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Phosphorus sorption and desorption properties of soils developed on basic rocks under a subhumid Mediterranean climate


C. Horta. E-mail:


Representative samples of A, B and C horizons from 11 pedons (Luvisols, Solonetzs and Cambisols) developed on intermediate and basic rocks were studied regarding evaluation of phosphorus (P) behaviour, with the aim of delineating site-specific management practices. For this purpose, (i) sorption properties were evaluated by Langmuir and Freundlich isotherms, (ii) soil samples were fertilized to a level of ca. 0.5 mg P/L, estimated by the Freundlich isotherm fitted to each horizon and (iii) the amount of phosphate-P desorbed to a dilute electrolyte mimicking fresh water over periods of up to 60 days at soil:solution ratios of 1:100, 1:1000 and 1:10 000 was measured. The pedons had a slightly acidic or alkaline reaction and a limited capacity to sorb P, sorption seemingly occurring by both adsorption and precipitation. The nature of the sorption processes seemed to change with depth: precipitation dominated in C horizons, adsorption in A horizons and both types of processes being balanced in B horizons. The short-term capacity of the soil to transfer P to water was related to the sorption curve parameters and the nature of the process seemingly controlling sorption. The ability of soil to desorb P increased with decreasing soil:solution ratio and when precipitation was the dominant process in sorption. After P fertilizer application, soils exhibited a high capacity to transfer P to water, thus making it advisable to use a replacement P strategy to prevent P accumulation in the topsoil and its downward movement.