Soil mineralogical studies have shown that potassium (K) bearing minerals are dominant in Iranian soils, particularly in arid to semiarid regions. Numerous investigations on the release kinetics of non-exchangeable and structural K have indicated that plants can take up K from K-containing minerals in K deficient soils. Rhizospheric environments and plant root exudates can cause the releases of K from minerals and make mineralogical changes. In addition to soil properties, the biochemical characteristics of rhizosphere depend on the type of plant. Among different parameters affecting the K dynamic in soil, the effect of particle size of micaceous minerals on the K supply and mineralogical alteration has not been studied. Besides, the role of crop cultivars has not been investigated either. Therefore, the objectives of this research were: (1) to investigate the role of different alfalfa cultivars on the K uptake from phlogopite, (2) to understand the effect of phlogopite particle size on the K release, (3) to examine the influence of alfalfa cultivar on phlogopite mineralogical changes, and (4) to study the relationship between the K uptake by plant and mineralogical bio-weathering. A pot experiment was carried out under greenhouse conditions with a completely randomized design and factorial arrangement and 3 replications for a period of 6 months. Treatments were consisted of four phlogopite particle sizes including 53 µm (D 1 ), 53- 62 µm (D 2 ), 62- 260 µm (D 3 ), 250 µm- 1 mm (D 4 ) and control (no phlogopite), three alfalafa cultivars incloudingRehnani (R), Hamedani (H) and Pickseed2065 MF and 2 types of nutrient solutions (complete and K-free). During the growing period, shoot was harvested 4 times and the root was collected at the end. The K concentration in plant samples was measured by flame photometry after dry ash extraction. For mineralogical investigations, samples were taken from the D 1 treatment and their clay fraction was examined using an XRD.The results showed that the Rehnani cultivar produced the highest amount of both shoot and root under the K-free nutrient solution. Growth media containing D 1 and D 3 treatments produced the highest level of shoot and root, respectively. The least quantity of dry shoot and root was obtained in control treatment under the K-free nutrient solution with the MF cultivar. The maximum K concentration of both root and shoot under both nutrient solutions belonged to the MF cultivar. The highest K concentration in shoot and root was measured in D 1 and D 2 treatments, respectively. In pots treated with K-free nutrient solution, the MF cultivar could take up the highest amount of K. The highest uptake of K by shoot and root was measured in D 1 and D 3 treatments, respectively. Under the K-free nutrient solution, the K concentration in shoot increases up to the 3rd cut and then decreases. The finer the phlogopite diameter, the higher the amount of K released. Coarse sized phlogopite particles seem to release K much slower but for a longer of time.X-ray diffraction analysis of Mg- saturated phlogopite and its likely weathering products indicated that in all the 3 cultivars under the K-free nutrient solution, the intensity of the 1.0 nm peak decreases while a new 1.4 nm peak forms. The highest ratio of 1.4 to 1.0 nm peak intensity was obtained for the MF cultivar supplied with the K-free nutrient solution. XRD analyses well show the formation of expandable minerals such as smectite and vermiculate. Keywords: Release of potassium, Phlogopite, Alfalfa cultivars, Vermiculite, XRD