| The effect of soil, clay and humic acid on nitrification was studied in laboratory and greenhouse experiments. Montmorillonite, kaolinite, illite and three soils: Cecil (clayey, kaolinitic, thermic, Typic Hapludults), Davidson (clayey, kaolinitic, thermic, Rhodic Paleudults) and a Venezuelan soil (fine, illitic, isohyperthermic, Fluventic Camborthids) were tested in these experiments. Humic acid was added at rates of 0, 400 and 1000 mg kg('-1) (soil experiments) or mg L('-1) (liquid cultures). Ammonium sulfate or KNO(,2) were used as N-substrates for Nitrosomonas and Nitrobacter, respectively.; Clays showed differential effects on the nitrification process. Their influence, especially of 2:1 clays, was very pronounced when applied to soils with low pH and poor buffer capacity. Two to 1 layer clays and soils containing this type of clay prolonged delay periods and favored a higher ammonium-oxidation rate. The latter was enhanced by treatments with humic acid interacted with clays. In general Nitrosomonas was affected more by clays than Nitrobacter. Humic acid exerted a beneficial effect on both corn plants and nitrifiers. Nitrite tended to accumulate in soils with high pH, but such a tendency was not evident in the presence of growing plants. Plant growth did not show any inhibitory effect on nitrification. |
