Potassium is one of the essential elements for plant growth which is mostly supplied by K-bearing minerals in soils. The role of these minerals, particularly micas, in K supply for plants is well known. However, little information is available on the rate of K release from micaceous minerals and consequent mineralogical changes with time. The objective of this study was to investigate the temporal changes of concentration and uptake of potassium from phlogopite and biotite and consequent biological alteration of these minerals in alfalfa rhizosphere. A pot experiment was conducted under greenhouse conditions. The experimental design was a completely randomized design with factorial combination and 3 replicates. The growth medium was a mixture of Hamadan quartz sand and micaceous mineral (Hamadan phlogopite or Amlashbiotite). During 200 days of alfalfa cultivation, plants were irrigated with either a complete or K-free nutrient solution and distilled water as needed. At 40, 75, 110, 140, 165 and 200 days after cultivation, 3 replicates of each treatment were harvested and their K concentrations and uptake were measured by flame photometer following dry ash extraction. Also, the mica particles and their weathering products in each pot were separated from the quartz sand and their clay fraction analyzed using X-ray diffraction (XRD). Results indicated that the K concentration in plant tissues significantly decreased with time. In addition, phlogopite was able to maintain plant potassium concentration in sufficient range for more than 110 days but this period for biotite was 75 days. Also, the parameters of growth and uptake rates had significant changes with time. At the early stage of plant growth, both growth rate and K uptake were low. These parameters increased with time as the ability of root to extract potassium also increased. At the end of experiment, because of the plant age and decline in potassium source, root power for potassium uptake reduced and, thus, the amounts of plant growth rate and K uptake rates also decreased. XRD data showed the transformation of phlogopite and biotite under both nutrient solutions during the experiment. Under K-free nutrients solution, phlogopite and biotite had much greater mineralogical changes with time as compared to the complete nutrient solution. It seems that considerable mineralogical changes in clay sized phlogopite occurred and rhizospheric conditions induced the transformation of phlogopite to vermiculite, smectite and chlorite. Beside this transformation for biotite was less than phlogopite and quantities of smectite and chlorite was very low. Temporal changes in the ratio of 1.4 to 1.0 nm XRD peaks was very similar to changes in growth and potassium uptake rates of alfalfa in 200-day growth period. At the early stage of plant growth until 110 days, this ratio was low. The rate of mineral transformation increased with time as the ability of roots to uptake interlayer potassium increas