Weathering of K-bearing minerals is the major source of potassium in soils which is of great importance under K deficient conditions. Many studies have been carried out on the effect of organic matter on soil physical, chemical, biological, and nutritional properties including the effect of organic matter on the availability of such elements as P, N and heavy metals. However, there is no information on the effect of organic matter on potassium release from micaceous minerals and their mineralogical changes. The objective of this study was to investigate the effect of organic matter on potassium uptake from micaceous minerals by alfalfa and the mineralogical changes. A pot experiment was carried out in a completely randomized design with factorial combination and three replicates. The growth medium was a mixture of Hamadan quartz sand, micaceous mineral ( Hamadan phlogopite, Amlash biotite and Hamadan muscovite) and organic matter (0, 0.5 and 1 % cocopeat). During 120 days of cultivation, plants were irrigated with either complete or K-free nutrient solution and distilled water as needed. At the end of cultivation, plant shoots and roots were separately harvested. The dry matters of plants were determined and their K concentration was 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). Under the K-free nutrient solution, a significant increase in shoot and root biomass occurred in pots containing trioctahedral micas (phlogopite or biotite) and organic matter as compared to those with no organic matter amendment. Also, under K-free condition, potassium concentration in shoot was above the threshold value only in phlogopite amended pots. There was no significant difference in shoot K concentration and uptake among different levels of organic matter in control treatment as well as in muscovite added treatment. Under both nutrient solutions treatments, significant increase of shoot and root K uptake occurred in pots containing trioctahedral micas (phlogopite or biotite) and organic matter, as compared to those without it. XRD data showed the transformation of phlogopite and biotite under both nutrient solution treatments, but no XRD detectable transformation of muscovite was detected. Organic matter amendment seems to have created considerable mineralogical changes in clay sized phlogopite and induced the transformation of phlogopite to vermiculite and a minor quantity of smectite and chlorite. Under complete nutrient solution, the addition of organic Keywords: K release, phlogopite, biotite, muscovite, mineralogical changes, organic matter