Predicting Reduction in Phosphorus Storage Capacity over Time in Soils with Manure Additions.
Myrlene Chrysostome, Vimala Nair, Willie Harris, and Donald Graetz. Univ of Florida, 2601 NW 23rd Boulevard #216, Gainesville, FL 32605
Phosphorus (P) concentrations in agricultural soils have substantially increased in the last decades, increasing the risks for surface and ground water contamination through P runoff and leaching. In sandy soils, the potential for P leaching is greater, especially in areas of intensive animal agriculture. The purpose of this study is to assess the reduction in P storage capacity at a site in the Suwannee River Basin (Florida and Georgia) receiving poultry manure additions. Four soil profiles in fields amended with poultry manure were sampled by horizon three times over 36 months. Initial P concentrations were low (less than 40 mg Mehlich 1-P kg-1). Approximately 82 mg Pkg-1 were added to the soils between the first and the third sampling periods. Physical and chemical characteristics of the soils were determined, including water soluble P (WSP), oxalate extractable P, Al, Fe, and Mehlich 1-P. The P saturation ratio (PSR) was calculated as the molar ratio of oxalate extractable P to oxalate extractable Fe and Al. The soil P storage capacity (SPSC) was calculated for individual horizon and by profile, using a PSR threshold of 0.15. The SPSC refers to the amount of P that can be safely added to a certain volume or mass of soil before the soil becomes a P source. The analysis of the surface horizon of the profiles showed an average SPSC decrease from -23 mg P kg-1 to -54 mg P kg-1 after the third sampling. The results also showed average decreases in SPSC (31%) for the entire profile and corresponding average increases in WSP (131%) and Mehlich 1-P (180%) over the three-year period. SPSC can be used to predict site-specific safe P-loading for sandy soils.