Document Type

Technical Report

Publication Date



Agriculture, Phosphorus, Water quality, Soil test, Surface runoff


Phosphorus in agricultural runoff is often a major cause of accelerated eutrophication of lakes and streams. Previous research has indicated that the amount of dissolved P (DP) in runoff is directly related to P content of the surface soil. Decades of fertilizer application at rates exceeding those of crop uptake have elevated soil test P (STP) levels in areas of intensive crop and livestock production, making this the major source of DP loss in runoff. The objective of our experiment was to relate STP content of Captina silt loam to P concentration and loss in runoff, and determine which STP method correlates best to P levels in runoff. The 57 grass plots used in this study had a wide range of STP levels. A representative soil sample was composited from the 0-2 cm depth of each plot, and STP content was determined by Mehlich III, distilled water, and iron oxide paper strip extraction methods. Simulated rain was applied at 100 mm h-1. Runoff samples were filtered and analyzed for DP content. Regression methods showed STP (regardless of extraction method) was directly related to runoff DP, but extractions with distilled water or iron oxide paper strips gave results that correlated most closely with runoff DP concentrations. Because plot runoff totals were highly variable, total DP load did not correlate well with STP.

Report Number

PUB 168