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The science of waiting for 50°

To hear Brandon’s Agribusiness Report on the science behind waiting to apply anhydrous ammonia until soil temperatures are 50° or less, click here.

With spring conditions a big unknown, some producers are applying anhydrous ammonia in the fall. Most producers making applications this fall will wait until the soil temperature is reliably less than 50°, and with good reason. Waiting is both an environmentally and economically smart decision.

Soil microbes will turn ammonium into nitrate, but the temperature of the soil has a significant impact on the rate of conversion, according to Iowa State University Extension Soil Fertility Specialist Dr. John Sawyer. Ideally the ammonium won’t be converted into nitrate until the spring, at which rate plants can make use of it, but if producers put anhydrous ammonia into the ground too early, it can be a costly mistake. Sawyer says there are two ways money can leave a farmer’s pocket as nitrate leaving his or her field.

A physical loss through leaching, as a nitrate is a negatively charged molecule, and so it can leach from the soil because the soil has a net negative charge, so that’s physical leaching, with water as it drains through the soil system, through the rootin zone, and then out either through groundwater or tile flow.
The other mechanism is conversion to nitrogen gas through what’s called ‘denitrification,’ and that’s again another microbial process where then microbes convert the nitrates to nitrogen gas, and then that gas, as N₂, is lost to the atmosphere.

Read more about soil temperatures and soil health on Iowa State Univeristy’s website by clicking here.