Status of fall-applied N this spring

Relatively warm and wet weather last winter and early this spring has prompted questions about possible losses of anhydrous ammonia-N applied last fall. To learn about the status of this N, soil ammonium and nitrate concentrations were measured at more than 30 test areas during the last week of April. The test areas were in Boone, Greene, Hamilton, and Hardin counties. The study involved intensive sampling at each test area, and it was done with the assistance of more than 30 students participating in an Iowa-Ukraine exchange program focusing on science, agriculture, and the environment.

Key findings of this study can be summarized in three points. First, substantial portions of the fertilizer N already had been converted to nitrate. More than two-thirds of the test areas had more nitrate-N than exchangeable ammonium-N in the surface foot of soil. Essentially all exchangeable ammonium should be converted to nitrate by mid-May. This N, therefore, is vulnerable to loss by leaching or denitrification before plants start taking up significant amounts of N in June.

Second, substantial amounts of the fertilizer N were present in the second foot of soil in many fields. This is reasonable because the ammonia was injected at a depth of 6-10 inches and downward movement of nitrate would have occurred as excess water moved through the soil profile. Sampling to a depth of only 1 foot, therefore, may underestimate availability of fall-applied N in many fields.

Third, observed concentrations of nitrate-N plus exchangeable ammonium-N varied greatly among fields. Some had concentrations within the range that should be expected in absence of significant losses of N. Some fields, however, seemed to have lost more than half of the N applied. We observed no trends that could be used to help predict which fields are likely to have low concentrations.

Weather conditions during May and early June undoubtedly will be important factors determining the frequency and severity of N deficiencies due to losses of fall-applied N in Iowa this year. Above-average rainfall could cause additional losses and increase N deficiencies. Favorable weather, however, could promote enough mineralization of N from soil organic matter to offset losses that have already occurred in most fields. For economic and environmental reasons, it seems inadvisable to apply extra N to compensate for possible losses of fertilizer N unless there is clear evidence of need.

The late-spring test for soil nitrate should be considered a useful tool for assessing the need for additional N to compensate for possible losses of fall-applied N. Each producer need only collect a few samples in each of a few fields. However, at least three "sets of eight cores" should be collected for each sample by using the pattern suggested in Iowa State University Extension publication Pm 1714, Nitrogen Fertilizer Recommendations for Corn in Iowa. You may obtain a copy of this publication from your local extension office or access it electronically at http://www.exnet.iastate.edu/Pages/pubs/. In addition, modifications are needed to compensate for above-normal concentrations of nitrate in the second foot of soil.

One possible modification is to collect samples representative of the surface two-foot layer of soil. If this modification is used, then the "critical concentration" of nitrate should be considered to be 17 ppm rather than 25 ppm (see Pm 1714). This modification is not practical if the second foot is saturated with water due to excess rainfall or if tools are not available to sample the second foot.

An alternative modification is to interpret the results of normal one-foot samples by using the recommendations given for manured soils (i.e., use the recommendations given in Table 3 of Pm 1714). The recommendations for manured soils were based on observations from many fields that received fall applications of animal manure and, therefore, had significant amounts of nitrate in the second foot of soil. These recommendations also enable adjustments for May rainfall. Based on ongoing analyses of data from many trials, I would use the recommendations in Table 3 for assessing N fertilizer needs on all soils already fertilized with anhydrous ammonia.

This article originally appeared on pages 79-80 of the IC-480(10) -- May 18, 1998 issue.

Updated 05/17/1998 - 1:00pm