Over the last several decades, agricultural sources of P have been suggested as a contributing factor to water quality degradation (U.S. EPA, 1996). Although industrial wastes, municipal wastes, and urban runoff contribute to P loading in certain watersheds, P applied directly to cropland through fertilizers and animal wastes can be contaminant sources of P in watersheds with predominately agricultural land uses (USDA-NRCS, 1994). Whereas fertilizer P use has increased slightly in the last decade (Terry and Kirby, 2000), substantial growth of confined animal feeding operations (CAFO) has occurred (USDA-NASS, 1999).
In general, P applied at recommended rates, soil test P generally remains the same or increases slightly with time depending on P rate, soil type, and crop removal (Havlin et al., 1999). Because manure rates are typically based on crop N requirement, concomitant P rates are two- to fivefold greater than crop needs. Continued overapplication of P will increase soil test P and subsequent risk of P loss to surface and groundwater. In North Carolina, P applied in animal waste exceeded P requirements of all agronomic crops in nearly 40% of counties, representing about 85% of CAFO in the state. (Crouse et al., 2001)
Because of recent research results regarding P use and water quality, the USDA-NRCS revised the Nutrient Management (590) Field Office Technical Guide to reflect impacts of P use on water quality (USDA-NRCS, 1999). The revised standard required each state to develop a method to quantify potential P loss from agricultural fields.
The objective of this paper is to describe primary P transport mechanisms and factors essential to include in developing relatively simple and practical tools to quantify P loss in agricultural fields (Figure 1).