In Sicily, weathering makes new soil at about a tenth of a millimetre yearly
Under Sicily's Mediterranean climate, rock breaks down into fresh soil at roughly a tenth of a millimetre a year. At that pace, a hand's depth of earth represents many centuries of patient chemistry, microbes and roots. Soil scientists sum up the ingredients in one odd acronym: CLORPT.
CLORPT stands for climate, organisms, relief, parent material and time, the five classic factors that together decide what kind of soil develops. Parent material is the raw mineral stock. Rock of any kind, igneous, sedimentary or metamorphic, supplies all of a soil's minerals and every plant nutrient except nitrogen, hydrogen and carbon.
Surprisingly little soil forms where its rock lies. In the United States perhaps three percent of soils are residual, weathered directly from the bedrock beneath them. Most have travelled. Wind carries silt and fine sand hundreds of miles to build loess, which is 60 to 90 percent silt and blankets parts of the American Midwest, northwestern Europe, Argentina and Central Asia; clay rarely blows because it clumps. Rivers, lakes and ancient seas leave their own deposits, as around the Great Lakes or in California's Imperial Valley, glaciers drop moraines, and gravity piles talus at the foot of cliffs. Peat and muck form in place from waterlogged plant remains; peat can be sterile while muck may be very fertile.
Breaking rock down happens in stages. Rock formed deep underground swells and cracks once relieved of pressure at the surface, heating and cooling split it further, and freezing water wedges cracks open. Chemistry then takes over, mostly through hydrolysis and carbonation, where carbonic acid converts calcite into soluble bicarbonate. The rate of dissolving roughly doubles with every 10 degrees Celsius of warming but depends on water, so a rock that crumbles within years in the tropics can sit untouched for millennia in a desert. Bacteria and fungi drive much of it by excreting organic acids.
Life builds on the process. Microbes feed on released nutrients and leave residues that become humus, and roots working with mycorrhizal fungi pull nutrients straight from rock. Pioneer species give way to richer plant and animal communities, topsoil thickens, and over time distinct layers, or horizons, appear.
Source: Soil formation
