Nitrification in soilless potting media

Research was undertaken to characterize the nitrification process in soilless potting media. In the evaluation of a short-term assay for the determination of nitrification activity, results showed that NO{dollar}sb2sp{lcub}-{rcub}{dollar} accumulation in the presence of chlorate (ClO{dollar}sb3sp{lcub}-{rcub}{dollar}) was linear between 6 and 18 hr of incubation. Ammonium oxidation rates were calculated from the slope of the line using linear regression. The difference between samples incubated with and without ClO{dollar}sb3sp{lcub}-{rcub}{dollar} was also linear between 6 and 18 hr and was used as an estimate of NO{dollar}sb2sp{lcub}-{rcub}{dollar} oxidation. Buffering of the assay at pH 7.0 had little effect on NH{dollar}sb4sp{lcub}+{rcub}{dollar} rates when compared to unbuffered assays. Chlorate was shown to block NO{dollar}sb2sp{lcub}-{rcub}{dollar} oxidation nearly 100% in Metro-Mix 220 (MM220), while only 80% and 75% in Metro-Mix 350 (MM350) and Metro-Mix 300 (MM300), respectively. Nitrapyrin was shown to block NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidation 100% in all media. Combinations of nitrapyrin and ClO{dollar}sb3sp{lcub}-{rcub}{dollar} indicated that NH{dollar}sb4sp{lcub}+{rcub}{dollar} and NO{dollar}sb2sp{lcub}-{rcub}{dollar} oxidation in these media were carried out by predominantly autotrophic organisms.; Ammonium oxidation rates increased rapidly in all media up to about 4-6 weeks of cropping, then declined to initial levels. Samples previously cropped with plants had higher oxidation rates than unplanted samples. Medium type and NH{dollar}sb4sp{lcub}+{rcub}{dollar}:NO{dollar}sb3sp{lcub}-{rcub}{dollar} fertilizer ratio significantly affected NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidation rates in media, but had no effect on the growth of either chrysanthemum or poinsettia plants. Metro-Mix 220 had significantly higher NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidation rates than either MM300 or MM350, while media fertilized with 25% NH{dollar}sb4sp{lcub}+{rcub}{dollar}:75% NO{dollar}sb3sp{lcub}-{rcub}{dollar} had higher rates than media fertilized with either 50:50 or 75:25. It appears that differences in NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidation can be partially explained by pH. High pH values were necessary, but not totally sufficient, for NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidation to occur.; Nitrite-oxidizing organisms were more abundant than NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidizers. Populations of both types of organisms varied widely in 18 different soilless media samples. Population numbers were similar to those typically reported in soils. Population estimates of NH{dollar}sb4sp{lcub}+{rcub}{dollar} oxidizers were not correlated with NH{dollar}sb4sp{lcub}+{rcub}{dollar}-oxidizing activity, implying low counting efficiency or heterotrophic nitrification.