Estimating Nitrogen Losses From Wet Soils
DR. LLOYD MURDOCK & DR. GREG SCHWAB
PRINCETON, KY. LEXINGTON, KY.
Wet soils cause nitrogen losses, and determining
how much nitrogen is lost is
necessary to choose the proper management
options. 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 (NO3-
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
NO3-N for each day of saturation. Use the
Table below to determine how much fertilizer
NO3-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. Because
of a series of heavy rains, 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 percent lost each day
for five days, 20% would have been lost. 88 lb
N x 20% = 18 lb N lost and 60 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 NO3-N in
the soil after flooding is a NO3-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. Δ
DR. LLOYD MURDOCK: Extension Soil Specialist,
University of Kentucky
DR. GREG SCHWAB: Extension Soils Specialist,
University of Kentucky