An experiment was carried by soil culture and sand culture on variation of cation exchange capacity(CEC) of roots, readily available K and slowly available K in rhizospheric and non-rhizospheric soils of flue-cured tobaccos (Nicotiana tabacum L.) different in genotype (i.e. Nongda 202, K326 and NC89), and activating effects of root exudates, and population of rhizospheric microbes on soil K. Results showed that in terms of soil readily available K in both rhizospheric and non-rhizospheric soils, the three treatments followed the order was Nongda202＞K326 ＞NC89. Root exudates from tobaccos, regardless of genotype, activated potassium in the soils. However, among the three Nongda 202 showed the strongest effect, and the differences between Nongda 202, K326 and NC89 were extremely significant. The number of fungi in the rhizospheric soils of the three genotypes of flue-cured tobacco increased over those in the non-rhizospheric soils, especially in the rhizospheric soil of Nongda 202, much higher than of K326 and NC89. However, the difference between the latter two was not significant. The differences among the three genotypes of tobacco in population of rhizospheric bacteria were extremely significant, but not in number of actinomycete. Nongda 202 was the higher than NC89 and K326 in CEC of the root exudates, but the latter two were not so different. So it is important to pay attention to physiological properties of the roots in selecting K-enriching tobacco varieties.