Potential Nitrogen Fertilizer Loss from Farm Fields,
Based on Production of 7 Major Crops
Source: Natural Resources Conservation Service - National Resources Inventory
Map Description:
The amount of nitrogen from commercial fertilizer applications that could potentially be available for runoff or leaching was estimated to show which areas of the country had the greatest potential for water quality problems related to nutrient loss from farm fields. Estimates were made by combining data on land use for 1992 from the National Resources Inventory (NRI) with data on commercial fertilizer use and crop yields from the National Agricultural Statistics Service. The NRI is a national survey of land use and soils characteristics that is based on about 800,000 sample points, 300,000 of which are on cropland . The calculation is based on the assumption that when crops take up less nutrients than are available from the amount applied, the excess is potentially available for leaching and runoff. State data on nutrient application rates and percent acres treated for 1992 were imputed to the NRI sample points according to the state and the crop grown. Crops included corn, soybean, wheat, cotton, barley, sorghum, and rice. A per- acre estimate of pounds of excess nitrogen was calculated for each 1992 NRI sample point as the difference between the rate of application per treated acre and the amount of nitrogen estimated to be taken up by the crop and removed from the field at harvest. The amount of nutrients taken up by the harvestable portion of the crop was estimated by multiplying the percent of nutrients in the harvested portion times the county per-acre yield.

Since yields can vary dramatically year to year, a five-year average yield was used, determined using county yield for 1988-1992. Nitrogen from animal waste is not included in these estimates. An additional adjustment was made for legumes grown in the previous years. If the previous legume crop was soybeans, 1 pound of nitrogen credit was assumed for each bushel of soybeans harvested. If one crop of alfalfa existed during the previous 2 years, the nitrogen credit was 90 pounds per acre. If alfalfa crops occurred in the previous 2 years, the nitrogen credit was 135 pounds per acre. Cropping history for the two years prior to 1992 was obtained from the NRI.

The average per-acre rate for the watershed was determined by dividing the excess nitrogen loadings per watershed (accounting for the percent acres treated) by the total acres of non- federal rural land in the watershed. Dividing by the acres of non-federal rural land provides an overall watershed level perspective of the significance of the excess nutrients in terms of the potential to degrade other environments if the nutrients migrated from the farm field. Consequently, watersheds with only a few acres of the 7 crops will generally not score very high, whereas watersheds with a high proportion of the watershed in the 7 crops and where nutrients are applied in amounts greater than amounts removed in the harvest would score high. Red areas of the map include 25 percent of the watersheds with the highest scores. These estimates reflect the amounts of applied nitrogen and phosphate fertilizer that are not taken up by the harvested crop and, as such, may be available for loss to the environment. Not all of these materials will actually move from the field, however.

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