Iowa State University's new nitrogen fertilizer recommendations for corn (outlined in publication Pm-1714) do not consider yield goals. This is an important change that deserves explanation because use of N recommendations based on yield goals has been widely accepted as a "best management practice."
Overall analysis of research results from many sites during the past decade has revealed an absence of any useful relationship between economic optimum N rates and yields. It makes no difference whether the analyses are based on published yield potentials, yields observed in a given year, average yields for a site over several years, or any other method of calculating "realistic" yield goals.
The results do not challenge the fact that the total amount of nitrogen utilized by a corn crop tends to increase with yield level. They show, however, that this fact is irrelevant when making N fertilizer recommendations. The reason is that much of the N taken up by a crop is supplied by the soil rather than by the fertilizer. In this context, nitrogen "supplied by the soil" originates from soil organic matter, decaying residues of previous crops, and residual inorganic N from fertilizer applied in previous years. Recently applied animal manures should be considered a type of fertilizer. Iowa soils often supply all the N needed by corn in some years.
The results can be explained by recognizing that differences among soils in their ability to supply plant-available N are not considered when N fertilizer needs are estimated by multiplying a yield goal by an efficiency factor. The efficiency factor merely indicates units of yield expected per unit of fertilizer. Whether categorized by mapping units or management history, soils clearly differ in the ability to supply N for crop growth. Differences among soils are caused both by differences in capacity to mineralize N and differences in capacity to retain N against losses. Yield potential, mineralization potential, leaching potential, and denitrification potential all interact unpredictably as one moves across a landscape.
Recent research using remote sensing and combines equipped with yield monitors and global positioning systems has provided compelling evidence that N recommendations based on yield goals do not predict spatial variability in N fertilizer needs within fields. Better recommendations are required to obtain the benefits of new applicators that vary rates of fertilization while moving across fields.
The old recommendations based on yield goals make no adjustments for important effects of weather. This includes year-to-year variability in the amounts of N supplied by a soil, as well as in the percentages of fertilizer N lost from that soil. Ignoring the effects of weather essentially means ignoring some of the most important factors affecting N fertilizer needs.
Recommendations that ignore differences caused by soil and weather variability were acceptable in the past only because better recommendations were not available. Research during the past decade, however, has provided soil and tissue tests that can be used to begin addressing these differences. Recommendations based on these tests are derived from observed relationships found in studies conducted across many sites and many years. The new recommendations can be defended by using current knowledge of N behavior in soil-plant systems. Unlike use of the old recommendations, use of the new recommendations generates information that enables continuous evaluation and improvement of management practices and recommendations.
The most important reason for eliminating recommendations based on yield goals is to help everybody understand what research really has shown. Although these recommendations were developed to help producers, it now seems these recommendations have been incorrectly interpreted to mean that observed yields usually are proportional to rates of N application. This incorrect interpretation makes it difficult to understand how variability due to soils and weather could be important. It creates difficulty understanding why routine application of extra N is not a good investment. It maintains a formidable barrier to recognizing that improvements in N management should be expected to increase profits for producers while reducing environmental problems. Single copies of the new publication, Nitrogen Fertilizer Recommendations for Corn in Iowa, Pm-1714, are available free of charge from any ISU Extension county office.
This article originally appeared on pages 82-83 of the IC-478(11) -- June 2, 1997 issue.