Abstract
Effects of medium, NH4: N03, ratio, and N fertilizer rate on the development of NH4 oxidation in soilless potting media were evaluated. In two separate experiments, NH4 oxidizing activity increased to a maximum (4 to 6 weeks of cropping) and then dropped off sharply. Ammonium oxidation activity varied significantly among types of soilless potting media. Media fertilized with 1 NH4-N: 3 N03-N had higher rates of NH4 oxidation than media fertilized with ratios of either 1:1 or 3:1. Nitrogen fertilization at 15 mm gave consistently higher oxidation rates than fertilization at 30 mm. In general, media samples that had been cropped with plants had higher nitrifying activity than unplanted samples. Ammonium oxidation rate over all observations was significantly correlated with medium pH (r = 0.50). pH values above 6.8 were necessary but not completely sufficient for relatively high rates of NH4 oxidation. Rates of oxidation were insignificant with pH values <5.6.
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