Assessment of the Georgia Phosphorus Index on farm at the field scale for grassland management

Abstract

In order to better manage agricultural phosphorus (P), most states in the United States have adopted a “P indexing” approach that ranks fields according to potential P losses. In Georgia, the Georgia P Index was developed to estimate the risk of bioavailable P loss from agricultural land to surface waters, considering sources of P, transport mechanisms, and management practices. Nine farm fields, managed as pasture or hay systems, were outfitted with 28 small in-field runoff collectors. Runoff P, soil P, and field management practices were monitored from 2004 to 2007. Fields varied from those rich in P (broiler litter or dairy slurry) to those without P amendments (inorganic nitrogen [N] or no amendments). Data relating to nutrient applications, soil properties, soil P, and management were used as input values to determine a Georgia P Index value estimating the risk of P export from each field. Results indicated that the Georgia P Index underrated the risk in only 2% of the cases when considering loads or mass losses of P, partly due to the influence of small annual runoff volumes and thus greater flow-weighted concentrations from some fields. While measured P export was generally low to moderate (<7.5 kg P ha⁻¹ y⁻¹ [<6.7 lb ac⁻¹ yr⁻¹]) from fields rated as a low or medium risk of P export, findings from this study indicated that the Georgia P Index, at times, overestimated the risk of P losses for hay systems and underestimated the risk of P losses for pastures when no amendments were applied.