concentration on the estimates of these parameters and on these two phenomena were studied on a loamy soil. Potassium sorption and desorption experiments were conducted for 16 h at five initial Ca concentrations (from 0 to 10(-1) M) and followed by the measurement of soil exchangeable K (ammonium acetate extraction). Soil K-Ca exchange properties and the contributions of exchangeable K and non-exchangeable K to K dynamics of the soil-solution system were estimated. The 'Ratio Law' applied for the medium range of Ca concentrations, i.e. 10(-1) M to 10(-3) M. But, it failed for some experiments at small initial Ca concentrations (0 M and 10(-4) M). This failure went with a decrease of the number of sites of great affinity for K in K-Ca ion exchange and/or a decrease of the amount of K not in exchange equilibrium with Ca but extracted by M ammonium acetate. Release of K increased and fixation of K decreased when Ca concentration increased. The relation between the change in the amount of non-exchangeable K during the experiment and the initial constraint (phi) was curvilinear on the large range of phi investigated. But, this relation was independent of Ca concentration. The K concentration of the solution for which neither sorption of K by the soil nor desorption of K from the soil occurred decreased and the slope of the sorption-desorption curve at this K concentration increased when the solution Ca concentration decreased. These two parameters can be considered the K concentration of the soil solution of the soil and the buffer power of the soil, respectively, only if the initial Ca concentration imposed during the sorption-desorption experiments is close to the Ca concentration of the soil solution of the soil. A predictive model of the soil buffer power based on ion exchange and release-fixation properties is proposed. Despite some discrepancies at very low Ca concentrations (<0.5 mM Ca) when 'Ratio Law' did not apply the agreement between calculated and observed values was good. The model permits the correction of the experimentally obtained buffer power for the bias related to the great solution volume: soil weight ratio commonly used during the sorption-desorption experiments.