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Title: Assessment of the Georgia P Index on-farm at the field scale for grassland management

Author
item Butler, David
item Franklin, Dorcas
item CABRERA, M - University Of Georgia
item RISSE, L - University Of Georgia
item RADCLIFFE, D - University Of Georgia
item WEST, L - Natural Resources Conservation Service (NRCS, USDA)
item GASKIN, J - University Of Georgia

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2010
Publication Date: 5/1/2010
Citation: Butler, D.M., Franklin, D.H., Cabrera, M.L., Risse, L.M., Radcliffe, D.E., West, L.T., Gaskin, J.W. 2010. Assessment of the Georgia P Index on-farm at the field scale for grassland management. Journal of Soil and Water Conservation. 65(3):200-210.

Interpretive Summary: In order to better manage agricultural phosphorus (P), most states in the USA have adopted a “P indexing” approach which ranks fields according to potential losses of P. 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. Available data is limited on runoff P from varying agricultural management practices. Data collected on-farm at the field-scale by scientists from USDA ARS J. Phil Campbell Sr., Natural Research Conservation Center, USDA ARS U.S. Horticultural Research Laboratory, The University of Georgia and the USDA NRCS was used to evaluate the performance of the Georgia P Index in grassland management systems. Nine farm fields, managed as pasture or hay systems were outfitted with a total of 28 small in-field runoff collectors (SIRCs) and runoff P, soil P, and management practices were monitored from 2004 to 2007. Nutrient treatment of 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, slope, 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 under rated the risk in 15% of the cases when considering flow-weighted concentrations and in only 2% of the cases when considering loads or mass losses of P. While measured P export was generally low to moderate [< 7.5 kg P/ha/yr (6.7 lbs/acre/yr] 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.

Technical Abstract: In order to better manage agricultural phosphorus (P), most states in the USA have adopted a “P indexing” approach which ranks fields according to potential losses of P. 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. Given the limited nature of available data on runoff P from varying agricultural management practices, data collected on-farm at the field-scale will be useful in testing the performance of the Georgia P Index in grasslands. Nine farm fields, managed as pasture or hay systems were outfitted with a total of 28 small in-field runoff collectors (SIRCs) and runoff P, soil P, and management practices were monitored from 2004 to 2007. Nutrient treatment of 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, slope, 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 under rated the risk in 15% of the cases when considering flow-weighted concentrations and in only 2% of the cases when considering loads or mass losses of P. While measured P export was generally low to moderate [< 7.5 kg P/ha/yr (6.7 lbs/acre/yr] 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.