Cumulative Contributions of Various Forms of Swine Manure to Soil Test Phosphorus in a Clay Loam Soil Under Long-Term Corn-Soybean.

Determination of long-term cumulative trends of manure form-specific effects on soil phosphorus (P) is essential to nutrient management practices and planning that assures maximized use efficiency while minimizing adverse impacts on water quality. A study was conducted to quantify the cumulative changes in soil test P (STP, Olsen-P) as a function of various forms of swine manure (liquid, LM; solid, SM; and liquid swine manure compost, MC) relative to chemical fertilizer P; and to determine the manure P source co-efficient in a Brookston clay loam soil under corn–soybean rotation from 2004 to 2011. Manure or chemical fertilizer was applied using a P-based approach at the same rate, 100 kg P ha^-1 year^-1 to the corn phase only. Post-harvest STP content increased linearly with the year of application in the 0-30 cm depth, while it remained unchanged in the depths below 30 cm.  The amount of manure-P needed to increase STP by 1.0 mg P kg^-1 was 20, 29 and 24 kg P ha^-1 yr^-1, respectively for LM, SM, and MC if only the surface 0-15 cm soil layer was considered. When the subsoil 15-30 cm layer was included, an amount of 16, 22 and 16 kg P ha^-1 yr^-1 manure-P was required respectively for LM, SM, and MC to increase each mg P kg^-1 of STP. By using a net P addition approach that subtracts  crop P removal from the total P applied, the amount of manure P required to increase each mg P kg^-1 of STP in the 0-30 cm depth were 10, 14 and 11 kg P ha^-1 for LM, SM, and MC, respectively. Manure P source coefficients, determined as the equivalency to chemical fertilizer P, were 1.04, 0.98 and 0.93 in the 0-15cm soil depth, and 1.01, 0.96, and 0.94 in the 0-30cm depth for LM, SM, and MC, respectively. Long-term effects of swine manure on STP varied slightly with its forms, but were all similar to chemical fertilizer P under the corn-soybean rotation cropping system.