When N is deficient in plants, we often see a reduction in new growth and yield, yellowing (chlorosis) of older leaves, and earlier leaf drop in the fall. New growth may have a red to red-brown color and the plant will have a lower crude protein content. The photo on the right shows a corn stand with symptoms of N deficiency.
An abundance of N exists on some dairy farms. With too much N in the farm system, we often see crops with very dark leaves and stems as in the bottom photo (e.g. for corn, from soil to tassle). An abundance of N can cause excessive vegetative growth, at the expense of grain or fruit yield. Excess N can also lead to increased lodging (i.e. stalks bending and breaking) and delayed maturity, resulting lower yielding and lower quality crops at harvest.
What else could happen if excessive N is available to the crop? Nitrogen that isn't taken up by the crop is more susceptible to losses to the air, groundwater, streams, etc. When N is lost from the root zone and out of reach of the crop's roots, it often represents a financial loss. The investment in additional fertilizer and/or manure applications will only give a return if it results in a significant improvement in crop yield and quality.
The goal is to make the most efficient use of the investment in nitrogen, which means not shorting the crop, but also not applying N in excess. To develop a plan for efficient N use, consider a few factors about each field.
The basics concepts behind this approach are found in the N cycle. Continue to the next page for a tour through the N cycle.