The ability of roots to weather phlogopite as shown by the release of interlayer-K and by the concomitant vermiculitization of the mica was investigated in the rhizosphere of Italian ryegrass (Lolium multiflorum), grown for 1 to 32 d on 2–105 μm mica as sole source of both K and Mg. The K-balance showed that ryegrass was able to induce a significant release of interlayer-K from phlogopite after 8 d of cropping. After 32 d, this root-induced release of K amounted to 191 g Kg−1 of total-K and contributed a major part of plant nutrition. Concurrently, the concentration of K in solution decreased markedly in the rhizosphere and X-ray diffractometry showed a strong vermiculitization of the phlogopite. The transformation of phlogopite into vermiculite was detected when the K-concentration in the rhizosphere decreased below a threshold of about 80 μmol dm−3. That suggests that the roots act by depleting the rhizosphere-K, and thus shifting the exchange equilibrium, so as to release interlayer-K and expand the interlayer space of the phyllosilicate leading to the transformation of the phlogopite into vermiculite.