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.
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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Last modified: October 1, 2002