Plants need nitrogen to grow and produce high-quality crops. How much will be required is difficult to predict with absolute certainty. What we do know is that the soil in crop fields can be a very important source of nitrogen for crop growth. Farmers and crop advisors will need to better understand a soil's ability to supply nitrogen to ensure that added nitrogen from fertilizer and manure is enough for optimum yields. Estimating the nitrogen contribution from soil is challenging as weather conditions and management can greatly influence the supply. However, some estimation is needed as mistakes in nitrogen fertilizer management can be costly.