Nitrogen (N) fertilizer is one of the most common field production inputs in agriculture. It is necessary to achieve high productivity but threatens ecosystem integrity as it is readily transported to water bodies and emitted to the atmosphere. This chapter reviews a series of studies conducted by investigators at the Soil Ecology and Management Lab in Raleigh, North Carolina, to understand how inherent soil N supply might supplement the need for N fertilizer inputs in corn and forage management systems. The chapter also discusses relevant research prior to 2013 and subsequent research on the correlation and calibration of soil-test biological activity with soil N availability. Laboratory incubations, greenhouse growth trials and yield response trials in the field provide clear evidence that soil-test biological activity can be considered a simple, rapid and robust technique to estimate soil N availability from mineralizable components. A new approach for adjusting N fertilizer recommendations is also provided.