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ANHYDROUS AMMONIA A detailed and comprehensive paper describing research conducted by two scientists at the National Fertilizer Development Center in Muscle Shoals, Alabama,* described how liquid ammonia, NH3, when injected into the soil, is transformed into a gaseous state and severely affects the structure and its water holding and drying ability. The research scientists described in detail what happens when anhydrous ammonia is injected into several soils. They found that the soil immediately around the retention zone that ammonia of NH3 has been injected is modified, or changed, in structure, causing a more rapid rate of water evaporation, or drying, than the normal soil without ammonia. They show how this tendency to dry out is increased as you proceed toward the center of the retention zone, or the point of release of the ammonia in a knife applicator. What is even more surprising is the fact that these patterns tend to persist over a period of several months, even when the soil is subjected to alternate wetting and drying. AGRONOMIC IMPORTANCE Although much of their work was done in the greenhouse, one set of experiments to evaluate the drying properties of anhydrous ammonia under actual field conditions was conducted. In the greenhouse studies, ammonia equivalent to 180 pounds of NH3 per acre was injected into pots at a depth of 3.25 inches. Numerous observations were made, testing the pH and soil drying properties of the soil immediately surrounding the point of injection. They found that the application of ammonia drastically changes the physical and chemical properties of the soil. For example, after injection of ammonia, when the soil was exposed to the atmosphere, any soil treated by ammonia resulted in a rapid rate of evaporation of water. This moisture loss, or rapid evaporation from ammonia-treated soil, may be of significant agronomic importance to farmers who wish to conserve and utilize rain water most efficiently. The actual field test was conducted with an ammonia applicator that injects ammonia seven inches deep at the rate of 180 pounds of NH3 per acre. After exposing a vertical cross section of the injection channel, the ammonia retention zone was tested for pH as well as for moisture holding capacity. The results showed that the drying effect on the soil lasted for at least 40 days and possibly longer. Refer to table 1, which shows the results of their field tests. Note that even ten days after the injection of anhydrous ammonia, the soil structure was still modified so that it would materially lose water faster. For example, at ten days after injection, a difference of 10.6% moisture in the treated zone, compared with 12.8% moisture in the untreated area, was noted. Under practical conditions, the ability of the soil to hold 2% more moisture may be significant in increasing or decreasing yields, as well as affecting materially the benefits that one might receive from anhydrous ammonia fertilizer. The injection of anhydrous ammonia creates a temporary high pH that can persist for a number of weeks. Refer to figure 1. Immediately after application, the pH at the center of the retention zone often exceeds 9.5. As time goes on, the ammonia moves out from the point of injection and the pH gradually drops and begins to approach the normal pH of the soil. In their test, they used a silt loam soil with a pH of 6.2. Two inches from the point of injection, the pH dropped from 8.8 to approximately 6.7 in five weeks’ time. Approximately three inches from the point of injection, the pH dropped from about 8.2 to 5.8 in five weeks’ time. Nitrification begins around the perimeter of the zone where sufficient dilution of ammonia, NH3, and reduction in pH has occurred, so as not to inhibit the nitrifying microorganisms. Referring to figure 1. One can see that the nitrification proceeds most rapidly outside of the two inch radius from the release point of anhydrous ammonia. This nitrification produces a lower pH - often so low, or the pH is reduced to such low levels, that can further inhibit the activity of the nitrifying microorganisms. SUMMARY A “seat of the pants” observation brings to mind the fact that it is often very difficult to get a significant response from high levels of anhydrous ammonia, either preplant or side-dress. This lack of response could well be due to the chemical reactions of anhydrous ammonia, NH3, in the soil as described above. In these days when we are striving for maximum yields, it would seem feasible that other sources of nitrogen, such as solutions 32 and 28, or ammonium phosphates be seriously considered instead of anhydrous ammonia. The applied cost per acre is often nearly the same for all sources, although the apparent low price tag on anhydrous ammonia usually does not take into consideration the high cost and the slow time consuming application expense. From a realistic viewpoint, long term usage of anhydrous ammonia could affect the crop producing ability of soil. Quite possibly, the price tag on anhydrous ammonia is not cheap after all.
Parr, J.F., and Khasawhen, F.E., Plant and Soil, XXIX, No.3; Drying patterns in Soil Following Anhydrous Ammonia Injection. (Dec. 1968) National Fertilizer Development Center, TVA, Muscle Shoals, Alabama |
