Efficacy of Soil Extracting Solutions for Assessing Potential Phosphorus Loss From a Dairy Waste Application Field Soil.

Elevated phosphorus (P) from manure application field (MAF) soils transported to surface waters is a concern throughout the world due to its role in eutrophication.  In Texas, Mehlich III is widely used to determine labile P.  Our goal was to determine why the common labile extractants Olsen, DTPA, and Mehlich III produce different values given the same soil sample.  Ten sites within a dairy MAF were sampled by genetic horizons from fine, mixed, Thermic, Udic Paleustalf (Windthorst) map units.  In addition to determining plant labile P by Olsen, Mehlich III, and DTPA methods, the soluble and total P was determined.  As expected, all P pools decreased with depth, but the Olsen, Mehlich III, and DTPA labile P concentrations were variable.  Mehlich III extracted more plant labile P at neutral and above to slightly alkaline pH levels.   Mehlich III also dissolved more carbonates.  Therefore, assessment of the fraction of labile P that Mehlich III extracts in excess of Olsen and DTPA may warrant further characterization of the carbonate associated P to assess its role in plant nutrition and environmental quality regulation.