An Environmental Phosphorus Monitoring Tool for Soils of the Eastern and Midwestern USA.

The loss of legacy phosphorus (P) from agricultural fields due to over-application of fertilizers and manures could result in eutrophication of receiving water bodies. To evaluate P loss risk, a practical approach, the soil P storage capacity (SPSC), has been proposed to estimate the capacity of agricultural soils to act as sinks or sources of P to runoff or leaching. The SPSC is based on a threshold molar ratio of extractable P/(Al+Fe ), called the soil phosphorus saturation ratio (PSR), above which water-extractable P (a surrogate for soil porewater P) abruptly begins to increase. The objective of our study was to test the consistency of the threshold PSR value across a geographic diversity of soils within the Southern /Central/ Chesapeake Bay regions of the US. The threshold PSR for the range of soils (186 samples) is 0.1 (confidence interval: 0.05 to 0.15; p<0.0001) based on P, Al, and Fe as extracted by the Mehlich 3 soil test, indicating that a common threshold is applicable across the geographic range of this study. We further related SPSC to field water quality data (runoff/leaching data) and identified locations where the soil was a P source (PSR > 0.1; SPSC is negative). Results thus far suggest that the PSR/SPSC concept could be used as an environmental P monitor for non-calcareous soils of the Eastern and Central US.