With tassels out, it may seem too late to worry about nitrogen management, but extension soil specialists at the University of Minnesota say there are some significant steps you can take. John Lamb and Daniel Kaiser point out that by this point in the growing season, the nitrogen that your crop needs should be in the soil where the plant can use it. And they suggest using the stalk nitrate test to evaluate how well your crop is utilizing nitrogen at this late stage in the growing season.
The test was develop at Iowa State University in the 1990s with the goal of evaluating nitrogen management through the growing season. "This is a post mortem test and it not predictive in nature," they state. To run the test, collect a sample from the lower section of the corn plant -- about 6 to 14 inches -- above the ground at 1 to 3 weeks after physiological maturity, or black layer. Any unused nitrate in the corn plant at the end of the growing season collects in the lower part of the stalk. Iowa State University research indicates the results from this test can be put into four categories: low (less than 250 ppm N); marginal (250 to 700 ppm); optimal (700 to 2000 ppm); and excess (greater than 2000 ppm).
The researchers indicate there are some drawbacks to the 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," they state. "Second, the research data in Minnesota and Wisconsin indicates that the variability of the nitrate-N concentration in the stalk between plant and replications in small plots is large. This means that you need to take a large number of stalk samples to ge an accurate basal stalk nitrate-N number," the note. The final source for error is where in the field the sample is taken. If not taken from a representative area, you will get a non-representative result, they observe.
Click here for more information on the test:
Lamb and Kaiser also say another way to evaluate your nitrogen program is to simply count the number of nitrogen deficient leaves from the ground to the ear. The number of N-deficient leafs is related to nitrogen sufficiency. A nitrogen deficiency in a corn leaf starts with yellowing along the mid-rib. As the deficiency progresses, the yellowing moves out toward the edges of the leaf. Eventually the leaf will turn brown and die. Since nitrogen is mobile in the plant, the leaf-yellowing process starts with the bottom leaf and progress up the plant. The higher up the plant the deficient leaves appear, the more deficiency. "In a perfect world, there should be some N deficient leaves at black layer," they note. "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," they say.
They add that the above tests are useful when used in a "good established nitrogen management plan. Without a good plan, the tolls are worthless," they observe. The next two links are good starting points if you're a Minnesota grower.
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