N Deficiency and Excess
N deficiency in crops
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 top photo shows a corn stand with symptoms of N deficiency.
What can occur with excess N?
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 excess 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 us a return if it results in a significant improvement in crop yield and quality.
Applying the right amount of N for a crop
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 the following factors about each field.
- First, estimate the crop's need for N, i.e. a gross N requirement.
- Next, N can become available to crops from many sources, including soil organic matter, plowed or killed sods (e.g. hayfields), and past and current applications of manure. Factoring the contribution of N from these soucres will help make the most efficient use of supplemental fertilizer.
- Then factor how efficiently the crop will use the N, based on the soil type.
- Finally, the need for supplemental N is determined by crediting the N available from all sources against the gross N requirement.
