The adsorption affinity(logK_a) and desorption potential(logK_d, or relative desorption potential logK_dr) of cadmium in soil colloids were proposed and deduced of theoretically, as well as confirmed with experiment in detail. The definitions adsorption affinity and desorption potential were given by logK_d=log[Pa/(100-Pa)W/V]
logK_d=log[Pd/(100-Pd)V/W]
logK_d=logK_d+4.60
respectively, where Pa is adsorption % of Cd by colloids from solution, Pd is desorption % of Cd fram colloids with 0.1 M NaNO₃, W is weight of colloids and Y is volume of solution in the adsorption,or desorption processes respectively. Both logK_a and logK_d (or logK_dr) are intensive factors that deal only with the nature and characteristics of soil colloids, and are independent of the ratio of solid to liquid and concentration of Cd addition for adsorption or the ratio of liquid to solid and content of Cd for desorption in the certain range. logK_a depends significantly directly on pH, and logK_d(logK_dr) is markedly inversely correlated to pH.Some experiments applying log Ku and logK_d (or log lid,) showed that 1.logK_a could be used as a indicator of relative selectivity of soils or colloids for Cd; 2. logK_d, could be used as a indicator of relative fixed capacity of soils or colloids for Cd; 3. logK_a and logK_d could be used to prodict Cd adsorption % or pollution extent of soil colloids; 4. Cd uptake by wetland rice was significantly correlated to logK_a and logK_d (or logK_dr);Cd concentration in brown rice was decreased with increasing of logK_a or decreasing of logK_a (or logK_dr).Adsorption affinity(logK_a) and desorption potential(logK_d, or logK_dr) can reflect the resultant effect of the factors that affect adsorption or desorption of cadmium in soil colloids. It would be expected that logK_a and logK_d might be used in some research fields, such as the chemistry of soil environment, soil physical chemistry, the chemistry of plant nutrient, water chemistry, interface chemistry,etc.