Inorganic and Organic Phosphorus in Soil Affected by Compost, Tillage and Cropping System.

Excessive fertilization with organic or inorganic phosphorus (P) amendments increases the potential risk of P losses to surface waters. An understanding of the P fractions in soil is essential for proper management of fertilizer amendments. The objective of this study was to evaluate the distribution of organic and inorganic P forms in soil as affected by compost amendments, tillage, and cropping system. Soil samples were collected from the surface (0-7.5 cm) layer of plots under a corn-soybean-wheat/clover rotation, with or without fall compost application, and managed with moldboard plow, chisel plow, or no-tillage since 1988. Seven fractions of organic P and four fractions of inorganic P were separated from subsamples of Clarion silt loam and Canisteo silty clay loam. Long-term compost application increased all inorganic P fractions, as well as labile organic P and biomass P in the soil. The low C:P ratio of the compost likely increased soil P mineralization, leading to the differences we measured. Tillage had little effect on the organic P fractions, but greater mixing of the soil significantly decreased most of the inorganic P fractions. No-till plots consistently had higher amounts of inorganic P and labile organic P. Total inorganic P was the only soil P fraction affected by cropping system, with the highest levels found when soybean was grown, followed by corn and wheat. In general, total inorganic P and labile organic P were the soil P fractions most affected by the various treatment combinations. Results of this study suggest that long-term application of compost with a low C:P ratio can increase the availability of soil P to both plants and the environment.