ESTIMATING SOIL PHOSPHORUS REQUIREMENTS AND LIMITS FROM OXALATE EXTRACT DATA

Journal of Environmental Quality, 2003 May-June issue 

E.M. D'Angelo*, M.V. Vandiviere, W.O. Thom, and F. Sikora

University of Kentucky, Soil & Water Biogeochemistry Laboratory, Department of Agronomy,

N-122 Agricultural Science Building North, Lexington, KY 40546-0091.

Excessive fertilizer and manure phosphorus (P) inputs to soils elevates P in soil solution and surface runoff, which can lead to freshwater eutrophication. Runoff P can be related to soil test P and P sorption saturation, but these approaches are restricted to a limited range of soil types or are difficult to determine on a routine basis. The purpose of this study was to determine whether easily measurable soil characteristics were related to the soil P requirements (P_req, the amount of P sorbed at a particular solution P level). The P_req was determined for eighteen chemically diverse soils from sorption isotherm data (corrected for native sorbed P) and was found to be highly correlated to the sum of oxalate extractable Al and Fe (R²>0.90). Native sorbed P, also determined from oxalate extraction, was subtracted from the P_req to determine soil P limits (P_L, the amount of P that can be added to soil to reach P_req). Using this approach, the P_L to reach 0.2 mg P L^-1 in solution ranged between -92 and 253 mg P kg^-1. Negative values identified soils with surplus P, while positive values showed soils with P deficiency. The results showed that P, Al, and Fe in oxalate extracts of soils held promise for determining P_L to reach up to 10 mg P L^-1 in solution, and potentially runoff, for many soils. The soil oxalate extraction test could be integrated into existing best management practices for improving soil fertility and protecting water quality.