ROOT-INDUCED RELEASE OF INTERLAYER POTASSIUM AND VERMICULITIZATION OFPHLOGOPITE AS RELATED TO POTASSIUM-DEPLETION IN THE RHIZOSPHERE OF RYEGRASS

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 multifloru m), grown for 1 to 32 d on 2-105 mum mica as sole source of both K and Mg. The K-balance showed that ryegrass was able to induce a significa nt release of interlayer-K from phlogopite after 8 d of cropping. Afte r 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 a nd X-ray diffractometry showed a strong vermiculitization of the phlog opite. The transformation of phlogopite into vermiculite was detected when the K-concentration in the rhizosphere decreased below a threshol d of about 80 mumol dm-3. That suggests that the roots act by depletin g the rhizosphere-K, and thus shifting the exchange equilibrium, so as to release interlayer-K and expand the interlayer space of the phyllo silicate leading to the transformation of the phlogopite into vermicul ite.