Recent research has shown that a new plant tissue test can be used to evaluate the nitrogen (N) status of both silage corn and grain corn in Connecticut. The test is called the “end-of-season cornstalk test,” and involves measuring nitrate concentrations in the lower portions of cornstalks at the end of the growing season. It is a powerful management tool that enables corn producers to distinguish between optimal and excess applications of N.
BASIS FOR THE TEST
Corn plants suffering from N deficiency remove nitrate from the lower cornstalks and leaves during the grain-filling period. The effects of N removal can be easily observed when the lower leaves turn yellow and die prematurely. Corn plants having more N than needed to attain maximum yield accumulate nitrate in their lower stalks at the end of the season. The test accurately measures the nitrate in cornstalks and will indicate deficient, optimal, or excess N availability during the growing season.
WHY THE TEST IS NEEDED
The results of corn producers’ use of the June nitrate test since 1988 suggest that many producers unknowingly apply more N than needed. This is understandable because corn plants grow equally well when supplied with profitable or unprofitable rates of N. The June nitrate test results from 1994, which are typical results, provide a good estimate of the amount of excess N available to corn: 62% of the fields tested required no top dress fertilizer, and 25% of the fields had more than double the amount of soil nitrate required for maximum yield.
The underlying reason for over fertilization is that corn plants show no visual symptoms that enable producers to recognize when above-optimal rates of N have been applied. This means that producers can apply too much N year after year and never suspect a problem with their N management. The end-of-season cornstalk test makes it possible for producers to avoid this pitfall. Many producers associate a dark green plant with optimal rates of fertilization, and they fertilize to maintain dark green plants late in the growing season. This practice needs to be questioned, however, because corn leaves tend to lose their deep green color late in the season at economically optimal rates of fertilization. The end-of-season cornstalk test, therefore, helps producers distinguish between fertilizing to maximize greenness and fertilizing to maximize profits.
HOW THE TEST IS DONE
The portion of each plant sampled is the 8-inch segment of stalk found between 6 and 14 inches above the soil. Leaf sheaths should be removed from the segments. Stalks severely damaged by disease or insects should not be used. It is extremely important to cut the stalks at the 6- and 14-inch height above the ground, because cutting lower or higher will result in incorrect assessment of nitrogen availability. Consistently cutting below 6 inches will result in higher than expected stalk nitrate concentrations, and consistently cutting above 14 inches will result in lower than expected stalk nitrate concentrations.
Fifteen 8-inch segments should be collected to form a single sample to be sent for analysis. These should be collected at random within an area not larger than 15 acres. Areas differing in soil types or management histories should be sampled separately. An easy method for collecting a sample is to collect 3 to 5 stalk segments from each of several small areas (less than an acre) that seem to be representative of larger areas within a field.
Samples should be sent to the Soil Testing Laboratory at the University of Connecticut for analysis as soon as possible after collection. Samples should be placed in paper (not plastic) bags to enable some drying and minimize growth of mold. The test results will be incorrect if the samples are stored for more than one day before lab. The time normally required to mail samples to the laboratory is not a problem. The UConn Soil Testing Laboratory will accept samples for this test starting in the fall. Because the laboratory does not yet have the equipment necessary for rapid analysis of the samples, the results of the test will be sent to farmers in the February after harvest. The results will indicate whether the availability of N during the growing season was low, optimal or excess.
TIME OF SAMPLING FOR THE TEST
Collect stalk sections from one week before you plan to chop the corn until one day after the corn is chopped. The easiest method to collect a sample is to walk or drive across the field up to 24 hours after harvest and cut the stalk sections from the stubble. If your stubble height is normally less than 14 inches tall, I recommend you raise the corn head for about 50 feet in a few places in the field and collect your stalk sections from the stubble in those areas.
The time for sampling cornfields to be harvested for grain is between one and 3 weeks after black layers have formed on about 80% of the kernels of most ears.
INTERPRETATION OF THE TEST RESULTS
Stalk nitrate concentrations can be divided into three general categories: LOW, OPTIMAL, and EXCESS. The LOW category indicates high probability that greater availability of N would have resulted in higher yields. It should be noted that concentrations in this category give little indication of the magnitude of yield increase that might be expected from more available N. Visual signs of N deficiency usually are present when nitrate concentrations are in this category. The OPTIMAL category indicates high probability that N availability was within the range needed to maximize profits for the producer. The EXCESS category indicates high probability that N availability was greater than when N was applied at rates that maximize profits for producers. The stalk nitrate concentrations for each of the three categories for both grain corn and silage corn are:
LOW = less than 250 ppm N
OPTIMAL = 250 to 2000 ppm N
EXCESS= greater than 2000 ppm N.
The concentration of nitrate in the stalk at the end of the season reflects all factors that influenced N availability and N needs during the growing season. Because many of the factors influence N availability after fertilizers are applied, it is unrealistic to expect any producer to attain OPTIMAL concentrations in all fields in all years. Indeed, experience has shown that the OPTIMAL category is difficult to consistently attain with existing management practices.
When interpreting the results of the test, consideration must be given to weather conditions that occurred during the growing season. Rates of fertilization that are most profitable for many years should be expected to often result in OPTIMAL concentrations, but in some years concentrations will be in the LOW or EXCESS categories. Lower-than-desired concentrations should be expected in years having unusually large amounts of in-season rainfall that results in unusually large losses of N and (or) high yield potential. Higher-than-desired concentrations should be expected in years when unusually low rainfall limits N losses and (or) yield potential.
Fertilization rates should be increased for areas that usually test in the LOW category and decreased on areas that usually test in the EXCESS category. The test does not directly indicate how much N rates should be increased or decreased, but continued use of the test for several years enables producers to make adjustments toward optimal rates. Concentrations in the EXCESS category indicate that use of the June nitrate test to guide N fertilization will probably increase profits for the producer.
WHO SHOULD USE THE TEST
All corn producers should consider using the test on a few fields each year. Those who learn that their fields usually test in the OPTIMAL range will not need to continue testing or change their N management. Those who learn that they usually apply too much N to some or all of their fields will find it profitable to adjust rates of application. Thoughtful use of the test for a few years should help producers optimize rates for their fields.
All producers who grow corn on manured soils should use the stalk test. Recent studies indicate that many producers greatly underestimate the amount of N supplied by animal manures and apply unneeded fertilizer. Use of the stalk test to evaluate rates of N recommended by use of the June nitrate test is encouraged. Use of both tests for a number of years on the same field should enable producers to build confidence in the lower rates of N that are frequently recommended by use of these new tools.
By Tom Morris and Beth Hooker
UConn Soil Nutrient Analysis Laboratory
Department of Plant Science and Landscape Architecture
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