Heated micaceous minerals are used as culture medium in greenhouses under different commercial brand names including vermiculite. Heating minerals affects the release of elements, such as potassium, by changing their structure. The aim of this study was to investigate the effect of heating on potassium release from muscovite and phlogopite, as a di- and a tri-ochtahedral mica, respectively, under greenhouse conditions and also during laboratory extraction. Particles of muscovite and phlogopite, with a diameter of less than 60?, were heated at 100 to 900 0 C for 5 hours. In order to study the heat-induced morphological changes, mineral surfaces were studied by a scanning electron microscope (SEM). The amount of extractable K from blank and heated minerals was then determined using two different solutions including ammonium acetate and sodium tetraphenyl boron for 3 time periods of 1, 5 and 15 minutes. To simultaneously study the effects of heat treatments and rhizospheric conditions on potassium release, a pot experiment was carried out as a completely randomized design with factorial combinations and three replications. Heated minerals were mixed with quartz sand to fill the pots as the culture medium for alfalfa, as a test plant. The blank culture medium contained pure quartz sand and unheated minerals. Pots were irrigated with two different nutrient solutions (+K and -K) and distilled water for 5 months. After harvesting, dry matter of shoot and root samples was measured separately. The K uptake by plant was then measured by flame photometer following the dry ash extraction. Results revealed different responses of micaceous minerals to heat treatments. SEM micrographs showed the exfoliation of minerals at lower and their flaking at higher temperatures. Heating from 200 to 350 0 C also caused the mineral structure to fold. The pot experiment indicated the significant influence of the heat, type of mineral and nutrient solution treatments on the plant dry matter, K uptake and concentration parameters. In K-free treatments, the plant K uptake was significantly influenced by culture medium. In phlogopite treatments, the shoot K concentration of the K-free pots was evaluated as sufficient except for the 700 and 900 0 C temperatures, whereas the sufficient shoot K concentration in muscovite treatments was only detected for plants supplied with the complete nutrient solusion. The highest plant available K was released at 100 and 200 0 C for muscovite and phlogopite treated plants, respectively. The structural changes, causing a considerable decrease in potassium release, also took place at above 700 0 C for phlogopite and 500 to 700 0 C for muscovite. The amount of total K uptake by plants from phlogopite under the K-free nutrient solution was considerably higher than that extracted by both extractants.