The Comparative Rate of Nitrification of Anhydrous Ammonia, Urea, and Ammonium Sulfate in Sandy Soils

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AbstractThree acid sandy soils, Arredondo, Leon, and Klej fine sands, were used in comparing the rates of nitrification for anhydrous ammonia, urea, and ammonium sulfate in the laboratory. Anhydrous ammonia stimulated the nitrification process more than equivalent applications of ammonium sulfate in limed and unlimed soil. The stimulating effect of urea was of the same general order as anhydrous ammonia. Anhydrous ammonia applied to the soil raises the pH into a more nearly optimum range for nitrification and initially provides a base for the neutralization of the nitric acid produced. This process can be effective only so long as high concentrations of ammonia are present in the soil; in these soils, which nitrified at low rates, this period of time was long enough to initiate a good rate of nitrification. The stimulating effect may also occur with field applications of anhydrous ammonia, since an adequate concentration of ammonia is retained in a relatively small volume of soil. The stimulation of nitrification by anhydrous ammonia has not been definitely shown under field conditions; this has probably been due to failure to properly delineate the retention zone into a central core where ammoniacal nitrogen was high enough to be initially toxic to the nitrifiers and an optimum zone between the toxic concentration and that in untreated soil.