Phosphorus management on extensive organic and low-input farms
P. S. CornishSchool of Natural Sciences, University of Western Sydney (Hawkesbury Campus), Locked Bag 1797, Penrith South DC, Penrith, NSW 1797, Australia. Email: p.cornish@uws.edu.au
Crop and Pasture Science 60(2) 105-115 https://doi.org/10.1071/CP07134
Submitted: 9 May 2007 Accepted: 21 February 2008 Published: 27 February 2009
Abstract
A synthesis of the Australian literature reporting soil and plant phosphorus (P) status under organic methods of broadacre farming provides clear evidence that available soil P is lower in organic systems, although there have been no reports of farm P balances that might help to explain the lower P concentrations. There is also evidence, which is largely circumstantial, to suggest that P deficiency significantly reduces productivity of broadacre organic farms, but few experiments prove this conclusively because of other confounding factors. An overview of international literature suggests similar findings for mixed farms. Nine case studies further examined the constraints imposed by P on broadacre organic and low-input farms in Australia. Two farms on fertile soils had negative P balances but maintained productivity without fertilisers by ‘mining available’ P reserves. Five extensive organic farms on inherently less fertile soils had positive P balances because P fertiliser was used. Four of these farmers reported low productivity, which was supported by comparisons of wheat yields with estimated water-limited potential yields. Low productivity appeared to be related to P deficiency despite the use of allowable mineral fertilisers, mostly reactive phosphate rock (RPR), on these farms. The apparent ineffectiveness of RPR is most likely due to the modest rainfall at these farms (380–580 mm/year). The highest research priority is to develop effective, allowable fertilisers. Until this has been achieved, or ways of using less labile P have been developed, there is a case for derogation in the Certification Standards to allow the use of soluble forms of P fertiliser on soils with low soil solution P and high soil P-sorption. Two low-input farms practicing pasture-cropping had approximately balanced P budgets and from this perspective were the most sustainable of the farms studied.
Additional keywords: Australia, P balance, grain, grazing, soil fertility, pasture-cropping.
Acknowledgments
The Rural Industries Research and Development Corporation sponsored a workshop with the case-study farmers and scientists who had expertise in soil–plant P relations and organic farming. My thanks go to the farmers for their valuable data, and to all workshop participants for their critical insights. Thanks also to Dr Paul Maeder and Dr Astrid Oberson for helpful comments on the manuscript, and to Ms Kate Hamer, formerly Certification and Compliance Manager for the National Association for Sustainable Agriculture Australia, for evaluating and commenting on the case-study data and conclusions from an industry perspective.
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