PHOSPHORUS SORPTION CAPACITY OF LOW ACTIVITY CLAY SOILS OF SOUTH WESTERN NIGERIA AND ITS USEFULNESS IN EVALUATING P REQUIREMENT OF RICE

Field and laboratory experiments were conducted on 15 low activity cla y soils in Ogun State of Nigeria to evaluate the relationships between P sorption capacity and some soil properties and the use of sorption indices in evaluating the P requirement of rice. Langmuir adsorption c apacity (b) varied from 30.9 to 414.3 mu g g(-1). Although adsorption capacity was related significantly to a number of soil properties, cit rate dithinonite bicarbonate (CDB) extractable Fe was the most importa nt variable accounting for 99% of the variation in adsorption capacity . The solution P concentration (SPC) required to achieve 95% maximum g rain yield of rice varied from 0.03 in a sandy clay soil to 0.19 mu g ml(-1) in a sandy soil, while the quantity of fertilizer P required to attain the solution P concentration (Standard Phosphate requirement, SPR) varied from 14.1 to 88.7 kg ha(-1). Highly significant power func tion relationships were obtained between SPC and b (r = 0.93) and betw een SPR and b (r = 0.93). The P buffering capacity (PBC) of the soils indicated that the soils are moderately buffered. However, SPR account ed best for the variation in grain yield of rice on the field (R-2 = 0 .90). The use of P sorption indices in estimating P needs of rice appe ars superior to the use of chemical extractants.