Some physical and chemical aspects of ammoniacal nitrogen in soils

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Virginia Polytechnic Institute

Free energy changes of oxidative inorganic nitrogen transformations in acid and basic media were calculated and interrelated. The free energy values of reactions involving nitrification were not always found to be in agreement with those reported in the literature. The primary reason for the deviations was that the pH of the medium had often not been considered in formulating these reactions. It was also found that thermodynamically, hyponitrous acid and nitrous oxide might well be intermediates in the process of nitrification. The existence of hydroxylamine as an intermediate between ammonia and/or ammonium ion and nitrite was not supported by the data obtained. It was shown that in acid solutions nitrite may thermodynamically undergo three reactions; it may decompose to nitric oxide, oxidize to nitrate or be reduced to nitrous oxide. In alkaline solutions nitrate seems to be the only product.

Ammonia volatilization and water evaporation from the soils investigated followed different functions and may be independent of each other. The functions were not affected by differences in soil texture, soil water, speed and relative humidity of air flowing over the soil surfaces. Ammonia losses from finer-textured soils were proportional to their original soil pH's. Norfolk fine sandy loam, pH 6.7, lost most ammonia indicating that soil texture is also an important factor in ammonia volatilization. A linear relationship was observed between rate of ammonia applied (up to 600 ppm) and ammonia volatilized. The rate of ammonia volatilization followed a first-order reaction. Deviations from a first-order reaction were observed when higher levels of ammonia were applied.

In regard to mechanisms of ammonia adsorption by soils it was found that at low concentrations, ammonia adsorption followed Langmuir's monomolecular adsorption theory. The differential slopes obtained for different sections of the curves were related to reactions involving "hydrogen", exchange sites and physical adsorption. The S-shaped curve obtained for high concentrations suggests the formation of polymolecular layers. Ammonia adsorption by neutral and basic soils also gave a typical Freundlich adsorption isotherm and furthermore closely simulated the behavior of a buffer medium. In fine-textured soils the amount of ammonia retained was inversely related to the original soil pH. The subsequent difference in adsorption capacity for ammonia between Yolo loam and Davidson clay, and between the buffer medium and Norfolk fine sandy loam seems to indicate that kind and amount of salt and/or weak acid may have a greater effect on ammonia adsorption than pH. Between texture groups, texture appears to exert a major influence on ammonia adsorption capacity.