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Sustained Soil Temperatures Below 50 Degrees Needed For Fall Nitrogen Application

By Clarke McGrath
In running plots and talking with growers the last week or so, the shift to more “fall-like” temperatures has people thinking, “Maybe I can run some fall NH3 while (if) I am held up waiting on trucks, grain drying, or other typical fall delays.”
Having been present for recent conversations that growers were having with their fertilizer dealers (“I want this certain toolbar”) and engaged in discussions about N-Serve and how much N to apply, growers are hoping for a good fall application season. Knowing that the window could be tight this fall- and again this spring given there is little to no room to store additional precipitation so it won’t take much rainfall to stop NH3 applications- they are anxious to get a jump on it.
The bottom line is that fall N applications shouldn’t necessarily be targeted to start the first day that temperatures reach 50°F; be sure that the trend is for sustained soil temperatures below 50° F and continued cooling. The dates when soils cool below 50°F vary considerably from late October to late November. So don’t be tripped up by temporary cold spells, especially early in the fall. Watch the 6- to 10-day weather forecast; a forecast for above average temperatures may signal soil warming that could mean we need to wait to start.
To help illustrate where we are at, I like to use the Iowa State University NPKnowledge website that shows soil temperatures over the last several days, and has a link to the 6- to 10-day temperature forecasts to help dial in the right timing to start NH3 applications
We’ll cover more of the agronomics in a week or two, because that will still give us plenty of time to talk about best management practices, N-Serve, and what N rate to run.

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