November 30, 1999 By Bruce Barker
The smell of anhydrous ammonia (NH3) wafting from a freshly fertilized field is enough to choke up any farmer, both literally and metaphorically. Preventing application losses is a key to maximizing fertilizer efficiency. “Previous studies have suggested that anhydrous ammonia retention by the soil may be influenced by soil moisture, clay mineral type and content, and soil organic matter content,” explains Rigas Karamanos, Viterra’s manager of agronomic services. “Anhydrous ammonia interaction with the soil plays an important role in its environmental fate, including whether there is volatilization loss and ultimately on the fertilizer use efficiency.
Karamanos was part of a laboratory experiment conducted by Monica Benke, Xiying Hao and Newton Lupwayi, with Agriculture and Agri-Food Canada at Lethbridge, Alberta, and Tee Boon Goh at the University of Manitoba. The experiment looked at the effect of soil texture, pH and moisture content on the retention of anhydrous ammonia injected into the soil.
Anhydrous ammonia was injected into 10 soils with varying soil properties: a pH range from 4.0 to 7.9, clay content from 2.1 to 75.4 percent, and soil organic matter from 1.8 to 8.7 percent. Soil moisture was at either 100 percent field capacity or air dry. Anhydrous ammonia was applied at 110 lbs of N per acre and compared to a control (only air injected). Nitrogen retention was measured at three hours, and one, two, four, seven and nine days after injection.
Measured one day after application, the researchers found that soil moisture did not influence N retention. Air dry soil retained 89 percent while the wet soil retained 86 percent. “We did not see any impact of soil moisture on the amount of N retained by the soil. This isn’t surprising because anhydrous ammonia quickly grabs on to any moisture available in the soil. Even with air dry soil, there is enough moisture to convert the ammonia to ammonium,” says Karamanos.
There were some soil texture differences. The maximum N retained at one day was 93 percent on an Orthic Gray Luvisol soil with a pH of 5.0, 75 percent clay and 50 grams per kilogram total carbon (a measurement of organic matter). The lowest amount of N retained at one day was 66 percent on a Dark Brown Chernozemic soil with a pH of 7.2, 12.7 percent clay content and 21 grams per kilogram total carbon. Karamanos says the N retention between these soils was significantly different.
The most important factor that influenced N retention was pH followed by organic matter content. Lower pH and higher organic matter retained greater amounts of N. There was no significant correlation between NH4 retention and soil clay content. “This could be related to the fact that clay type was similar among the soils used.”
Anhydrous quickly converted to stable mineral-N forms in the soil
When anhydrous ammonia is injected into the soil, it rapidly converts to ammonium (NH4), a positively charged cation that is bonded to soil particles and organic matter. This conversion usually happens in a reaction with water (H2O). Ammonium-N cations strongly adhere to mineral and organic matter particles in the soil, helping to prevent gaseous or leaching losses. Subsequently, the ammonium cation can be nitrified into nitrate forms (the NO2 and NO3 negatively charged cation), which are plant-available forms.
Soil structure will make a difference
Moving the results from the laboratory to the field, Karamanos cautions that soil conditions during application will override the laboratory results. Soil that is too wet or too dry may not provide the right conditions for the soil to flow around and close up behind the anhydrous ammonia applicator knife. Under these circumstances, there will be physical, gaseous losses of the anhydrous ammonia, not because the soil cannot capture the ammonia quick enough, but because the ammonia is not actually injected into the soil. Cloddy soils that do not close up, and wet soils where the furrow stays open may have gaseous losses, as indicated by a white vapour emitting from the soil behind the openers, and an overwhelming ammonia smell. “Under most field conditions, anhydrous ammonia retention in the soil is very high. Losses are only likely to occur when soil conditions don’t allow sealing in of the ammonia gas during application,” explains Karamanos.