Rhizosphere properties are known to differ fromthose of bulk soil, but differences are not always predictable and may be influenced by the kinds of plant roots and by soilmineral composition. Parent material primaryminerals have a great influence on the soil secondarymineral suite, and the secondary phyllosilicates can influence soil solution composition. We hypothesized that 1) bulk soil solution and rhizosphere soil solution differ, 2) soil clay
mineral Si content, hence parent material mineral composition, controls soil solution Al activity, 3) soil solution composition of soils acidified by agricultural fertilization practices differs from soil solution composition of soils acidified by long-term weathering, and 4)monocots and dicots differ in their rhizosphere effects. We compared the effects of a monocot (fescue, Vulpia myuros) and a dicot (tomato, Solanum lycopersicum) on rhizosphere solution
composition in soilswith awide range of Si content. The greenhouse pot experiment was conducted using soils derived from parent materials of sialic, mafic, and mixed lithology. Differences between non-acidified, agriculturally-acidified, and naturally-acidic phases of soil within each parent material were compared so that a soilmember of each parentmaterial could be evaluated for its influence on rhizosphere chemistry. Mineral stability diagrams based on soil clay-fraction mineralogy for each parentmaterial type served as guides to interpret rhizosphere effects on soil solution composition. These diagrams demonstrated that rhizosphere solution extracts
are farther from equilibrium than are bulk soil solution extracts, that rhizosphere influences are greater in non-acidified than in agriculturally-acidified soils, and that the dicot changed the rhizosphere more than the monocot, relative to bulk soil. The sialic soil diagram suggests that conditions are not favorable for hydroxyl-Al
interlayered 2:1 material (HIM) formation in this parent material, presumably because solution Si competes with 2:1 interlayer space for solution Al. The diagrams also suggest amontmorillonite→ beidellite→ kaolinite transformation in themafic soil. We can draw no clear conclusions about which solid-phase sink controls solution Al in the non-acidified and agriculturally acidifiedmixed soils, but SRO aluminosilicate formationmay attenuate the Al3+ activity in the naturally acidic soil.