Effect of Manure P Speciation On Changes in Soil Test Level.

Understanding how and why soil test phosphorus (P) levels increase with manure and fertilizer application is important to assist in improving estimates of P loss in the Wisconsin P Index. An incubation study investigated the change in soil test P (ΔSTP) after 42 different animal manures (dairy, beef, swine, chicken, turkey, goat, sheep, and horse) or fertilizer were applied at a rate of 40 mg total P kg^-1 to 25 different agriculturally important soils of Wisconsin. Both liquid and solid dairy and swine manure were represented, while for the other species only solid manures were included. Total phosphorus (P) in animal manure ranged between 2.8 and 48.7 g P kg^-1, dry matter basis. Initial soil test P (STP) ranged between 14 and 69 mg P kg^-1. Treated soils were incubated for 10 weeks at 60% of water filled pore space. After incubation, soils were extracted in Bray P-1, the ΔSTP was calculated by subtracting the control STP level from each the treated soil. The ΔSTP for the TSP treatment averaged between 13 to 36 mg P kg^-1. Liquid swine manure increased STP as much as or more than TSP, while solid swine manure increased STP less than or equal to TSP. Unlike swine manure, both solid and liquid dairy manure increased STP similarly to each other and less than or equal to TSP. There were significant interactions between P sources and soil series, indicating that each manure may result in different ΔSTP when applied to different soils. This highlights the importance in understanding how and why manure changes STP differently on different soils. To further investigate the reasons for the results found, enzymatic fractionation of organic P and chemical speciation of inorganic P fractions in the manures are being conducted. These analyses are expected to help understanding how manure P speciation influences the different ΔSTP observed on different soils.