Rapid Weathering of a Trioctahedral Mica By the Roots of Ryegrass

The weathering of a trioctahedral mica (phlogopite, 2-100-mu-m particle size) was investigated in the rhizosphere of 'Turilo' Italian ryegrass (Lolium multiflorum Lam.). A special culture vessel made it possible to simulate a one-dimensional macroscopic rhizosphere by developing a root mat on a polyamide net. This root mat was laid down on a mica-agar substrate that was cut into slices 0.5 mm thick (parallel to the root mat) after 1, 2, 3, and 4 d of contact. The phlogopite mica was the sole source of both K and Mg for the plants. Whereas there was no significant release of Mg as a consequence of the roots' activity, K was released and concomitantly the mica lattice expanded after 3 d of continuous exposure to the root mat, in response to K uptake by plants. After 4 d, the vermiculitization of the phlogopite was detectable up to 1.5 mm from the root surface (i.e., the root mat) by x-ray diffractometry. Such rapid mineralogical changes induced by roots support the view that primary minerals such as trioctahedral micas might contribute significantly to the supply of K to the plants, at least around the most active parts of the roots.