Modern agriculture has become heavily dependent on fossil-fuel-based commercial nitrogen (N) for maintaining soil fertility and high crop yields. In the past, farmers have not had the means to monitor soils routinely for N concentrations, nor have they been able to match yield goals to those concentrations. In addition, the nutrient value of manure applied to fields often is not taken into account before commercial N fertilizer is applied. As a result, N has been over-applied on many midwestern farms, prompting concerns about the cost of commercial inputs into farming and the amount of nitrate in groundwater.