Patterns in optimal rates of N fertilization within cornfields

New technologies make it possible to vary rates of N application within cornfields. Studies are in progress to develop new N recommendations suitable for use with these new technologies.

The studies are conducted in fields of cooperating producers who have yield-monitoring combines. The fields are managed in accordance with the producer's normal practices except for application of N. The fields are aerially photographed soon after planting, and digitized soil maps are superimposed on the photos. Spatial patterns in soil color indicate differences in organic matter and moisture content. When corn plants are 6-12 inches tall, soil samples are collected at several test areas within the field for determination of soil nitrate concentrations, organic matter, and other soil properties of interest. Various rates of fertilizer N are applied as strips that will be harvested as individual swaths of a 6- or 8-row combine.

Remote sensing, tissue testing, and yield monitoring are used to measure response of the corn to applied N. Aerial photographs of the crop canopy in late August show spatial patterns in N response. Responses can be seen because N-deficient corn has a lighter color than corn having optimal amounts of N, but corn with excess N usually has the same color as corn with optimal amounts of N.

Comparisons of color among strips within small areas, therefore, can be used to identify spatial patterns in optimal rates of N fertilization. The end-of-season test for cornstalk nitrate is used to assess N status at numerous test areas within the field. Geographic information systems are used to overlay all data collected and relate observed yields and yield responses to soil mapping units or soil characteristics.

The spatial patterns shown in Figure 1 are typical of the patterns found in many fields. Responses to fertilizer N often are greatest on soil mapping units rated as having the lowest yield potential. Although the observed spatial patterns in N response somewhat conform to soil mapping units, the spatial patterns in response and yields often significantly deviate from the mapping units. Spatial patterns in crop response are being related to organic matter concentrations, landscape position, and other factors that may be useful for dividing the fields into areas that should be sampled and fertilized as different management areas. Results to date suggest that many fields need be divided into no more than two or three different management areas to obtain most of the benefit of variable-rate application of N.

Data from approximately 50 fields have been collected during the past four years and are being analyzed to develop N recommendations that are most likely to maximize profits when used with variable-rate application technologies. In addition to within-field variability in soil characteristics, major factors being considered are antecedent weather, management practices, prices for grain and fertilizer, and other relevant factors that vary from field to field or year to year.

Results of precision farming N-response experiment in Iowa

This article originally appeared on pages 19-20 of the IC-480 (4c Precision Ag Edition) -- April 9, 1998 issue.

Updated 04/08/1998 - 1:00pm