An Algorithm for Estimating Amounts of Rock Phosphate Needed to Meet Crop Phosphorus Requirements in West African Soils

An incubation study was conducted (i) to investigate the influence of soil properties on the dissolution of Tilemsi rock phosphate (PNT) and extractability of phosphorus (P) after dissolution and (ii) to develop an algorithm for estimating amounts of rock P needed to meet crop P requirements in West African soils. Subsequently, the amount of phosphate rock (PR) that had dissolved and changes in Bray-P and Olsen-P were respectively determined by measuring the increase in extractable P (Δ NaOH-P) and changes in plant-available P (Δ Olsen-P and Δ Bray-P). The extent of dissolution of PNT ranged from 3.3 and 38.2% of the total P added to the soils. The greatest PNT dissolution was obtained in Niessumana and Longorola soils, which are acid lowland soils. The highest amount of plant-available P was also measured in these soils. Exchangeable acidity, calcium (Ca) saturation, and effective cation exchange capacity appeared to be the factors controlling the PNT dissolution. The amount of rock phosphate dissolved in contrasting Malian soils could be predicted by a modified MacKay's equation of the form: Y = A + (B * e−c * time), where Y = amount of PNT dissolved, measured by NaOH extraction; A = 78.1627 + 30.62149 acidity − 0.63335 CaSat; B = −16.58786 − 21.74709 acidity + 4.07675 ECEC and c = curvature coefficient. The amount of P to reach the optimal yield for a given crop, which is the difference between P critical level and measured P in a given soil, could be estimated using the plant available-P/NaOH-P ratio and the above modified MacKay's equation.