A conceptual framework for improving the P efficiency of organic farming without inputs of soluble P fertiliser

The issues for P availability to agricultural plants are 3-fold: the solution concentration of P during early growth (intensity factor); the quantity of Pin the soil 'bank' to meet plant and animal needs (capacity or quantity factor); and the rate at which P becomes available from mineral and organic sources (kinetic factor). These three needs can be met by: (1) applying traditional (manure, compost) and novel (biosuper) sources; (2) modifying or selecting plants for their root architecture, phytase activity, carboxylate excretion, and P translocation ineffciency; (3) encouraging rhizosphere conditions which favour phytase and carboxylase activity; (4) developing symbioses with mycorrhiza and Penicillium species; (5) developing exoenzyme products which release inositol P; (6) adjusting soil pH to 6-7 for maximum availability of native mineral P sources or lowering pH for maximum availability of reactive rock P; (7) developing rotations which maximise organic P cycling. These 7 broad groupings of strategies to improve P nutrition each operate by 1 or more of the 3 mechanisms of quantity, intensity, and kinetic factors. The possible application of these strategies to 'organic' farming is outlined in this review. However, a successful application of these strategies might also improve the P efficiency of conventional agriculture.