This chapter looks at the formation of soil clays from the standpoint of development of alteration profiles. The alteration process is initiated by the diffusion of hydrogen ions into the pre-existing high-temperature silicate crystals susceptible to such chemical processes such as tectosilicates. Water flow must be sufficiently low so that the diffusing elements released from dissolving phases can reach the zones of lower chemical potential where new crystals are forming, while retaining a state of relatively high concentration at the solution–growing crystal interface. The essence of neogenesis is the dissolution of primary minerals or of unstable initial alteration products of these minerals to yield their elemental components in solution which, given time, recombine and precipitate out of solution as new secondary minerals. The presence of clay minerals in soils is largely driven by the chemistry engendered by plants and other life forms associated with them in the upper portions of an alteration sequence.