From Havlin, Beaton, Tisdale and
NH3 contains 82%
N, the highest amount of any N fertilizer. In some respects NH3 behaves
like water, since they both have solid, liquid, and gaseous states. The
great affinity of anhydrous NH3 for water is apparent from its
solubility. As a result, NH3 is rapidly absorbed by water in human
tissue. Because NH3 is very irritating to the eyes, lungs, and skin,
safety precautions must always be taken with anhydrous NH3 use. Safety
goggles, rubber gloves, and an NH3 gas mask are required safety
equipment. A large container of water attached to the NH3 tank is also
required for washing skin and eyes
exposed to NH3. Under normal atmospheric conditions, anhydrous
NH3 in an
open vessel boils and escapes into the atmosphere. To prevent escape, it
is stored under pressure and/or refrigeration (-28F), as is often done
at large, modern bulk-storage facilities. When liquid NH3 is released
from a pressurized vessel, it expands rapidly, vaporizes, and produces a
white cloud of water vapor. This cloud is formed by the condensation of
water in the air surrounding the liquid NH3 as it vaporizes.
Because anhydrous NH3 is a gas at
atmospheric pressure, some may be lost to the above-ground atmosphere
during and after application. If the soil is hard or full of clods
during application, the slit behind the applicator blade will not close
or fill, and some NH3 will escape to the atmosphere. Anhydrous
convertors are often used to reduce the need for deep injection and
pre-application tillage. The convertors serve as depressurization
chambers for compressed anhydrous NH3 stored in the applicator or nurse
tank. An- hydrous NH3 freezes as it expands in the convertors,
separating the liquid NH3 from the vapor and greatly reducing the
pressure. The temperature of liquid NH3 is about -32C (-26F).
Approximately 85% of the anhydrous NH3 turns to liquid; the remainder
stays in vapor form. The liquid flows by gravity through regular
application equipment into the soil. Vapor collected at the top of the
convertor is injected into the soil in the usual manner.
RETENTION ZONES. Immediately after
injection of NH3 into soil, a localized zone high in both NH3 and NH4 Is
created. The horizontal, roughly circular- to oval-shaped zone is about
I-X to 5 in. in diameter, depending on the method and rate of
application, spacing, soil texture, and soil moisture content. Vertical
movement is normally about 2 in., with most of it directed toward the
soil surface. A number of temporary yet dramatic changes occur in NU3
retention zones that markedly influence the soil chemical, biological,
and physical conditions in the retention zone. Some of the conditions
that develop include
1 .Increased concentrations of NH3 and
NH4+ (1,000 to 3,000 ppm).
2. pH increases to 9 or above.
3. N02- increases to 100 ppm or more.
4. Osmotic suction of soil solution that exceeds 10 bar.
5. Lower populations of soil microorganisms.
6. Solubilization of OM.
Free NH3 is extremely toxic to
microorganisms, higher plants, and animals. It can readily penetrate
cell membranes, which are relatively impermeable to N"4+. There is
a very close relationship between pH and concentration of free or
non-ionized NH3 and NH4+. Between pH 6.0 and 9.0, there is a 500-fold
in- crease in NH3 concentration (Fig. 4.35). Figure 4.42 summarizes
schematically the effects of pH, osmotic suction, and/or NH4+
concentration on the formation of N02- and N03-- The influence of high
osmotic suction or NH4+ in the soil solution is primarily on
Nitrosomonas bacteria. Activity is retarded by pH values above 8.0,
especially in the presence of large amounts of NH3. N02- accumulates at
pH values between 7 and 8, whereas below pH 7, N03- becomes abundant.
NH3 is lost to the atmosphere if it does riot react rapidly with water
and various organic and inorganic soil components. Possible NH3
retention mechanisms are as follows:
a. NH3 + H+ ---NH4+
b. NH3 + H20 --- NH4+ + OH-
c. Reaction of NH3 with OH- groups and tightly bound water of clay
d. Reaction with water of hydration around the exchangeable cations on
the exchange complex.
e. Reaction with OM.
a. NH4+ fixation by expanding clay minerals.
b. Adsorption by clay minerals and organic components through H bonding.
The relative importance of these
mechanisms varies from soil to soil and is also influenced by
environmental conditions. The capacity of soils to retain NH3 increases
with soil moisture content, with maximum NH3 retention occurring at or
near field capacity. As soils become drier or wetter than field
capacity, they lose their ability to hold NH3. The size of the initial
NH3 retention zone decreases with increasing soil moisture. Diffusion of
NH3 from the injection zone is impeded by high soil moisture, be- cause
of the strong affinity of NH3 for water. The NH3-holding capacity of
soils increases with the clay content. NH3 Movement is greater in sandy
soils than in clay soils since NH3 can diffuse more freely in the larger
pores in coarse-textured soils. Soil textural differences in NH3
retention are often obscured by other properties, such as OM and
moisture content. As might be expected, NH3 retention increases with
increasing depth of injection and varies considerably, depending on soil
properties and conditions. Studies have shown that an injection depth of
5 cm was effective for a silt loam soil, but placement at 10 cm was
necessary in a fine, sandy loam soil. In dry soil, NH3 loss declines
with increasing placement depth (Fig. 4.43). At a given rate, the NH3
applied per unit volume of soil decreases with de- creasing injection
spacing. With the greater retention achieved with narrow spacings, there
is less chance of NH3 loss, particularly in sandy soils with limited
capacity for holding NH3- The OM component of soils contributes
significantly to NH3 retention. At least 50% of the NH3-holding capacity
of soils is attributed to OM. The nature and extent of
changes in soil properties with NH3 applications can have an important
bearing on crop responses to N fertilizers. The high concentration of
NH3 and NH4+, which produces high soil pH and high osmotic potential,
results in a partial and temporary sterilization of soil within the
retention zone (Table 4.24). Bacterial activity is probably affected
most by free NH3, while fungi are depressed by high pH. Partially
sterilized conditions at the center of the retention zone are known to
persist for as long as several weeks.
A rapid recovery in the activity of bacteria and actinomycetes generally
occurs. As a consequence of reduced microbial activity, nitrification of
NH4+ to N02- and N03- will be reduced until conditions return to normal.
High concentrations of NH3, NH4+, and N02- can severely damage
germinating seedlings (Fig. 4.44). Concentrations in excess of 1,000 ppm
of NH3 near the seed were associated with substantial reductions in corn
plants. Deeper injection offsets the harmful effects of high rates of
NH3 more than extending the time for the NH3 effects to dissipate.
Closer spacing of the NH3 injection would also reduce the injurious
effect of large amounts of NH3- The OH- produced by the reaction of
anhydrous NH3 in soil will dissolve or solubilize soil OM. Most of these
effects on OM are only temporary. Solubilization of OM may temporarily
increase the availability of nutrients associated with OM. Contrasting
beneficial and harmful effects on soil structure have been reported
following the use of anhydrous NH3. Several long-term studies have shown
no difference among N sources on soil physical properties. Impairment of
soil structure is not expected to be serious or lasting except in
situations involving low-OM soils, in which any alteration or loss of OM
would likely be harmful.