Phased addition of organic and phenolic acids with phosphate fertiliser increases P availability in an acid soil

Abstract

The role of carboxylic acids in increasing soil phosphorus (P) availability has been well established, using both organic and phenolic acids. However, the practical application of this knowledge in farming systems is still a developing research area. This study determined the amount of carboxylic acid required per unit P fertiliser for increased solution P concentrations and the optimum order of application, with carboxylic acids applied before (phased addition), or in conjunction with (co-addition), the P fertiliser. Two inorganic P fertilisers (potassium dihydrogen phosphate, KH2PO4; diammonium phosphate, DAP) were applied to an acid soil in conjunction with seven carboxylic acids (oxalic, malic, citric, 4-hydroxybenzoic, protocatechuic, 4-hydroxycinnamic, and caffeic acids) in a series of short-term adsorption and desorption experiments. When the carboxylic acids were applied to soil without P, they all increased solution P concentrations. When applied with P fertiliser, the highest solution P concentrations were measured when solutions of oxalic, citric, or protocatechuic acids were equilibrated with soil before the addition of DAP fertiliser (phased addition). In comparison, phased addition of KH2PO4 fertiliser with malic or citric acids resulted in the greatest potential for subsequent desorption of P from the soil. It is proposed that coating of DAP fertiliser granules with an organic/phenolic acid blend may enhance P fertiliser efficiency in acidic cropping soil through reduced P loss via adsorption onto soil surfaces.