Phosphorus Fractionation and Availability Dynamics in Highly Weathered Soils of Akwa Ibom State, South South Nigeria

This study investigates phosphorus fractionation and availability dynamics in highly weathered Ultisols of Akwa Ibom State, located in Nigeria's South South geopolitical zone. Phosphorus management in highly weathered tropical soils is complicated by high fixation capacity from iron and aluminium oxides, but the relative importance of different soil phosphorus pools to crop nutrition is not well characterised for the specific soil types of the South South zone. Using the Hedley sequential phosphorus fractionation procedure, soils from 20 representative profiles in Ini, Uyo, and Ikot Abasi local government areas were partitioned into resin-extractable P, bicarbonate-P (labile), NaOH-extractable P (Fe/Al-associated), HCl-P (Ca-associated), and residual P fractions. Phosphorus sorption isotherms were determined at six P concentrations. In a parallel greenhouse pot experiment, five phosphorus sources (triple superphosphate, Sokoto rock phosphate, compost, biochar, and no P) were compared for maize phosphorus uptake from three representative soils. Findings revealed that NaOH-extractable iron-associated P was the dominant fraction, accounting for 48 to 68 percent of total P in all sampled soils. Resin-P was critically low (below 5 mg/kg) in 85 percent of profiles. Rock phosphate showed significantly lower dissolution rate than triple superphosphate but comparable plant uptake when applied with compost, attributed to organic acid-mediated dissolution enhancement. The study recommends combined application of organic amendments with rock phosphate as an economical phosphorus management strategy for Akwa Ibom State food crop production. Keywords: phosphorus fractionation, highly weathered soils, Akwa Ibom State, phosphorus availability, Hedley fractionation.