Monday, November 5, 2007

Effect of Animal Manure on Phosphorus Sorption to Soils.

Carl Bolster, USDA-ARS, Animal Waste Management Research Unit, 230 Bennett Lane, Bowling Green, KY 42104 and Karamat Sistani, USDA-ARS, 230 Bennett Lane, Bowling Green, KY 42104.

Sorption studies are commonly used to obtain important parameters controlling the fate of phosphorus (P) in the environment. In most cases P is added as an inorganic salt to a pre-defined background solution such as CaCl2; however, in many areas the application of P to agricultural fields is in the form of animal manure. Given that manure leachate differs significantly in ionic composition, organic matter content, and pH from a CaCl2 solution, it is unclear whether sorption parameters obtained using a CaCl2 matrix will be representative of the sorption behavior of manure-derived P in the environment. Therefore, the purpose of this study was to determine what effect P source (manure vs. KH2PO4) and solution matrix (manure extract vs. CaCl2) has on the sorption behavior of P. Single-point P addition experiments were conducted with KH2PO4 in a 0.01 M CaCl2 solution and compared with those conducted in animal manure extracts. Factors expected to affect P sorption such as pH, ionic strength, concentration of dissolved organic carbon, and ionic composition were also measured. Results show that the sorption behavior of manure-P is different from that of inorganic-P and that the differences are dependent on soil texture and manure type. Though not conclusive, our results suggest that organic matter caused dissolution of metal oxides in the dairy manure solution leading to reduced sorption whereas high pH values in the swine effluent solution led to enhanced sorption though calcium phosphate precipitation. Therefore, applying sorption parameters obtained from CaCl2-P sorption studies to estimate manure-P sorption and transport in the environment may yield erroneous results.