Soils in parts of Kansas are now cool enough to allow producers to apply anhydrous ammonia for their 2017 corn crop, and have been for the past couple weeks in northern Kansas.
Figure 1 Weekly mean 4-inch soil temperatures for the period of Nov. 22-28. Soil temperatures in individual fields in any given area will vary with differences in vegetative cover, soil texture, soil moisture, and other factors. Source: K-State Weather Data Library.
This practice has some appeal. For one thing, fall fertilizer application spreads out the workload so that there’s more time to focus on corn planting in the spring. Secondly, wet spring weather sometimes prevents producers from applying lower-cost anhydrous ammonia in the spring ahead of corn planting, and forces them to apply more expensive sources after planting. Equally important for many producers have been issues with ammonia availability at times in the spring.
Despite those advantages, producers should be aware that there is potential for higher nitrogen (N) loss in the spring following a fall application, as a result of nitrification of the ammonium during late winter and very early spring and subsequent leaching, or denitrification. If anhydrous ammonia has already been applied this fall for the 2017 corn crop, this article will discuss what to expect if soil temperatures have not been cold enough until now.
Reactions of anhydrous ammonia in the soil
When anhydrous ammonia is applied to the soil, a large portion of the ammonia is converted to ammonium (NH4+), and can be bound to clay and organic matter particles within the soil. As long as the nitrogen remains in the ammonium form, it can be retained on the clay and organic matter, and does not readily move in most soils except sandy soils with very low CED, so leaching is not an issue.
At soil temperatures above freezing, ammonium is converted by specific soil microbes into nitrate-N. Since it is a microbial reaction, it is very strongly influenced by soil temperatures. The higher the temperature, the quicker the conversion will occur. Depending on soil temperature, pH and moisture content, it can take 2-3 months or more to convert all the ammonia applied in late summer/early fall to nitrate.
By delaying application until cold weather, most of the applied N can enter the winter as ammonium, and over winter losses of the applied N will be minimal
Traditionally, producers should wait until soil temperatures are less than 50°F at a depth of 4 inches before applying ammonia in the fall or early winter. It’s not that nitrification stops below 50 degrees, but rather that soils will likely become cold enough soon to limit the nitrification process. In many areas in Kansas, soils may stay warmer than 50 degrees well into late fall, and only freeze for short periods during the winter.
The use of a nitrification inhibitor such as N-Serve can help reduce N losses from fall N applications under specific conditions, particularly during periods when soil temperatures warm back up for a period after application.
One should also consider soils when considering fall application. Fall applications of N for corn should not be made on sandy soils prone to leaching, particularly those over shallow, unprotected aquifers. Rather, fall N applications should focus on deep, medium to heavy textured soils where water movement is slower.
When is N lost?
When considering fall application of N, keep in mind that loss of N during the fall and winter is not normally our problem in Kansas. The conversion of “protected” ammonium to “loss prone” nitrate during the fall and winter can be minimized by waiting to make applications until soils have cooled, and by using products such as nitrification inhibitors. The fact that essentially all the N may remain in the soil as ammonium all winter, coupled with our dry winters, means minimal N is likely to be lost over winter.
However, soils often warm up early in the spring and allow nitrification to get started well before corn planting. Generally, if the wheat is greening up, nitrification has begun! Thus one of the potential downsides of fall application is that nitrification can begin in late February and March, and essentially be complete before the corn crop takes up much N in late May and June.
The bottom line is this: If anhydrous ammonia is to be applied in the fall, there are a number of factors that must be considered, including soils, temperature and soil moisture. Consider the following guidelines:
Dorivar Ruiz Diaz, Nutrient Management Specialist
Mary Knapp, Weather Data Library