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 clay 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 capacity (b) varied from 30.9 to 414.3 µg g^–1. Although adsorption capacity was related significantly to a number of soil properties, citrate dithinonite bicarbonate (CDB) extractable Fe was the most important variable accounting for 99% of the variation in adsorption capacity. The solution P concentration (SPC) required to achieve 95% maximum grain yield of rice varied from 0.03 in a sandy clay soil to 0.19 µg 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 function relationships were obtained between SPC and b (r=0.93) and between SPR and b (r=0.93). The P buffering capacity (PBC) of the soils indicated that the soils are moderately buffered. However, SPR accounted best for the variation in grain yield of rice on the field (R²=0.90). The use of P sorption indices in estimating P needs of rice appears superior to the use of chemical extractants.