Improving Bioavailability of Phosphate Rock for Organic Farming

Abstract

The sustainable use of nutrients in agricultural food production represents a major emphasis for international research, and evidence that clearly demonstrates the imbalance between nutrient inputs and outputs exists. Nutrient surpluses exist and are most commonly associated with intensive livestock production and present a particular range of environmentally related issues. Nutrient deficiency can also develop, and organically managed systems highlight the difficulties that are involved in maintaining agronomically acceptable concentrations of soil phosphorus (P). A restricted range of P-containing sources, often having poor solubility, exacerbate these difficulties, and obvious benefits would arise if the availability could be “naturally” enhanced. Slow rates of phosphate rock (PR) solubilization under prevailing soil conditions reduce the general agronomic usefulness and potential benefits that any direct applications might provide. Being able to improve rates of dissolution through some control of the solubilization process would offer widespread potential advantages, particularly with respect to better matching patterns of P supply with crop demand. A variety of pre and postapplication opportunities exist to improve the solubility of rock phosphate. Some of these have particular relevance to organic agriculture where phosphate rock represents an important and acceptable “external” source of P. A range of post-application, farm management practices that include green manures and rotations using crops with favorable traits that improve P utilization have been successfully employed. Here, we emphazise pre-application techniques, especially the co-composting of phosphate rock with various organic by-product materials that include livestock manures and residual vegetable matter. A range of laboratory incubations have demonstrated the underlying mechanisms involved with solubilization. The significance of microbially induced production of organic acids and acidity during composting is particularly important in this respect. While co-composting with phosphate rock offers a great potential that could be developed for use at the individual farm scale, the key controlling factors and underlying mechanisms are far from being fully understood. A possible time sequence of reactions that might be envisaged include an initial production of protons and organic acids leading to the mineralogical dissolution and release of Ca and P, followed finally by an extended period during aging of the compost where secondary reactions appear to influence the form of P. The consequences of composting conditions and individual processes on immediate and longer-term bioavailability of P once field applied are still poorly defined.