The weathered material that gives rise to the present-day soils was disturbed by premafrost and glacial action, so they are not quite all that old, but are at least 8,000 to 10,000 years old. Most are older. The formation of a complete soil cannot be observed in one human lifetime. The lithosphere with its upper weathered mantle is shown. We are not generally conscious of soil and what occurs there as we walk over the landscape.
Some water is lost to the groundwater table by deep percolation.
The greatest fluctuations in soil moisture also occur near the surface. Thus, the deeper portion of the soil may remain moist for a longer period of time. Gaseous diffusion occurs between the atmosphere and the porous soil, with the greatest rate at the surface, creating optimum conditions for soil-inhabiting organisms. Production of organic acids during these cycles enhances weathering of inorganic materials. Weathering is most intense at the surface.