The corn is tasseling, we are praying for rain, and the week of the 4th of July was hot and miserable. It must be time to think about evaluating this year's nitrogen management program and making decisions about next year's nitrogen needs.
Since the corn is tasseling, you should have all of your nitrogen on whether you are growing corn in dryland or irrigated situations. Some varieties can take up some nitrogen after tasseling, but this should not be a reason for late N application. The nitrogen should be in the soil where the plant can use it. There are a few tools that have been developed to evaluate the N status of the current corn crop at the end of the growing season. They include the stalk nitrate test and counting number of N deficient corn leaves.
The stalk nitrate test was developed at Iowa State University in the 1990's. Its goal was to evaluate the nitrogen management for corn during the growing season. This is a post mortem test and is not predictive in nature. It involves collecting lower section of corn stalk (6 to 14 inches above the ground) at 1 to 3 weeks after physiological maturity (black layer). Unused nitrate in the corn plant at the end of the growing season accumulates in the lower part of the corn stalk. This is the reason for sampling this part of the corn plant. As the amount of excess nitrogen for corn production increases, (possibility from too great of an application of nitrogen fertilizer or manure), the larger the concentration of nitrate-N occurs in the lower corn stalk. Research from Iowa State University indicates the results from this stalk nitrate-N test can be put into four categories; low (less than 250 ppm N), marginal (250 to 700 ppm N), optimal (700 to 2000 ppm N), and excess (greater than 2000 ppm). Because of the differences that are measured in stalk nitrate-N concentration, the basal corn stalk test has gained a considerable notoriety from regulatory agencies as a great tool to reduce excess N application.
There are some huge drawbacks to this diagnostic test. One is that if there is a yield limiting event, such as drought, the concentration of nitrate-N in the stalk can be elevated because of the plant stress and not from the over application of nitrogen. Second, the research data in Minnesota and Wisconsin indicates that the variability of the nitrate-N concentration in the stalk between plants and replications in small plots is large. This means that you need to take a large number of stalk samples to get an accurate basal stalk nitrate-N number. This is not the easiest thing to do in a corn field right before harvest because of lack of time and limited access. The final source of error is where in the field do I take these stalk samples? If they are not taken from a representative area, you will get a non-representative result.
For more information on the corn stalk nitrate-N test, check out http://www.extension.iastate.edu/Publications/PM1584.pdf.
Are there other ways to evaluate my nitrogen program? A simple one is to count the number of nitrogen deficient leaves from the ground to the corn ear. The number of N deficient leafs is related to nitrogen sufficiency. A nitrogen deficiency in a corn leaf starts with yellowing along the midrib. As the deficiency progresses, the yellowing will move out towards the edges of the leaf. Finally the yellow area on the leaf will turn brown and die. Since nitrogen is mobile in the plant, this process of leaf yellowing starts with the bottom leaf first and progresses up the plant. The further up the plant the deficient leaves are, the more nitrogen deficient a plant is. In a perfect world, there should be some N deficient leaves at black layer. If all the leaves are green, then there was too much for the corn plant. Also you do not want all the leaves up to the ear to be nitrogen deficient. In this case, there was not enough N for the plant. While this system is very simple, it may be as effective as the stalk nitrate test without the hassle of sampling stalks. Remember, this is also not a predictive test!
As a crop consultant and grower, these tests can be a good evaluation tool for a good established nitrogen management plan. The important words are a good established nitrogen management plan. Without a good plan, the tools are worthless. The first thing to do is get the management plan figured out, implement it, and adjust to your local conditions. The important ingredient is to use reasonable research based N nutrient guidelines and follow the research based Nitrogen Best Management Practices.
In Minnesota, the starting place of this information is: