Applying NH3 this fall for 2012 corn crop
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. This generally means that anhydrous ammonia applications should be delayed until the second week of November north of I-70.
Where is fall application an acceptable practice?
Traditionally, daily high soil temperatures of 50°F at a depth of 4 inches are considered the maximum for the application of ammonia in the fall. It’s not that nitrification stops below 50 degrees, but rather that soils will soon become cold enough (in all likelihood) 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. As a general rule, applying ammonia in the fall for corn is probably not a good idea south of I-70 because soil temperatures are not normally cold enough on a consistent basis during the winter to prevent nitrification in the fall. North of I-70 applications can be made in soils which are, or soon will be, cold enough to limit nitrification. 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 soil type 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 be confined primarily to deep, medium- to heavy-textured soils where water movement is slower.
When is N lost?
More N is likely to be lost from a fall application of anhydrous ammonia during early spring than during the fall and winter. Loss of N during the fall and winter is not normally our problem in Kansas. The conversion of ammonium to 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 of N 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. If N is applied closer to the time of corn planting, or after corn has been planted, a higher percentage of the N is likely to still be in the ammonium form during the wet periods of late spring. More of the N from fall applications than spring applications can be lost to heavy May and June rains through leaching and denitrification because of the higher portion of N present as nitrate with fall-applied N.