The recolonization by Nitrosomonas and Nitrobacter of a soil sterilized by gamma radiation was extremely slow compared with the rates of recolonization by heterotrophic bacteria and fungi. As a consequence, ammonium N formed during and following irradiation was not nitrified for at least 11 weeks and plants growing on irradiated inoculated soil were ammonia fed. Under these conditions plant growth was better in irradiated-inoculated soil than in unsterilized soil. After 3 weeks, wheat from irradiated inoculated soil had three times the asparagine content found in wheat from unsterilized soil. Soil that has been irradiated then inoculated could be a useful tool for studying both nitrogen turnover in soil and the availability of nitrate and ammonium to plants growing in soil under nearly natural conditions.