Ecological effects of germanium addition to soil on microorganisms and their critical value were emphatically discussed in this paper. A pot experiment was carried out with results demonstrating that low Ge concentration had stimulating ef fect on soil bacteria, fungi, actinomycetes, but when the content of Ge in the soil increased, the stimulating effect began to al ternate with inhabiting one. In the pot experiment, the luminescence of photobacterium phosphoreum spp. T3 showed a strongly negative relationship to the concentration of Ge. If "100%" and "80%" of the luminescence of Strain T3 is deemed as critical values for two grades, the critical concentrations of Ge in soil were est imated to be 50.2 mg kg^-1 and 5.8 mg kg^-1, respectively.