CHANGES IN SOLUBLE AND EQUILIBRIUM PHOSPHATE CONCENTRATION IN SELECTED SOILS FROM ITALY

Soil phosphate in solution (Psol) and equilibrium P concentration (EPC o) are important soil P parameters both for agronomical and environmen tal purposes. Solution P is the main source from which plant roots ads orb 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 extrac table P (Mehlich 3-P and Olsen P) were determined following equilibrat ion of 13 Italian soils with five rates of fertilizer P (0 to 100 mg k g(-1)). The slope of the regression equation (Y=a+bX) for the relation ships between Psol and EPCo (Y), and soil extractable P (X) decreased with increasing soil P sorption. Furthermore, at the same level of Meh lich 3 and Olsen P, higher sorbing P soils had lower values of Psol an d EPCo. As a result, changes in Psol and EPCo per unit increase of soi l extractable P were closely related (R-2 of 0.86 to 0.93) to soil P S orption Index(SI). Equilibrium P Concentration could be estimated from soil solution P for the studied soils.