In its most straightforward description, a field nitrogen balance is the difference between the nitrogen accumulated in the crop over a growing season (nitrogen uptake) and the amount of nitrogen made available to the crop (nitrogen supply) within the boundary of a farm field. Thus, the nitrogen balance is the amount of nitrogen applied and released to the soil solution not taken up by the plant. The bigger this number, the greater the amount of nitrogen that is available for loss to the environment. Soil organic matter mineralization will take place in every growing season. Crop residues, past and current manure applications, and fertilizer additions will supply nitrogen as well. All these sources need to be taken into account when calculating a nitrogen balance. While estimates of nitrogen from soil, crop residues, and previous years’ manure application can be somewhat uncertain, nitrogen balances calculated without these components are incomplete and could trigger the incorrect assumption that more fertilizer is needed. Field testing can help farmers become more comfortable with crediting these various sources of nitrogen.