Soil Temperature and Fall Applied Nitrogen Fertilizer

There is general recognition by crop producers that fall applied nitrogen should be planned so that it stays in the ammonium form throughout the fall, winter and early spring. Otherwise, when present in the nitrate form, there is a substantial risk for loss of nitrogen. As stated many times in the past, this loss can be the consequence of leaching or denitrification. The key to preserving nitrogen in the ammonium form is application of nitrogen fertilizer after the soil temperature drops below 50 degrees F. Those who keep track of soil temperatures in the fall also realize that temperatures do not remain below 50 degree F once that temperature has been reached. Highly influenced by air temperature, solar radiation, and moisture, soil temperature can easily move above and below the 50 F mark for several days. So, it's important to be aware of the average last date for measurement of soil temperatures higher than 50 degrees F. These dates are shown on the map below.

Soil Temperature Map
The dates depicted in the map above are estimated from the soil temperature records at eleven locations around the state (both National Weather Service and University of Minnesota sites) in conjunction with extrapolation of mean air temperature values and their relationship to soil temperatures. A bare surface condition (tilled with absence of crop residue) is assumed.

George Rehm, Mark Seeley - University of Minnesota Extension Service

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URL: http://climate.umn.edu/doc/soil_temp_fall_N.htm
Last modified: October 1, 2002