Corn stalk nitrate test and N management
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The nitrate N concentration in the lower portion of the corn stalk at the time black layer formation occurs in the kernels is a good indicator of the nitrogen status the crop experienced throughout the growing season. The stalk nitrate test may also provide an indication of whether or not excess residual nitrogen is left in the soil. When used over a number of years, the stalk nitrate test can help identify N efficient fields or management systems, thus enabling fine tuning of N inputs.
As corn approaches maturity, plants stressed for N will move nitrate from the lower cornstalk to the ear resulting in a low stalk nitrate concentration. When corn plants have sufficient N or more than sufficient N for maximum yield, nitrate accumulates in the corn stalk. Extensive studies done by Purdue, Iowa State and Penn State universities have shown the usefulness of this test in distinguishing between sufficient and excess N situations. Their interpretation guidelines are given in Table 1. Both sets of studies showed that a stalk nitrate N concentration above 2,000 ppm is indicative of excessive nitrogen having been available to the corn crop. Quite often this is associated with the application of animal manure, but may also be related to over application of fertilizer N. In the Purdue studies, maximum yields were associated with stalk nitrate N concentrations above 450 ppm. Iowa State uses 700 ppm as the transition value. Above this value is considered the zone of “luxury” N consumption, i.e., no response to applied N. Values below 450 ppm have been associated with inadequate N being applied for maximum yield.
In N rate studies conducted at MSU over a three-year period, the stalk nitrate N concentration reflected the corn grain yield response and indicated when excess N had been applied. The total N concentration of the ear leaf at silking and the corn grain at harvest only showed when the N rate was too low or adequate, but did not reflect when excess N was applied. Hence, the stalk nitrate test is a better diagnostic tool. Even though it is post-mortem, it can be useful for long term adjustment of N management practices. Farmers are encouraged to try this test on a few fields with different N management practices. It can tell a lot about how N is being utilized by a corn crop. Over a few years, one can develop a good data base for evaluating the appropriateness of various N management practices. With the high cost of fertilizer N, elimination of excess N use improves the net return and provides a positive environmental situation.
Caution: Rainfall has been quite variable across Michigan this year. In areas that have been very dry, corn plants are beginning to dry up and the ears are “maturing” due to lack of water. Under these conditions, the stalk nitrate test may not give a reliable indication of the N status of the plant.
Table 1. Interpretation of the corn stalk nitrate N test. (Concentration as ppm nitrate-N)
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Doing the test
The optimum time to take stalk samples is one to three weeks after black layer has formed in 80 percent of the corn kernels, although Penn State has found that samples can be taken starting at one-quarter milk line formation. Cut an eight-inch segment of the stalk, between six and 14 inches above the ground, from 15 stalks within the area of interest. As with soil sampling, sample within uniform areas. Sample areas differing in soil type, topography, management practices or other properties separately. Remove any portions of leaves that may remain attached. Split the stalk segments to facilitate drying. Lay the samples out to dry, preferably in front of a fan or refrigerate the stalk segments until the samples can be sent or delivered to a testing lab. Place the samples in paper bags. Do not use plastic bags as this will prevent drying and may result in spoilage. The testing lab will oven dry and grind the stalks prior to analysis. This service is provided by the MSU Soil and Plant Nutrient Lab (517-355-0218) and other private labs. Cost is $12 at the MSU lab.