Long-term kinetics for phosphorus sorption-desorption by high phosphorus soils from ireland and the Delmarva Peninsula, USA

Soil testing to determine phosphorus (P) availability to crops is a well es tablished process. Today, however, there is increasing emphasis on relating existing or new soil tests to the potential for P loss from soils to surfa ce waters. The objective of this study was to determine how well shortterm soil P measurements (water soluble P (WSP), Mehlich-1 P, degree of P satura tion (DPS), and 1-day desorbable P) predicted long-term P release and P sor ption in relation to soil properties. Topsoils and subsoils with widely dif fering properties were collected from four sites in Northern Ireland, the R epublic of Ireland, and the U.S. mid-Atlantic coastal plain, with topsoils and subsoils sampled at each site. All soils were analyzed for water solubl e P, Mehlich-1 P, oxalate extractable Al, Fes and P (Al-ox, Fe-ox, P-ox), d egree of P saturation (DPS = (P-ox/(0.5)[Al-ox+Fe-ox]) x 100, free [Al-ox+F e-ox] = 0.5[Al-ox+Fe-ox]-P-ox), long-term desorbable P (using Fe-oxide-fill ed 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-t erm (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, followe d 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 capacit y were free [Al-ox+Fe-ox] (r = 0.73*) and free Al-ox (r = 0.80*), indicatin g that amorphous Fe and Al are the major soil components responsible for lo ng-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 [Al-ox+Fe-ox].