Greg Schwab and Lloyd Murdock
Wet soils cause nitrogen losses. In cases where high intensity rain results
in high runoff, leaching losses will probably be
low. The primary nitrogen loss mechanism in
saturated soils is denitrification, which occurs
when soil nitrate nitrogen (NO
3-N) is converted
to nitrogen gas by soil bacteria. Two to three
days of soil saturation is required for bacteria to
begin the denitrification process.
Well-drained upland soils that have been wet from a series of rains probably have not experienced much
denitrification. Soils in lower landscape
positions that stay saturated longer will likely
lose more N. Losses can be calculated by
estimating 3 to 4 percent loss of fertilizer NO
3-N
for each day of saturation. Use the Table below
to determine how much fertilizer NO
3-N was in
the soil.
EXAMPLE: Determining the Amount of
N Loss
A farmer applied 175 lb nitrogen (N)/A as urea to
corn grown on poorly drained soil. Three weeks after application
the field became saturated for seven days. How
much N was lost?
Step 1. Determine the amount of applied N
that was in the nitrate (NO3‐N) form.
According to the table, 50% of the urea will be in the
NO3‐N form three weeks after application. 175 lb N x
50% = 88 lb N.
Step 2. Determine the amount of N lost.
Remember that two days are needed for the bacteria
to begin the denitrification process. Therefore,
denitrification occurred for five days (seven days
total saturation minus two days to start the process).
With 4% lost each day for five days, 20% would have
been lost. 88 lb N x 20% = 18 lb N lost and 157 lb N
remaining. The N loss calculated in this example is
not as high as most people would assume. A soil N
test can verify this estimation.
Nitrogen Soil Test
An additional tool for determining NO
3‐N in the soil
after flooding is a NO
3‐N test. The soil sample should
be taken down to 12 inches deep, and several
samples should be taken in each field of both the low
and higher ground. The samples should be mixed
well and a subsample sent for nitrate analysis.
If the nitrate‐N is less than 11 ppm, there is a low
amount of plant‐available N in the soil. Therefore,
there is a good chance corn will respond to a
sidedress application of N ranging from 100 to 150
lbs N/acre.
If the nitrate‐N is between 11 and 25 ppm, there is a
greater amount of plant‐available N in the soil,
indicating corn may or may not respond to sidedress
N. The recommended sidedress N application at this
soil test level is 0 to 100 lbs N/acre. If the soil test
nitrate‐N is close to 11 ppm, then higher sidedress N
rates would be used. Lower rates would be used as
nitrate‐N approaches 25 ppm. The test is least
accurate in this range, so the test results can only be
used as a broad guide.
If soil test nitrate‐N is greater than 25 ppm, there is
adequate plant‐available N in the soil, which
indicates corn will probably not respond to sidedress
N application.
Nitrogen Broadcast Prior to Rain
Farmers sometimes broadcast fertilizer nitrogen on a
field within 24 hours of a heavy rain. In most cases,
very little nitrogen is lost to runoff, especially if the
field was under no‐till soil management. The
nitrogen fertilizer begins to dissolve almost
immediately after being applied to the soil surface
and will dissolve completely in a short period of time.
As rain begins, the first water that falls moves into
the soil, taking most of the fertilizer nitrogen with it.
Once in the soil, most of the fertilizer nitrogen is
protected from runoff. The only exception is a very
intense rain soon after application that also erodes
topsoil from sloping areas. Even in this situation, the
loss would probably be less than one third of the
fertilizer applied.
