PHOSPHORUS SORPTION AND DESORPTION INDEXES IN SOIL

Information about phosphorus (P) sorption and desorption indices in so ils is useful for environmental evaluations. The objective of this stu dy was to develop P sorption and desorption indices for soils in the B eaurivage River watershed, Quebec, Canada. Thirty samples from the A, B, and C horizons of soils of the five main soil series were taken fro m sites under forest, dairy farms, and farms with a known surplus of m anure. The maximum P sorption capacity (M) was estimated by a 48 h con tact with 0.005M CaCl2 solutions of increasing P concentration while P desorption was measured in water (Pw). The P sorption index (Psi) was measured by contact of 1.5 g P/kg soil for 40 h in 0.01 M KCl at a so il to solution ratio of 1:40. Regression analysis was used to generate equations predicting M, Psi, and Pw from readily available data on so il chemical and physical properties. The maximum P sorption capacity ( M) was very closely related to the Psi (r>0.97*) in all horizons. The M and Psi sorption parameters were related to organic C (OC), Mehlich 3-extractable P (M3P), and ammonium oxalate extractable Fe (FEOX) and Al (ALOX) in the A horizon; to M3P, ALOX, and the sand content in the B horizon; and to OC and ALOX in the C horizon. The Pw was related to pH, OC, M3P, exchangeable Ca in the A horizon; to M3P and sand conten t in the B horizon; and to pH, M3P, and FEOX in the C horizon. The rel atively low coefficients of correlation and the close correlation foun d between M and Psi suggest that the latter should be experimentally m easured rather than predicted to gain information on the P sorption po tential of the soils from this watershed. The results of the present s tudy indicate the M3P can be used in conjunction with other readily av ailable soil data to predict the P desorbability of vulnerable watersh eds.