Consider TemperatureWhen Applying Anhydrous
URBANA, ILL.
Now that soil temperatures
4 inches below the
surface are cooling down
to 50°F or lower, some will
soon be busy applying anhydrous
ammonia in preparation
for next year’s corn crop.
I think it is worth repeating
the reason for this temperature
threshold for fall nitrogen applications. It
is important to wait until soil temperatures fall
below 50°F because the activity of nitrosomonas
bacteria – responsible for transforming ammonium
to nitrite, which is then rapidly transformed
to nitrate – is reduced substantially.
Where temperatures do not get cold long
enough in winter (in Illinois, roughly south of
Route 16), fall N application is not an option because
the risk of N loss is too great. Because soil
temperatures vary as a result of many factors
(including soil color, amount of crop residue,
and moisture content), it is best to check the
temperature in the field of interest. You can a
general idea of soil temperatures at this link.
The question I have been asked most often in
the last week or so is whether soils are too dry
to apply anhydrous ammonia. As Illinois farmers
know, some parts of the state received very
little rain in July and August. While some locales
have received rain lately, there are areas
that are still very dry.
Is it too dry to apply anhydrous ammonia? It
doesn't take much moisture for ammonia (NH3)
to react with H+ ions from water to convert to
ammonium (NH4+). The biggest concern should
be not so much whether there is enough moisture
in the soil to react with ammonia, but
rather how moisture conditions impact the sealing
of the ammonia knife track. When soils are
dry, sometimes big clods are formed, leaving
large channels through which ammonia can
move quickly and escape to the atmosphere –
the loss can be substantial. If soil conditions are
such that the knife track is sealed with loose
soil, you can expect ammonia to be retained.
How deep should I apply anhydrous ammonia?
In dry soils anhydrous ammonia will move
in the soil a little farther than when there is
good moisture content. In very dry soils you can
expect it to move about 5 to 6 inches from the
point of application. Applying to a depth of 8
inches would be advisable in such conditions to
ensure that ammonia does not diffuse all the
way to the soil surface in search of water. That
said, deep application can mean a longer time
next year for corn roots to reach the nitrogen
band, so I suggest applying no deeper than
about 8 inches. If you apply deeper than that, it
would be advantageous to reduce your fall nitrogen
rate and apply that nitrogen near the soil
surface in spring so it is close to the newly developing
root system.
How long should I wait to till after applying
anhydrous ammonia? You can combine shallow
tillage (field cultivation, disking, etc.) with ammonia
application in fine-textured soils so long
as the soil has adequate moisture and ammonia
is applied behind the tillage operation at
sufficient depth to ensure that ammonia diffusion
does not reach the soil surface (see the earlier
discussion). If deeper tillage is needed after
the application, it is important to wait at least 5
to 8 days so the ammonia has time to react with
soil water and form ammonium. The reaction is
typically very fast, but the speed depends on soil
conditions.
How do I know if I am doing the right thing?
Once you evaluate your situation and decide
how to best ensure a successful anhydrous ammonia
application (rate, depth, tillage, etc.), the
best and easiest test is to go back to the field
after an application path and smell for ammonia,
as the human nose is very sensitive to its
aroma. Though you can’t quantify how much is
being lost, if you can smell ammonia a few
hours after application, you should wait for better
conditions. If there is no smell, nitrogen loss
from the application is probably minimized.
Some worry about losing ammonia in the
white puffs they see during application, but
what you see is water vapor-ammonia gas is colorless.
The cloud of water vapor forms because
anhydrous ammonia when it is applied rapidly
cools the warm air around it, causing water
condensation. Again, as long as ammonia
smells do not persist after application, any
clouds you see should not be a major concern.
Consider Your Options
Most of the points I have outlined relate to
minimizing nitrogen loss during or soon after
application. It is important to realize that
volatilization losses are not the only concern
with fall nitrogen applications. Because of the
long time between application and when the
corn plant uses nitrogen, fall applications are
typically riskier in terms of nitrogen loss than
preplant or sidedress applications, so they
should be considered carefully. In issue 22 I
provided additional information on applying nitrogen
in the fall. Δ
DR. FABIÁN G. FERNÁNDEZ: Extension Professor
Crop Sciences, University of Illinois