It has been known for more than 100 years that the soil can supply some nitrogen (N) for crop production, but that additional N is needed to maximize corn yield. If one were to know precisely how much N the crop needs to take up for maximal growth along with the amount of N that the soil could supply, then the difference between these two amounts would constitute the N fertilizer need. Determining the amount of N needed to produce a corn crop can be estimated by determining the amount of N in the above ground biomass (total N uptake). Measurements of total N uptake are relatively easy to obtain, though one would need to use data from previous years as an estimate for the present year. Based on data collected from the North Central Region total N uptake for corn is estimated at 1.35 lb N/bu (Murrell 2005). It should be noted that this is only an estimate and that total N uptake may vary by field, year, and perhaps hybrid. The trickier part of this mass balance approach is estimating soil N supply. Estimation of soil N mineralization has challenged several generations of soil scientists. Thus knowledge of soil N mineralization remains the key to fine tuning N recommendations for corn. The objectives of this paper are to explore factors affecting soil N mineralization and the potential of soil tests as a means to adjust N recommendations to improve profitability and minimize the potential for water quality degradation.