The formation of a soil from raw parent material or from a pre-existing soil encompasses the concept of soil genesis, or pedogenesis. Soil genesis involves both progressive and regressive processes (Johnson and Watson-Stegner 1987). It includes all processes operative within the soil, whether they act to promote horizonation, preserve it, or even destroy it. And it is the underlying principle behind many soil classification systems which endeavor to group soils of similar genesis, based on observable morphology or chemical properties (Cline and Johnson 1963, Smith 1986). The link between soil genesis and classification, shown in one respect in Fig. 7.6, can also be envisioned as follows: soil-forming factors → soil-forming processes → diagnostic horizons and materials → soil taxonomic system (Bockheim and Gennadiyev 2000).

We emphasize pedogenic processes in this book for several reasons First, we must understand at least some things about soil genesis if we are to classify, and indeed manage, soils in a logical manner. Second, only by knowing the processes that formed soils can we ever hope to predict how they may change, as inputs (e.g., precipitation, heat, ions) and human uses change over time (Bockheim and Gennadiyev 2000). Lastly, only by knowing how soils formed can we map them or explain their past, present or future distributions. Thus, soil genesis is an integral part of soil geography.

Forming maintaining or destroying a soil involves an extremely complex set of processes, much akin to building, maintaining or destroying a house.