Phosphorus - the predicament of organic farming
Today on many sites organic farming might neglect phosphorus (P) fertilisation due to soil reserves build up by P fertilisation of former farming systems. Additionally organic farming has restricted itself to the use of non solubilised rock phosphate as mineral fertiliser source that only has limited plant availability on agricultural soils with adequate pH. Also recycling of P from the food chain back to organic agriculture is not consequently realised. These predicaments of organic farming endanger its future sustainability. The article gives an overview on the state of knowledge of the use of topsoil and subsoil P reserves by plants in organic production, on possibilities to activate them by biological measures for direct use by plants and for their redistribution on farms. As in legislation phosphate rock is defined as prevailing mineral fertiliser source for organic farming, a detailed view is given on the use of its P in dependence of plant variety. Also attempts to improve its fertiliser value by mixtures with organic matter are reviewed. It is obvious that plant roots and soil organisms can activate P from the soil and keep it in biological turn-over. This is a promising concept to use soil reserves, but quantification of time-spans offering sufficient P supply just by this approach is not possible. Therefore, while promoting root density, root penetration and biological activity in soils, site specific analyses of plant available P in soil, analyses of field and farm P-budgets as well as of plants P supply are necessary to decide on P-fertilisation. Soluble P sources, preferably from recycling processes, should be developed for future fertilisation demands of organic farming.