Having an adequate supply of nitrogen (N) is vitally important to a crop for two reasons: First, crop plants need N to form chlorophyll in leaves, and if there is a shortage of chlorophyll, the crop will not be able to effectively harness energy from sunlight to photosynthesize and form sugars that are needed as energy sources to power plant growth, as well as form carbohydrate starches in grain or other storage organs (e.g., tubers in potato); secondly, N is a critical component of amino acids, which are the building blocks for protein formation in plants. A shortage of N results in lower-than-wanted protein contents of crop grains, or leaf and stem tissues of forage crops.
Having enough nitrogen is vitally important for cereal crops, or grass forages, that do not have the capacity to accumulate fixed N through a symbiotic association with rhizobia, as do legume crops (e.g., soybean). These non-legume crops receive N fertilizer applications at one of the three following times, or for some crops, a combination of the possible times:
- At planting
The rate of N applied to crops only receiving ‘preplant’, or only ‘at planting’ N applications, is usually determined based on target crop yields, taking into account residual mineral N determined by preplant soil sampling and testing, and also an estimate of N mineralized from soil organic matter in a typical growing season.
In-crop N applications include:
- Top-dress granular or liquid N fertilizer;
- Side-dressing row crops by placing N in a band to the side of the crop row, using anhydrous ammonia, dry granular N, or liquid N; and
- Foliar application of a liquid N source (e.g., urea ammonium nitrate [UAN], or liquid urea solution).
An important decision made by most farmers is the rate of N fertilizer to apply so a crop has adequate, but not excessive, available N. Historically, blanket N applications were applied to whole fields. Recently, however, variable N rates can be applied based on sub-field management zones, which are assessed separately. This crop N–level assessment can be done in one of the following ways:
- A simple visual leaf greenness assessment, done by an experienced farmer or agronomist. This can be done by comparing crop leaves to a standardized leaf color chart, developed and calibrated for the specific crop species and variety.
- Laboratory analysis of the N content of samples of the last fully expanded crop leaves collected within a field management unit.
- A leaf chlorophyll meter (e.g., Minolta SPAD meter) to measure chlorophyll content of the last fully expanded crop leaves.
- Remotely sense the spectrum of wavelengths reflected off crop leaves. The greater the N level and chlorophyll content, usually, the more red and near-infrared wavelengths are absorbed. These remote sensing measurements can be done in increasing resolution using multi-wavelength cameras or sensors mounted on:
- Unmanned aerial vehicles (UAV);
- In-field application equipment, for on-the-go measurement and fertilizer application; or
- Handheld units containing cameras or sensors.
No matter how the N status of a crop is assessed, there needs to be field research and calibration done to determine crop yield and protein response at the various possible N levels observed in fields. This is done by measuring crop response to a range of N rates, beginning at zero N added, in set units of N rate increases (e.g., 20 or 30 lbs N/ac), up to a level of excessive N where no further yield or protein response is measured.