New fertiliser options for managing phosphorus for organic and low-input farming systems

Plant-available phosphorus (P) has been found to be limiting crop and pasture production in Australian dryland, broadacre, organic farming systems. The present review examines the mechanisms that act to provide organic sources of P to soil or mobilise P stored within the soil. A range of products is available to exploit one or more of these mechanisms to achieve a claimed improvement in P fertility. These products are described, and where possible, scientific research of their effectiveness is reviewed. The use of microbial inoculants, although successful in laboratory and glasshouse experiments, has returned varied results in field trials. The addition of organic fertilisers, such as composted or elemental sulfur (S) enriched reactive phosphate rock (RPR), tended to produce more reliable results. The variable nature of the composting process creates complexity in the production of composted RPR. The increased dissolution of RPR by the oxidation of added S has been successful in increasing available P content above that of RPR alone. This is especially significant to low-rainfall areas where RPR tend to be ineffective. This paper highlights the need for development and optimisation of the many organic fertilisers and additives available to organic producers. In all cases, products still require rigorous field and economic evaluation so that organic producers can be confident in making decisions that are informed, correct, and profitable with regard to P fertility. The alleviation of P deficiency is vital to the increased adoption and sustainability of boardacre organic farming in Australia.