LONG-TERM KINETICS FOR PHOSPHORUS SORPTION-DESORPTION BY HIGH PHOSPHORUS SOILS FROM IRELAND AND THE DELMARVA PENINSULA, USA1

Abstract

Soil testing to determine phosphorus (P) availability to crops is a well established process. Today, however, there is increasing emphasis on relating existing or new soil tests to the potential for P loss from soils to surface waters. The objective of this study was to determine how well short-term soil P measurements (water soluble P (WSP), Mehlich-1 P, degree of P saturation (DPS), and 1-day desorbable P) predicted long-term P release and P sorption in relation to soil properties. Topsoils and subsoils with widely differing properties were collected from four sites in Northern Ireland, the Republic of Ireland, and the U.S. mid-Atlantic coastal plain, with topsoils and subsoils sampled at each site. All soils were analyzed for water soluble P, Mehlich-1 P, oxalate extractable Al, Fe, and P (Alox, Feox, Pox), degree of P saturation (DPS = (Pox/0.5[Alox+Feox]) × 100, free [Alox+Feox] = 0.5[Alox+Feox]−Pox), long-term desorbable P (using Fe-oxide-filled dialysis membranes), and long-term P sorption for "remaining P sorption capacity" (from a solution maintained at 5 mg P L−1). Long-term desorbable P followed a pattern of initial fast P release followed by a slower release of P that was still in progress after 39 days. Water soluble P, Mehlich-1 P, and the DPS were all correlated with the cumulative amount of P desorbed in 39 days (r = 0.82*, 0.79*, and 0.83*, respectively). However, for short-term (1 day) desorbable P, correlations followed the order WSP (r = 0.94***) > DPS (r = 0.83*) > Mehlich-1 P (r = 0.72*). When P was added to the soils, all of the soils exhibited an initial period of rapid P sorption, followed by a period of slower sorption still in progress after 38 days. The soil components found to be related most closely to remaining P sorption capacity were free [Alox+Feox] (r = 0.73*) and free Alox (r = 0.80*), indicating that amorphous Fe and Al are the major soil components responsible for long-term (38 days) P sorption. Overall, a single oxalate extraction for Al, Fe, and P proved to be most useful for predicting both long-term P release, through calculation of the DPS and for predicting the ability of the soils to sorb more P by calculating free [Alox+Feox].