Extractable Soil Phosphorus Concentrations As Determined By Molybdate Colorimetric Procedures and ICP: Influence of Soil Properties.

Evaluating agronomic soil phosphorus (P) availability is based on two foundational concepts: i) Extraction and measurement of a labile pool of molybdate-reactive P (presumed to be mostly orthophosphate), and ii) Relating this concentration of P to a crop-specific calibration curve to estimate P fertilizer need.  While soil test extractants and specific P guidelines vary by state (e.g., adjustments for pH, crop type, broad cast vs. banded, etc.), soil organic P (P_o) is generally unaccounted for in agronomic and environmental recommendations.  Research has shown that P_o mineralization can contribute substantially to the plant-available P pool and to losses in surface and subsurface runoff water.  We compared P concentrations in soil extracts (either the Modified Morgan or Morgan solution) measured by standard molybdate colorimetric procedures versus P concentrations measured by ICP across a wide range of agricultural and riparian samples.  Samples were collected by standard agronomic sampling procedures in the field and represent mostly Ap horizons.  In general, ICP measured greater P concentrations in extracts across all samples.  Differences were greater in lower P testing soils with higher organic matter content.  For example, in one riparian data set, P concentrations measured by ICP were as much as 1.7-fold greater than measured by molybdate colorimetric procedures, indicating the potential importance of labile organic P and/unreactive P complexes.  On a large agricultural soil sample set from Vermont (n = 29,437), P measured by ICP was only 7% greater on average compared P measured by molybdate reaction. Results suggest further research is needed to better characterize labile organic P forms in soil extracts.