A major environmental problem with aluminum is its toxicity to plants growing in highly weathered acid soils. Aluminum is one of the more abundant elements in soils, making up approximately 7% of the solid matter in an average soil. The solubility of aluminum in soils is initially controlled by minerals that are present that have the highest solubility. Pedogenic processes slowly remove the more soluble minerals, so that the more stable ones supporting the lowest activity of aluminum ultimately control its solubility. The widespread presence of aluminosilicates in soils is accompanied by a reciprocal solubility relationship between aluminum, silica. The presence of hydroxy-interlayered vermiculite in a number of these soils suggests that an amorphous aluminum hydroxide phase forming in the interlayer could also be an active component in these systems, since some of the data fall only slightly below this solubility line.