Phosphorus desorption kinetics in relation to phosphorus forms and sorption properties of Portuguese acid soils

Although much information exists on the quantitative relationships between phosphorus (P) sorption and soil properties, comparatively less attention has been given to the influence of soil properties and P forms on the kinetics of P desorption and to the relationships between desorbable and sorbed P. The aim of this study was to rationalize the P desorption properties of a group of 29 acid soils representative of agricultural areas of Portugal. The soils differed widely in basic properties, total P concentration (91-1730 mg kg(-1)), Olsen P (2.5-116 mg kg(-1)), and relative contents of the P fractions-defined operationally in accordance with the seven-step fractionation scheme of Ruiz et al. (1997). P sorption capacity was accurately predicted from the concentrations of oxalate-extractable Al and Fe, as is generally the case with acid soils. Desorption of sorbed P to an anion exchange resin could be described by a combination of two kinetic equations of the Michaelis-Menten type for fast and slowly desorbable P pools. Although the concentrations of both pools were correlated with those of various P fractions, they could not be unambiguously assigned to specific chemical P species. The high correlation found between fast desorbable P and Olsen P testifies to the usefulness of this agricultural P test for acid soils; however, Olsen P tended to overestimate and underestimate fast desorbable P for Olsen P values lesser and greater than similar to 80 mg kg(-1), respectively. The average ratio of fast desorbable P to sorbed P was 0.21 and that of total desorbable P to sorbed was P 0.60; both ratios increased with increasing degree of P saturation in the soil.