I have been getting several calls recently about fall anhydrous application. First and foremost -- when should we go? We want to start when soil temperatures are at 50 degrees or lower, and falling. Why? Nitrogen (N) in the ammonium form, stays in place, but when it gets warmer it more rapidly moves into the nitrate form -- which is water soluble and can move when excess water is in our soil profile. In recent years it is usually early November before the soil temperature gets below 50 degrees and stays there. Yesterday, October 20, the average temperature was in the upper 40's. Will it stay there? To check the most recent soil temperature information, go to:extension.agron.iastate.edu/NPKnowledge/soiltemphistory.html.
Now for the tougher question, will we get anhydrous ammonia to seal in the soil this fall? Ammonia (NH3) is applied as a gas, and is an unstable compound in the soil. It wants to steal another "H" ion and become ammonium (NH4). It does this by grabbing an H ion from whatever water is there, but additionally the ammonia can react with organic matter and clay in the soil to bind it in place. Are you remembering your high school chemistry? Even when our soils are this dry, they do still contain some water -- we call it "plant unavailable water" that is held tightly to soil particles and isn't available for plants. Ammonia can still interact with it. Because the soil is currently drier this fall, I suspect that the zone where the ammonia is trapped and held by the soil is a larger area than when there is adequate water available. So, it might get closer to the soil surface.
Voids in the soil allow the gas to escape towards the surface if they exist and the gas is not in contact with soil. It is dry enough that I suspect end rows, and maybe some fields, will create "chunky" situations where air voids are formed by the tillage done with the anhydrous ammonia applicator knife. I think this is the biggest risk for ammonia loss we have this fall.
Another concern, if the ammonia gets closer to the surface than normal and we stay dry, we might see something that we have not seen in 10 years, injury to corn seedlings in the spring from fall anhydrous application.
Anhydrous ammonia application depth needs to be monitored carefully this year. We have to get the soil to seal. Use your nose to smell if anhydrous ammonia gas is escaping. If you cannot get the depth or the sealing you need, either set the equipment in a different way to get it to work or you will need to wait until you have better soil and moisture conditions.
As always, handle anhydrous ammonia safely -- this is a dangerous product!
For more discussion on this topic, see this ICM newsletter article from October 23, 2000 found at:www.ipm.iastate.edu/ipm/icm/2000/10-23-2000/anhydrous.html.
This newsletter information and previous issues, can be found on-line at: www.extension.iastate.edu/plymouth/info/....