The mineralization of the nitrogen of soils during incubation: Influence of pH, total nitrogen, and organic carbon contents

The ammonia, nitrite and nitrate which accumulated in 19 soils (containing less than 0.25% of total nitrogen and which had not recently received inorganic nitrogen fertilizers) during three weeks' incubation, both in the presence and absence of added chalk, at 28° and optimum moisture content, were determined. Ammonia accumulation was generally high in acid and low in neutral and alkaline soils. The addition of chalk resulted in less ammonia accumulation in all but one of the soils. Small amounts of nitrite accumulated in six of the soils; this occurred only when some acid soils were incubated with added chalk. All the soils made a positive response in nitrate and total mineral nitrogen accumulation to chalk additions, the response decreasing as the original soil pH increased. In the absence of added chalk total mineral nitrogen accumulation was significantly correlated with total nitrogen but not with organic carbon, whereas in the presence of added chalk it was poorly correlated with both factors. Partial correlations showed a similar trend. In the absence of added chalk, nitrate accumulation was significantly correlated with total nitrogen and organic carbon, and in its presence with total nitrogen only. The only partial correlation that was significant was that between organic carbon and nitrate accumulation in the absence of added chalk. The carbon/nitrogen ratio was not correlated with nitrate or total mineral nitrogen accumulation per unit of total nitrogen present. A comparison of the amounts of nitrogen mineralized during three and six weeks' incubation indicated that air‐drying of soils causes a certain amount of partial sterilization. If the incubation method is to be used as a measure of the availability of the organic nitrogen in soils, more satisfactory results should be obtained by incubating the original soil and determining total mineral nitrogen accumulation than by incubating with added chalk and determining nitrate only.