Environmental Engineering Reference
In-Depth Information
Plate 19.1
Carboniferous limestone is weathered by physical
and chemical processes to give a regolith of rock fragments
and clay minerals. Morphogenesis produces the solum of a
dark humic (Ah) horizon above a lighter-coloured weathered
(Bw) horizon.
Photo: Ken Atkinson.
Of these four components, only the resistant residue is relatively stable and changes only
slowly. The organic matter is decomposed by the soil's fauna and microorganisms to
produce humus. The clay minerals and hydrous oxides undergo further alteration,
depending on the amount of leaching and the type of ions in the weathering solution; new
minerals can crystallize and previous ones alter. Ions in the weathering solution can be
thrown out of solution and precipitated in the solum if chemical conditions allow
(calcium carbonate, gypsum, soluble salts, secondary quartz from silica).
The final stages of soil formation consist of the processes of morphogenesis, i.e. the
production of a distinctive
soil profile
with its constituent layers or
horizons
(see
