Should the end-of-season test for cornstalk nitrate be used where dry soil conditions have caused above-normal firing? This question is being asked this year because the guidelines for using this test (outlined in ISU Extension publication Pm-1584) indicate that higher-than-desired nitrate concentrations should be expected when low rainfall has limited either nitrogen (N) losses during the season or reduced the yield potential.
Use the cornstalk test under such conditions, but collect fewer samples if yields are unusually low (i.e., less than about 50 percent of normal yields). Do not use the test on stalks that have little or no grain.
Remember that the test gives site-specific assessments of N status. It determines whether corn plants had too little, too much, or optimal amounts of N near the end of the growing season. You should expect the test to indicate plants had more N than needed if weather resulted in less-than-average N losses and lower yields. However, the plant may indicate something not expected!
The end-of-season test for cornstalk nitrate should be considered a tool for detecting failures in N management. The idea of searching for failures in N management is unappealing to most producers, but the alternative is to ignore failures and repeat them again and again.
Early research with the cornstalk test focused on using it to detect situations where too much N was applied. This is important for environmental reasons and because mounting evidence suggests that high rates of N application sometimes reduce yields during dry years. If extra N reduces yields in some years, this effect must be considered when selecting rates of fertilization in all years. It is important, therefore, to learn how high nitrate concentrations go during dry years.
Recent research has shown that the cornstalk test detects situations where losses of fertilizer N result in N deficiencies that limit yields. Although the test does not measure N losses, losses of N can be indicated when adequate amounts of N are applied and plants have inadequate N at the end of the season. Losses of fertilizer N early in the season can result in yield losses during dry summers because N deficiencies reduce the ability of plants to forage for water in soils. This problem should be suspected if concentrations of stalk nitrate are low after dry summers. Above-normal firing can be caused by lack of either moisture or N, but the stalk test distinguishes between these possibilities.
Keep simple records to show exactly where stalk samples are collected, management practices at that site, and other information that may be important. It is important, for example, to note weed pressure because competition from weeds tends to decrease concentrations of nitrate in cornstalks. The stalk test can be used to optimize nitrogen management amid variability in weather only if samples are collected at several locations each year for several years. This cannot be done, however, if samples are collected only when weather conditions are optimal.
You can use the cornstalk test to evaluate the quality of your N management by measuring outcomes on your fields. This is a major change from the traditional practice of judging quality of N management by comparing practices to those suggested in general guidelines. This change will cause some confusion during the next few years, but it should enable marked improvements in N management as producers move toward site-specific practices in the next decade.
This article originally appeared on pages 174-176 of the IC-478(22) -- September 15, 1997 issue.