Cornfields that have had applications of animal manures or commercially prepared nitrogen fertilizers should be checked in early June for nitrate levels. The results can be used to evaluate and improve nitrogen (N) management practices.
Animal manures contain large amounts of nitrogen needed during corn production. Surveys show, however, that most corn producers do not make adequate adjustments when they purchase and apply commercially prepared N fertilizers. This finding suggests a general lack of appreciation for the potential of animal manure as a fertilizer and as an environmental pollutant.
An effective way to demonstrate the value of manure as a source of plant-available N is to use the late-spring test for soil nitrate after animal manures and commercial fertilizers have been applied. Corn producers usually are surprised by what they learn. The test often shows that nitrate concentrations are two or three times higher than those needed by the crop. In many fields, however, the test reveals that large amounts of nitrogen from manure and fertilizer were lost before it was needed by the crop.
It is well established that much of the nitrogen in manure can be lost as ammonia soon after application, and that the percentages of N lost vary greatly with site conditions. It also is well established that carbonaceous materials in animal manures promote substantial losses of plant-available N by immobilization and/or denitrification under some conditions. Marked variability in losses of plant-available N by these mechanisms helps explain why corn producers consider animal manures to be an unreliable source of N for corn production.
Problems caused by variability in losses of plant-available N from manured cornfields were insurmountable in the past, but they can be addressed today. An effective approach is to apply no commercially prepared N fertilizer before corn is about six inches tall, and to use the late-spring test to make site-specific assessments of N fertilizer needs. This approach makes it possible to apply N fertilizer only where needed. Details for using this soil test are provided in a new ISU Extension publication, Nitrogen Fertilizer Recommendations for Corn in Iowa, Pm-1714.
Information provided by the soil test has two noteworthy benefits. First, it will help producers avoid costly applications of unneeded commercial fertilizers after manures are applied. Second, it should help producers select rates and methods of application that make manure a more reliable source of N for corn production. The overall effect should be a substantial net reduction in the amounts of N applied to soils and lost from soils.
Only small investments of time and money are needed to use the late-spring test to evaluate current N management practices. Collecting and testing two or three samples from each of three or four fields should provide a reasonable evaluation for most producers. Mounting evidence suggests that crop and livestock producers cannot afford the economic and environmental costs of not conducting routine annual evaluations of N management practices on their manured fields.
This article originally appeared on pages 64-65 of the IC-478 (9) -- May 19, 1997 issue.