Lowering the water solubility of phosphorus fertilisers impacts leaching, plant growth and residual soil phosphorus

Abstract

Context: Leaching of phosphorus (P) from water soluble agricultural fertilisers applied to sandy soil can adversely impact downstream water quality. Less soluble fertilisers may reduce P leaching and increase production. However, repeated application of low water soluble P (LWSP) fertiliser with high citrate soluble P (CSP) has potential to increase P leaching and P accumulation. Methods: We examined the effect of LWSP fertilisers (single superphosphate, SSP; fertilisers low in water soluble P, WSP) on leaching losses in soil with low P retention/low P status; with/without bauxite residue amendment and with/without plants. Key results: On low P retentive soils with a low P status, P leaching was reduced by reducing the WSP of fertiliser. Plants and soil amendment reduced P leaching further, but P loss remained proportional to WSP applied. Under field conditions, a subset of LWSP fertilisers greatly increased pasture dry matter production and increased soil test P values. Soil test P increases were positively correlated to fertiliser citrate soluble P content. Soils already containing at least 80% of the required plant available P did not require a WSP fertiliser to provide early season P. Conclusions: LWSP fertilisers and soil amendment have potential to reduce P leaching in low P retentive soils. Fertiliser formulations targeting reduced WSP and high CSP require attention. Implications: LWSP fertilisers may increase soil P residues reducing future P needs; however, ongoing soil testing is required for assessment of soil P accumulation, requirements and leaching potential.