Limitations
These nutrient budgets were developed to demonstrate general trends in cropland nutrient flows in the Mid-Atlantic region. The following limitations should be considered when viewing the budgets:
- Nutrient inputs and outputs for non-agricultural areas and pasture were not considered.
- Manure and fertilizer transfers among states, counties and watersheds were not considered. Although manure and fertilizer transfers may have occurred, they were not documented in the data used for the budget calculations. Eventually estimates of such nutrient transfers will be incorporated into the budgets.
- State-level budget trends were calculated based on nutrient inputs and outputs for the state as a whole. This approach can mask differences in budget trends in different areas of a state. For this reason, state-level trends should not be used to draw conclusions about nutrient budgets for individual counties, regions or watersheds.
- In some cases, animal or crop data were not published in the U.S. Census of Agriculture to avoid disclosure of information about individual farm operations. Undisclosed data were counted as zeros. Such missing information reduces the accuracy of observed budget trends. County budget trends may be less accurate than state budget trends due to undisclosed information at the county level.
- Farm-level budgets were not developed because information about individual farms is not published in the U.S. Census of Agriculture. Nutrient balances at the state, county or watershed scale should not be used to draw conclusions about nutrient balances on individual farms.
- No assumptions are made about the fate of nutrients that are supplied in excess of crop utilization. As such, positive nutrient balances should not be equated with nutrient “loadings” or pollution.
- We assumed uniformity in manure coefficients and crop coefficients to simplify budget calculations. Such assumptions are not necessarily accurate for all farms, counties, watersheds, regions, states or years, but are an objective and practical starting point.
- Only the major crops and major animals were included in the budgets. In some cases a crop or animal that is important in a particular county or watershed might not be included in the budget.
- Only recoverable manure nutrients were considered in these budgets. Other manure nutrients (such as those produced by grazing animals) were assumed not to be available to be managed by farmers and applied to cropland. However, these uncounted nutrients may still have the potential to negatively affect water quality.
- Although the animal and crop data are from the same source (U.S. Census of Agriculture) for all states and budget years, year-to-year comparisons may be complicated by changes in data collection methods in the Census.
- Fertilizer data were based on national and state industry sales reports (fertilizer data sources) because the fertilizer data necessary to develop nutrient budgets were not reported in the U.S. Census of Agriculture. The use of two different types of data sources (the U.S. Census of Agriculture and fertilizer sales reports) may have reduced the accuracy of the budgets.
- Each year's nutrient budget is a snapshot that is not necessarily representative of all years. Weather-related variations in year-to-year crop yields (which affect nutrient budgets) were not considered.
