Inconsistencies in results of soil nitrate testing

Each year brings a new crop of observations that the late spring test gives inconsistent results and, therefore, cannot be a useful tool for improving nitrogen (N) management. Common causes of these inconsistencies deserve attention.

A commonly observed problem is that different samples collected at the same time from the same area indicate substantially different nitrate concentrations. This problem often can be traced to inappropriate sampling methods. It often can be reduced by collecting more soil cores per sample than many people consider necessary. The underlying need is greater appreciation for the extent to which soil nitrate concentrations vary on a scale that is smaller than corn root systems and larger than probes used to collect soil samples.

Collecting more cores per sample, however, does not always solve this problem. Extensive studies have shown that soil nitrate concentrations in some fields show marked variability in patterns that reflect the width of fertilizer applicators. In such situations the problem that needs to be corrected is non-uniform applications of fertilizer. Inconsistent nitrate concentrations when samples are properly collected should be considered evidence of a nitrogen management problem before it is considered evidence that the soil nitrate test is fundamentally flawed.

Another commonly observed problem is that samples collected after relatively large rainfall events tend to have nitrate concentrations lower than samples collected immediately before the rainfall. A common question is: "How can you expect farmers to believe the soil test if samples collected a few days apart call for greatly different N rates?" The answer is that everybody needs to recognize that large rainfall events should be expected to cause losses of N and that these losses should be expected to change optimal rates of N fertilization.

The late-spring soil test is recommended because it reveals unexpected losses of N when used to check N management practices, and it minimizes losses of N when used to guide fertilization in late spring. Research during the past decade indicates that, for economic and environmental reasons, corn producers cannot afford to continue using N management practices that do not address unusual rainfall events and normal year-to-year variability in weather.

Users of the soil nitrate test should expect that isolated problems or errors will cause incorrect results for some samples. This problem can be minimized by basing N management decisions on results that make sense when considered in the context of samples collected at different locations and times. Occasional unusual results should be dismissed as errors, but large numbers of unusual results should be considered evidence of a sampling or management problem that needs to be corrected.

The late-spring test for soil nitrate is a management tool that enables growers to assess the amounts of plant-available N just before corn plants begin rapid uptake of N. This tool is far from perfect and ongoing efforts are needed to improve the reliability of this tool. It needs to be recognized, however, that the test often gives results that are inconsistent with popular ideas even when they are consistent with what is known about the behavior of N in soils. Such inconsistencies should not be ignored because tools that reveal flaws in popular ideas pave the way for substantial improvements in N management.

This article originally appeared on page 72 of the IC-478(10) -- May 26, 1997 issue.

Updated 05/25/1997 - 1:00pm