Changes in soluble and equilibrium phosphate concentration in selected soils from Italy

Abstract

Soil phosphate in solution (PsolCpand equilibrium P concentration (EPCo) are important soil P parameters both for agronomical and environmental purposes. Solution P is the main source from which plant roots adsorb P, whereas EPCo give information about the amount and direction of changes between soluble and particulate P that occur during transport of sediment in stream flow. Changes in Psol and EPCo with soil extractable P (Mehlich 3‐P and Olsen P) were determined following equilibration of 13 Italian soils with five rates of fertilizer P (0 to 100 mg kg‐1). The slope of the regression equation (Y=a+bX) for the relationships between Psol and EPCo (Y), and soil extractable P (X) decreased with increasing soil P sorption. Furthermore, at the same level of Mehlich 3 and Olsen P, higher sorbing P soils had lower values of Psol and EPCo. As a result, changes in Psol and EPCo per unit increase of soil extractable P were closely related (R2 of 0.86 to 0.93) to soil P Sorption Index (SI). Equilibrium P Concentration could be estimated from soil solution P for the studied soils.