Mounting evidence indicates that fall-applied nitrogen (N) is converted to nitrate much more rapidly than has been generally believed in Iowa. Observations made last December add to this evidence.
Soil samples were collected in mid-December from three fields that received anhydrous ammonia in mid-November. The fields were in Greene County, and soil samples were collected from a total of 25 test areas within the fields. The test areas were selected to represent the range in soil characteristics within each field. Samples were collected to a depth of 18 inches in and between injection bands within each test area.
Concentrations of ammonium and nitrate were measured in each sample. Fertilizer N was distinguished from soil N by subtracting concentrations found between bands from those found in bands. Percentage of nitrification of fertilizer N was calculated by expressing amount of fertilizer nitrate as a percentage of fertilizer N found as exchangeable ammonium and nitrate.
Percentage of nitrification increased with increase in soil pH (Figure 1). The average percentage of nitrification was 21 percent for soils having pH values less than 7.0 and 50 percent for pH values greater than 7.0. Observations made over the past few years suggest that nitrification in all parts of these fields will be essentially complete by mid-April.
Figure 1. Relationship between soil pH and percentage of fall-applied ammonia converted to nitrate by mid-December.
Rapid conversion of fall-applied N from ammonium to nitrate is undesirable because nitrate is likely to be lost from fields by leaching or denitrification when soils have excess water. Iowa soils are most likely to have excess water during March, April, and May. Minimal losses of this N should be expected before plants grow if little rainfall occurs before next July.
If substantial amounts of rainfall occur during late winter and early spring, losses of N should be expected to vary greatly with soil pH. In much of Iowa, cornfields may show N deficiencies in irregular spatial patterns that are associated with the high-pH soils. Losses of N also can be expected if excess rainfall first occurs during April and May, but soil pH will not be an important factor affecting amounts of N lost.
It is impossible to accurately predict how much N will be lost from Iowa cornfields before crops are planted next spring. Evidence collected during the past few years, however, suggests that more than half of the N will be lost from many fields if we have above-average amounts of rainfall.
Data concerning the effects of early season rainfall on losses of N are presented in the article Early season losses of nitrogen  that appeared in this newsletter on February 26, 2001. Examples of losses observed in recent years are presented the article Soil pH and losses of fall-applied ammonia  that appeared in this newsletter on August 21, 2000.
Fall applications of N offer the advantage of efficient application of N to soils, but efficient application to soils should not be confused with efficient use by crops. This distinction is important because the costs of N losses before crops grow often are much greater than the dollars paid for the fertilizer. Producers pay these costs even when they do not know that the losses occurred.
This article originally appeared on page 3 of the IC-488 (1) -- January 21, 2002 issue.