ENUMERATION AND INOCULATION OF NITRIFYING BACTERIA IN SOILLESS POTTING MEDIA

Autotrophic nitrifying organisms were enumerated in soilless potting m edia using the most probable number (MPN) technique. Populations of NH 4+ and NO(2)(-)oxidizing organisms varied widely between two soilless media-Metro-Mix 220 and 350, Estimates for NH4+ oxidizing organisms ra nged from 0.7 to 7.8 x 10(5) organisms/cm(3), while NO2- oxidizers ran ged from 1.3 to 9.5 x 10(5) organisms/cm(3). Population numbers were s imilar to those typically reported in soils. There was a significant e ffect of medium type, NH4+ N : NO3-N fertilizer ratio, and planting on MPN counts of both groups of organisms, with significant interaction between several of the factors. Estimates of NH4+ oxidizers were not l inearly correlated with NH4+ oxidizing activity, implying low counting efficiency, heterotrophic nitrification, or rate-limiting substrate N H4+ level. In a separate study, a soilless potting medium was inoculat ed with pure cultures of either Nitrosomonas europaea or Nitrobacter a gilis. Rates of NH4+ and NO(2)(-)oxidation increased, respectively, as inoculum volume increased. Inoculation with nitrifying bacteria may h elp in the overall management of N in the rhizosphere and be feasible alternatives for the prevention of either NH4+ or NO2- phytotoxicity w ith fertilizers containing urea or NH4+.