Soils across Kansas are still running above 50°F at the 4-inch depth (Figure 1). It is best to delay anhydrous ammonia applications until soil temperatures drop below this threshold. Applying anhydrous ammonia in the fall ahead of the following corn crop has some appeal to producers. For one thing, fall fertilizer application spreads out the workload, so there’s more time to focus on corn planting in the spring. Secondly, wet conditions in the spring sometimes prevent producers from applying lower-cost anhydrous ammonia ahead of corn planting and force them to apply more expensive sources after planting. Equally important for many producers have been issues with anhydrous ammonia availability at times in the spring.
Figure 1. Average soil temperature (°F) at 4 inches for the 7-day period ending on October 26, 2023. Soil temperatures in individual fields in any given area will vary with differences in vegetative cover, soil texture, soil moisture, and other factors. (Kansas Mesonet)
Despite those advantages, producers should be aware that there is potential for higher nitrogen (N) loss in the spring following a fall application due to nitrification of the ammonium during late winter and very early spring and subsequent leaching or denitrification.
Reactions of anhydrous ammonia in the soil
Anhydrous ammonia has a strong affinity for water (hydrophilic) and readily reacts with water in its surrounding environment. This hydrophilic nature can be detrimental if the ammonia comes in direct contact with plants or exposed skin, but it can also be advantageous when applied correctly as a fertilizer.
When anhydrous ammonia is injected into the soil, the ammonia gas (NH3) reacts rapidly with moisture in the soil and is converted to ammonium (NH4+). This ammonium is no longer in a gas form, and, being positively charged, it can be bound to clay and organic matter particles within the soil. This bound ammonium does not readily move in most soils, and leaching is not an issue except for some sandy soils with very low cation exchange capacity (CEC).
While this process does require moisture, the amount of water needed is quite low. The physical properties of dry soils cause the most common problems when applying anhydrous ammonia to dry soils. Poor closure of the injection furrow and voids and cracks in the dry soil can allow the ammonia to escape to the surface before converting it to ammonium. Using deeper injection depths and wing sealers in dry soils increases the amount of soil the gas comes into contact with and can significantly reduce ammonia losses back through the surface. Closing disks can also help seal the injection furrow and prevent losses at the injection site. More information on applying anhydrous to dry soils is available in this previous eUpdate article: https://eupdate.agronomy.ksu.edu/article_new/can-dry-soils-affect-anhydrous-ammonia-applications-510-4.
At soil temperatures above freezing, ammonium is converted by specific soil microbes into nitrate-N (NO3-) in a process called nitrification. Since this is a microbial reaction, soil temperatures strongly influence it. The higher the temperature, the quicker the conversion will occur. Depending on soil temperature, pH, and moisture content, converting all the ammonia applied in the fall can take 2-3 months or longer 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. 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. Nitrification does not cease below 50°F, but the soil will likely become cold enough to limit the nitrification process. In many areas of Kansas, soils may stay warmer than 50 degrees well into late fall and only freeze for short periods during the winter.
Using a nitrification inhibitor can help reduce N losses from fall N applications under specific conditions, particularly when soil temperatures warm back up for a period after application.
One should also consider soil physical properties 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. Instead, fall N applications should focus on deep, medium- to heavy-textured soils where water movement through the profile is slower.
When is nitrogen lost?
When considering fall applications of N, remember that loss of N during the fall and winter is not usually a 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 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, allowing nitrification to start well before planting corn. 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 early March and essentially be complete by late May and June.
If anhydrous ammonia is to be applied in the fall, several factors must be considered, including soil texture, temperature, and soil moisture. Consider the following guidelines:
Figure 2. Hutchinson 10SW Mesonet station 2023 2-inch soil temperature compared to climatology under grass cover. Soil temperatures in individual fields in any given area will vary with differences in vegetative cover, soil texture, soil moisture, and other factors. (Kansas Mesonet)
Dorivar Ruiz Diaz, Nutrient Management Specialist
Bryan Rutter, K-State Soil Testing Lab Manager
Christopher “Chip” Redmond, Kansas Mesonet Manager
Peter Tomlinson, Environmental Quality Specialist