Phosphorus Loss Assessment in North Carolina

Abstract

Increased concern about potential losses of phosphorus from agricultural fields receiving animal waste has resulted in the implementation of new state and federal regulations related to nutrient management. In response to strengthened nutrient management standards that require consideration of phosphorus (P), North Carolina has developed a site-specific P indexing system called the Phosphorus Loss Assessment Tool (PLAT) to predict relative amounts of potential P loss from agricultural fields.

The objectives of this study were threefold: (i) to predict the percentage and types of farms that will be forced to change management practices due to implementation of the PLAT index (ii) to evaluate the predictive ability of PLAT using sensitivity/uncertainty analysis; (iii) to establish whether the method of predicting soil P used in PLAT, Mehlich-3 soil test P, is an adequate proxy for a more descriptive measure of soil P, namely the oxalate degree of P saturation.

Based on a statistically random sampling of agricultural sites in all 100 counties, approximately 8% of producers in the state will be required to apply animal waste or inorganic fertilizer on a P basis rather than a nitrogen basis, with the percentage increasing for farmers applying animal waste (~27%). The tool predicted the areas in the state that are known to be disproportionately vulnerable to P loss due to histories of high P applications, high densities of animal units, or soil type and landscapes that are most susceptible to P loss. Statistical evaluation of the tool showed that soluble loss pathways, surface runoff and subsurface drainage, impact predictions of P loss more than either loss of P through erosion or applied source. Sensitivity to input factors related to the different methods for estimating soluble losses between naturally and artificially drained conditions varied depending on input values. Predicted P losses from organic soils were drastically more sensitive to soil test P than other soil types. Results from this analysis will help determine which areas to focus resources on in an attempt to improve upon the accuracy of PLAT's predictions of P loss.

This study showed that PSRox was significantly related to Mehlich-3 STP, especially when soils are grouped according to P retention properties. Mehlich-3 extracted 49% of Pox, 88% of Alox, and 15% of Feox. Phosphorus saturation measurements did not appear to be valid for organic soils, as they did not follow the assumption that amorphous iron and aluminum are responsible for the majority of P retention. Phosphorus saturation as determined by Mehlich-3 extraction was significantly correlated to P saturation as determined by oxalate extraction. However, caution should be used before substituting Mehlich-3 PSR for oxalate P saturation due to the inability of Mehlich-3 to account for extractable Fe and thus a soil's P sorption capacity.