Root‐induced release of interlayer potassium and vermiculitization of phlogopite as related to potassium depletion in the rhizosphere of ryegrass

SUMMARY 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.