The solum is taken by some to be "true soil" (Plaster 1992, and the Britannica Academic edition), defined by the O, A, E, and B soil horizons. This definition unnecessarily separates the O, A, E, and B horizons from the C horizon which lies below the solum. Plaster (1992) and others (Brittanic Academic edition) not only consider the solum to be the "true soil", they explicitly separate solum from the C horizon which lies below the B. The C horizon of soil has recently been found to be commonly intensely rooted by plants, the home of many millions of microbes per gram of soil, the environment of intense rhizogenic weathering, and an environment significantly affected over time by climate, biota, and geomorphologic process.
The soil is defined to include the O, A, E, B, and C horizons and is often much deeper than the solum. The C horizon is commonly the most voluminous horizon of the soil system.
Solum and Solium
I just thought I should explain my revert of a good-fath edit in the etymology: solum is not related to sedere, but solium seems to be. My understanding is that they have converged from different origins to very similar final forms. Chuck Entz (talk) 23:25, 19 May 2012 (UTC)