Corn is at dent. The latest round of rainfall has secured the yield and test weight should be good. Before harvest is a good time to look back on how well your nitrogen strategy worked.
This has been an interesting growing season. Higher than normal rainfall through June and August put nitrogen at risk — a fact that showed up in varying degrees of yellow. What probably saved many cornfields was the cooler than normal temperatures combined with higher than normal rainfall levels that allowed more nitrogen to be mineralized from the soil.
I have been surprised that nitrogen deficiencies weren’t more evident than they were considering the rainfall we had this summer. The likely reasons are better nitrogen management, nitrogen stabilizers and more nitrogen mineralized from soil organic matter.
I have been taking note of fields where I saw corn yellowing, when it started to yellow and how far up the plant leaves fired. Nitrogen is mobile in the plant. The corn plant can cannibalize it from lower leaves and export it up the plant to higher leaves to meet nitrogen demands. The capacity of the lower leaves to fix carbon and produce sugar is lost if this happens. At the same time, these leaves can be more of a liability than an asset when resources are limited.
Look at the number of green leaves below the ear as the crop moves from milk to dough stage. If the plants have at least four green leaves below the ear, nitrogen was adequate this season. Lower leaves will eventually begin to fire as the ear fills out and demand for nutrients increases.
This year it was dry in July and I did notice some premature firing by mid-August on lighter soils. Firing due to lack of water doesn’t follow any progressive or identifiable yellowing pattern.
Nitrogen loss is easier to identify because it starts at the tip of the leaf and follows a V-pattern toward the base of the leaf. Firing from nitrogen loss typically begins with leaves at the base of the plant and works upwards. The plant will lose those early leaves and save those at the top and above the ear.
You can always pull out the soil probe and sample the top 12 and 24 inches of soil and run a nitrate test to see how much residual nitrate remains. A soil sample and soil analysis is only a predictor of potential nutrient levels and possible deficiencies. Accuracy depends on how well the soil sample reflects the soil in the field.
A soil test can tell you if the crop had too little, enough or too much nitrogen during the season. Remember organic matter will always mineralize nitrogen — so a measurement of about 10 or 15 lbs. typically reflects mineralization, while a reading of 40 lbs. tells you there was more than enough nitrogen available. If only 5 lbs remain, your soil probably lacked organic matter and residual nitrate and the crop was shorted.
The cornstalk nitrate test pioneered by Alfred Blackmer at Iowa State University in the mid-1990s is also a good post-mortem test. Guidelines say to cut an 8-inch stalk segment from the base of the plant, beginning 6-inches above the soil line, removing leaves and leaf sheaths from the segment. Like with soil sampling, you have to collect enough representative samples to accurately reflect the field. These samples are then submitted to a laboratory for nitrate analysis.
According to Blackmer, nitrogen is deficient if the sample reads less than 250 ppm; marginal from 250 to 700 ppm; optimal from 700 to 2,000 ppm and nitrogen is in excess if greater than 2,000 ppm. Other universities have calibrated this test for their states and have slight different ratings.
Read more about the stalk test here:
I have used the stalk and soil nitrate test in the past, pulling samples in September. The stalk nitrate test better reflected what the corn experienced. I didn’t learn much from the soil nitrate test. However a high stalk and soil nitrate test would confirm that you probably over-applied nitrogen for that season based on the corn yield.
Nitrogen continues to be one of our most needed and expensive inputs. It makes sense to make sure the crop gets exactly what it needs. Read the corn leaves this fall to plan for next spring.