Samples of several California soils were incubated in a controlled environment chamber after addition of varying amounts of urea in solution. Samples were analyzed after time intervals ranging from 1 day to 8 weeks. In short‐term experiments analyses for residual urea were performed at frequent, short intervals, and in longer‐term experiments nitrification of the hydrolysis product was followed. At 45°F., 200‐ and 400‐ppm. amounts of urea were completely hydrolyzed after 7 days, although some urea remained after 3 days. At 75°F., no urea remained after 2 days when the initial application was 400 ppm or less. At 800 ppm., considerable urea remained at 3 days but none at 14 days. Urea was rapidly nitrified in all soils at moderate levels of application but was inhibited in the poorly buffered Hanford sandy loam by the 800‐ppm. application, due to the high pH produced. Leaching experiments with soil columns showed that urea moves less rapidly than nitrate, partly because of rapidly hydrolysis to ammonia and partly because urea itself is held by weak absorption forces.

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Incubation studies of factors influencing nitrite oxidation in four California soils were conducted at two temperatures-45° and 75° F. In acid soils nitrite transformations were found to be highly sensitive to nitrite concentration in the soil and were inhibited by very low levels. Nitrite had less effect on the second step of nitrification in alkaline soils. Nitrite oxidizers were shown to be very sensitive to low temperature soil conditions.Although nitrite disappeared rapidly following addition to acid soils, little nitrate was produced during the incubation period. Much of the nitrite-nitrogen was lost from the system. Experiments using acid soils which had been sterilized showed similar losses in added nitritenitrogen and lead to the conclusion that these losses result from nonbiological processes. Studies to determine the nature of these losses suggest nitrogen gas as the principal product.Investigations of respiration rates of the soil population where various concentrations of nitrite were added reveal that nitrites inhibit the respiratory activity of the soil microorganisms as a whole and not merely the nitrifying group. Very low concentrations were inhibitory in acid soils; however, in a calcareous soil more than a hundredfold increase in nitrite concentration was required to achieve the same magnitude of respiratory inhibition.

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Immobilization of tracer nitrogen added as ammonium sulfate or as nitrate was followed by analysis of inorganic nitrogen in Moreno sandy loam and Sacramento clay during the course of incubation experiments. In Sacramento clay receiving ammonium sulfate plus straw the nitrifying bacteria were able to compete effectively with the immobilizing flora for nitrogen, whereas in the Moreno soil this was not the case. In both soils immobilization attained a maximum in 6 to 10 days. There was some increase in inorganic nitrogen thereafter, but none of this resulted from remineralization of the tracer nitrogen added initially. In greenhouse experiments competition for added tracer nitrogen between a growing crop and the immobilizing flora emphasized preferential utilization of ammonium nitrogen by soil microorganisms and of nitrate nitrogen by the crop. In Moreno sandy loam, overall recovery of the added tracer was higher under treatments favoring immobilization than in pots receiving only fertilizer.