We evaluated soil reserve potassium mobilized by plants in a greenhouse K exhaustion experiment from 15 smectite-dominant soils in relation to soil K status determined by chemical estimates and electro-ultrafiltration (EUF) fractions of K and soil mineralogy. Cumulative K uptake from the soils over a period of 420 days of cropping varied from 160 ± 11 to 823 ± 78 mg kg−1, and the plant mobilization rate of soil reserve K varied from 0.151 ± 0.009 mg kg−1day−1to 1.880 ± 0.164 mg kg−1day−1. Soils with higher K estimates by chemical and EUF methods showed greater K uptake and a higher plant mobilization rate of soil reserve K. The plant was used as an extractant in place of the usual chemical extractants for studying release kinetics. Kinetic studies indicated that mobilization of soil K by plants could be explained best by the parabolic diffusion equation (r= 0.985-0.998; SE=0.016-0.059) among the different mathematical models used, suggesting that plant mobilization of interlayer K is a diffusion-controlled exchange process. Release rate constants varied among the soils from 45.2 h−1/2× 10−3to 62.1 h−1/2× 10−3. The relationship of the mobilization rate of reserve K with soil mineralogy indicated that mica in silt, as well as in the clay fraction of soils, was a major source of K for K uptake by plants.