Reactions of Partially Acidulated Phosphate Rock with Soils from the Tropics

Abstract

Abstract Equal amounts of water‐soluble P in the form of granulated partially‐acidulated phosphate rock (20% by H 3 PO 4 ) and concentrated superphosphate (CSP) were added to two Nigerian savanna soils and an Oxisol from Colombia. The amount of water‐extractable P was higher in soils treated with partially‐acidulated phosphate rock. Addition of phosphate rock to CSP resulted in an increase in the water‐extractable P. The results suggest that the presence of phosphate rock in partially acidulated phosphate rock or in the mixture of CSP and phosphate rock slowed down the immobilization of watersoluble P by reacting with some of the acidity produced during monocalcium phosphate hydrolysis, thus reducing the amount of acid available to solubilize soil Al and Fe. Reaction of acid with phosphate rock could release additional P to the watersoluble pool. The relative effectiveness of partially acidulated phosphate rock was highest in the Oxisol which had the highest capacity to sorb P.