Phosphate retention and extractability in soils of the humid zone in West Africa

Phosphate adsorption and desorption studies provide insight for developing P management strategies. Very few studies have concentrated on P desorption which provides information on the reversibility of adsorbed P. Phosphate a dsorption and desorption studies were carried out on seven rice-growing soi ls from three countries in West Africa, with the objective of relating thes e processes to P management strategies for the soils. The standard P requir ement (SPR) of soils, defined as the amount of P to be added to attain an e quilibrium solution concentration of 0.2 mg P l(-1), varied from 42 to 175 mg P kg(-1). Out of the seven soils, four required low amounts and the othe r three needed relatively higher P applications. The extractability of the adsorbed P in different extractants was: 0.01 M KCl 32-78%, Olsen P 21-112% , and Bray 1 P 15-40%. Differences in the amounts of P desorbed by the soil s suggested that the critical P levels needed for P management must be diff erent. Management options to increase P availability in the soils are sugge sted. (C) 2001 Elsevier Science B.V. All rights reserved.