Selecting Forms of Nitrogen Fertilizer
Nitrogen (N) can be lost from the soil through three principal pathways, (1)
DENITRIFICATION, (2) LEACHING, and (3) SURFACE VOLATILIZATION.
The form of N a gardener chooses should depend on how serious a problem he has
with the above N losses. Cost of N is another consideration when choosing a
fertilizer source.
DENITRIFICATION occurs when nitrate-N (N03-) is present in a soil and
there is not enough oxygen (02) present to supply the needs of the bacteria and
microorganisms in the soil.
If 02 levels are low, bacteria and microorganisms strip the oxygen
from the nitrate and the end result is the production of N gas (N2) or nitrous
oxide (N20), which volatilizes from the soil.
Three conditions that create an environment that promotes denitrification are
wet soils, compaction and warm temperatures for high activity of soil
microorganisms.
LEACHING losses of N occur when soils have more incoming water (rain) than
the soil can hold. As water moves through the soil, nitrates (N03-)
in the soil solution are picked up and moved with the water. Ammonia (NH4+)
forms of N have a positive charge and are held by the negative sites on the clay
in the soil; therefore, NH4+ forms of N leach very little. In sands where there
is very little clay, ammonium forms of N can leach. Relatively coarse soils
(sands and some mucks) are the only ones in which significant leaching of N is
important.
One way to minimize N leaching and denitrification is to minimize the time
the N is in the soil before plant uptake. This cuts down on the time when
conditions are favorable for losses. Most of the N is needed by corn after the
plant is three to four weeks old (June 1).
SURFACE VOLATILIZATION of N occurs when urea forms of N break down and
form ammonia gases and where there is little soil water to absorb them. This
condition occurs when urea forms of N are placed in the field but not in direct
contact with the soil. This situation can occur when urea is spread on corn
residues or 28% is sprayed on heavy residues of cornstalk or cover crop.
The rate of surface volatilization depends on moisture level, temperature and
surface pH of the soil. If the soil surface is moist, the water evaporates into
the air. Ammonia released from the urea is picked up in the water vapor and
lost. On dry soil surfaces, less urea-N is lost. Temperatures greater than 50oF
and pH's greater than 6.5 significantly increase the rate of urea conversion to
ammonia gases. Applying urea-type fertilizers when weather is cooler slows down
N loss. If the surface of the soil has been limed within the past three months
with two tons or more of limestone per acre, DO NOT apply urea-based fertilizers
unless they can be incorporated into the soil.
To stop ammonia volatilization from urea, the urea must be tied up by the
soil. To get the urea in direct contact with the soil it must rain enough to
wash the urea from the residue or the farmer must place urea-based fertilizer in
direct contact with soil by tillage, banding or dribbling.
If the residue is light, 0.25 to 0.5 inch of rain is enough to dissolve the
urea and wash it into the soil. If the residue is heavy, 0.5 inch or greater of
rainfall is required.
The common N fertilizers are anhydrous ammonia (82% N), urea (45- 46% N),
solutions (28-32% N), ammonium sulfate (21% N) and ammonium nitrate (34% N).
Anhydrous ammonia (82%) converts to nitrate N the slowest of any form of N
fertilizer. Therefore, it would have the least chance of N loss due to leaching
or denitrification. It must be injected into the soil; therefore, it would have
no loss due to surface volatilization. The disadvantage of anhydrous ammonia is
that it is hazardous to handle. It must be injected into the soil, and on steep
slopes erosion can be a problem.
Urea (45-46%) converts to nitrate N fairly fast, usually in less than two
weeks in the spring. Denitrification on wet or compacted soils can be serious.
Leaching can be a problem in coarse soils. In no-till situations surface
volatilization can be a problem if the urea is not placed in contact with the
soil and it is dry for several days after spreading.
Twenty-eight to 32 percent (28-32%) N solution is usually made up of urea and
ammonium nitrate. The nitrate in this product is subject to leaching and
denitrification from the time it is placed in the field. The urea components are
subject to the same loss mechanisms as urea.
N solutions can be banded on the soil surface easily by dribbling. This
method of application minimizes the amount that sticks to the residue;
therefore, surface volatilization is minimal.
Ammonium sulfate (21%) is a nitrogen source with little or no surface
volatilization loss when applied to most soils. Ammonium sulfate is a good
source of sulfur when it is needed. Its disadvantage is that it is the most
acidifying form of N fertilizer - it requires approximately 2 to 3 times as much
lime to neutralize the same amount of acidity as formed by other common N
carriers.
Ammonium nitrate (34%) is 50% ammonium N and 50% nitrate N when added to the
soil. The ammonium N quickly converts to nitrate N. For soil subject to leaching
or denitrification, ammonium nitrate would not be preferred. Ammonium nitrate
has no urea in it; therefore, surface application would be a good choice where
volatilization of urea is expected
02/01/2009
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