Research directions : improving plant uptake of soil phosphorus, and reducing dependency on input of phosphorus fertiliser

Long-term use of fertiliser in southern Australia has generally raised the organic and inorganic phosphorus (P) status of soils, although much of this P is not readily available for plant uptake. Inputs of fertiliser P have necessarily exceeded outputs because soluble P fertilisers react with soil constituents and are readily ‘fixed’ as adsorbed P, sparingly-soluble precipitates of P, and organic P, some of which is recalcitrant to mineralisation. Only 10–20% of applied P is directly used by plants in the year of application in most Australian soils, and subsequent use of the residual P rarely exceeds 50% (Holford 1997; Bolland and Gilkes 1998; Bünemann et al. 2006). The balance is mostly found in slowly available forms that are widely regarded as ‘unavailable’. Despite the build-up in soil P, the National Land and Water Resources Audit (NLWA 2001) considered that most soils used for mixed farming were below target concentrations for economic production. The implication is that fertiliser applications will continue with inevitable losses to ‘unavailable’ pools. NLWA (2001) also pointed out that as nutrient concentrations in soil rise to achieve maximum productivity, so the risk of nutrient leakage to the off-farm environment increases. The prospect of reducing fertiliser inputs and using some of this ‘bank’ of residual P is very attractive to farmers faced with narrowing terms of trade and rising concern over the environmental risks associated with rising nutrient concentrations in soil. It is also of interest to the rising number of organic and ‘low-input’ farmers, who were the inspiration for this workshop. The paper addressed the broad question of whether the bank of residual inorganic P and the recalcitrant forms of organic P might be utilised more effectively in productive agricultural systems. The result was a proposal for a program of research that would lead to a fundamental change in the approach to P management on Australian farms based on improved understanding of P dynamics in agro-ecosystems, and with a focus on accessing the pool of slowly available P.