Phosphate Equilibria in Soils in Relation to Added P, Sesbania aculeata Incorporation and Cropping - A Study of Solubility Relationships

Laboratory and field experiments were conducled on ten soils varying in pH and calcium carbonate content. A significant lowering of phosphate potential was observed after application of 36 mg phosphate kg⁻¹ and incorporation of dhaincha (Sesbania aculeata) into the soils. Lime potential was only slightly affected. Phosphorus solubility is other indicated an under-saturation with respect to octocalcium phosphate in most of the neutral and alkaline soils while the acid soils appeared to be in equilibrium with strengite. After cropping, more stable P compounds were found in the untreated soils. Cropping depleted the less stable P compounds resulting in the persistence of less soluble P compounds compared to incubated samples in the absence of plants. Less stable P compounds persisted in the phosphate treated soils. The effect of incorporation of sesbania was only marginal.