Thursday, November 16, 2006 - 8:30 AM

Developing Accurate Nutrient Mass Balances for Delaware Agriculture.

Amy Shober1, Joshua Mcgrath2, and J.T. Sims2. (1) Gulf Coast RECst REC, Univ of Delaware, Dept of Plant & Soil Science, University of Florida/IFAS, 14625 CR 672, Wimauma, DE 33598, (2) Univ of Delaware, 650 Johns Rd, Smyrna, DE 19977

In 1999, Delaware passed a nutrient management law in response to water quality degradation.  While all sources of nutrients are considered in the law, the major environmental concerns were nitrogen (N) and phosphorus (P) losses to water associated with areas of intensive animal agriculture.  Since this law was passed, nutrient management plans have been developed and implemented for over 157,000 ha of agricultural cropland.  In addition, there has been wide scale implementation of best management practices (BMPs) designed to improve water quality, optimize nutrient use, and maintain a profitable agricultural industry in the state.  We assessed the impact of the Delaware nutrient management law on agriculture and water quality by comparing state and countywide soil surface nutrient balances, calculated based on both crop nutrient requirements and crop nutrient removal, for 1997 vs. 2005.  Statewide nutrient surpluses (crop nutrient requirements) decreased from approximately 92 kg N ha-1 and 31 kg P ha-1 in 1997 to 66 kg N ha-1 and 19 kg P ha-1 in 2005.  Decreases in the nutrient surplus were predominantly due to reduced use of commercial fertilizers and lower concentrations of P in poultry litter as a result of addition of phytase to poultry feed.  Nutrient surpluses were not distributed evenly throughout the state.  Most of the excess nutrients were located in Sussex County (82-104 kg N ha-1 and 29-40 kg P ha-1), an area of intensive poultry production.  In contrast, nutrient surpluses were much lower in the predominantly urban New Castle County.  While state and county scale balances provide useful information about the effectiveness of management changes on nutrient surpluses, they cannot accurately pinpoint critical nutrient management areas.  Therefore, we developed a GIS-based nutrient balance for Delaware at the watershed scale to identify areas where implementation of BMPs would provide the most benefit to long-term water quality.