Nutrient Content, %

From Havlin, Beaton, Tisdale and Nelson (1999)

Anhydrous NH₃ 
Anhydrous 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 NH₃ 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:

1. Chemical
a. NH3 + H+ ---NH4+
b. NH3 + H20 --- NH4+ + OH-
c. Reaction of NH3 with OH- groups and tightly bound water of clay minerals.
d. Reaction with water of hydration around the exchangeable cations on the exchange complex.
e. Reaction with OM.

2. Physical
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.