Season long availability of nutrients, especially nitrogen (N), is essential in corn production. To ensure adequate supplies of N are available to the crop, producers can utilize several N deficiency monitoring methods. By monitoring N status, farmers can more effectively manage additional nitrogen applications, saving time and money.
Actively scouting a corn crop enables farmers to notice nutrient deficiencies when symptoms first appear. For nitrogen deficiencies, look for v-shaped yellowing beginning at the leaf tip and progressing along the midrib. Symptoms appear on lower leaves first and work their way up the corn plant. Other symptoms include spindly stalks, stunted growth and small ears.
Pre-sidedress Soil Nitrate Test
The pre-sidedress soil nitrate test (PSNT) determines the amount of nitrate-N available in the soil, which can be used to decide if enough N is available to meet crop requirements. Producers collect soil samples just prior to sidedressing, the time corn begins rapid N uptake. The PSNT is best suited for fields with high N mineralization potential, such as fields high in organic matter, fields previously in legumes or fields where manure is applied. One item to note, farmers should not apply preplant N fertilizer to fields when using PSNT to monitor N levels.
Leaf Tissue Analysis
A leaf tissue analysis reveals the nutrient levels in plants when the sample is collected. Based on the analysis, farmers can evaluate whether or not nutrient supplies are adequate to meet crop needs. Samples can be taken throughout the season, however waiting until later in the growing season, closer to tasseling, provides a better indication of possible N deficiency.
A chlorophyll meter measures the amount of N a plant has by estimating the chlorophyll content or greenness in the corn leaf. Producers collect an average reading from 20 to 30 plants. Then, the average is compared to the average reading from an N-reference strip, an area in the field where higher amounts of N is applied and is considered to have non-limiting supplies of N. This allows growers to determine the amount of additional N to apply. For the chlorophyll meter test, corn must have reached the V6 stage, yet, it is suggested to wait until later vegetative stages to determine fertilizer needs. By waiting growers have a more accurate reference for additional N rates, however, waiting requires using high clearance equipment to apply fertilizer.
Crop sensors read the canopy reflectance of a red or a green band plus an infrared band. As with chlorophyll meter readings, an N-reference strip helps sensors identify differences between the reference strips and the remainder of the field. Using the N-reference strip as a basis, the sensors can more accurately guide additional N applications.
Color images can be taken from aircraft or satellite to determine color differences in a corn field. To get an accurate image of the field, the corn must have sufficient canopy, which is usually when corn is at least waist high. A benefit of aerial imagery is growers can see the variations across the field as a whole, but also down to the row where crop color changes. Similarly to other N monitoring methods, taking images later in the growing season will result in more accurate fertilizer decisions.
In-Season Diagnostic Tests for Self-Learning and Monitoring Nitrogen in Corn. Silva, G. Michigan State University. http://fieldcrop.msu.edu/uploads/documents/In_season_diagnostic_tests_factsheet_revised_SLS_GHS.pdf