Fractionation and Bioavailability of Phosphorus in Agricultural Soils Along the Benue Trough Under Different Fertilisation Regimes

Phosphorus availability is frequently a limiting factor for crop productivity in highly weathered tropical soils, yet the chemical forms governing phosphorus bioavailability under varying fertilisation regimes in Benue Trough soils are not well characterised. This study investigates phosphorus fractionation using the modified Hedley sequential extraction procedure in agricultural soils of the Benue Trough, North Central Nigeria, under four fertilisation treatments: unfertilised control, organic compost, single superphosphate, and a combined organic-inorganic treatment. Sampling was conducted at three crop growth stages in a replicated field experiment established across two soil types, namely ferralsols and vertisols. Fractions determined include labile resin-Pi, bicarbonate-extractable Pi and Po, hydroxide-extractable Pi and Po, sonic-hydroxide Po, hydrochloric acid-extractable Pi, and residual P. Crop phosphorus uptake was measured and correlated with extractable fractions. Principal component analysis was applied to identify the phosphorus fractions most predictive of plant uptake. Results show that the combined organic-inorganic treatment significantly increased labile and moderately labile phosphorus fractions in both soil types compared to single fertiliser treatments. Resin-Pi was the most strongly correlated fraction with crop phosphorus uptake (r = 0.89). Vertisols showed greater phosphorus sorption capacity, partially immobilising applied superphosphate into less bioavailable hydroxide-extractable forms. Long-term organic amendment improved Po mineralisation potential, building a supplementary bioavailable pool. The findings support the adoption of integrated fertiliser management strategies to optimise phosphorus use efficiency in the Benue Valley. Keywords: phosphorus fractionation, bioavailability, Benue Trough, fertilisation, soil chemistry