Soils

Soils, ORIGIN OF. The name "soil" is generally applied to the disintegrated surface of rocks penetrated, or at least penetrable, by plant roots. Soils may be of two widely different origins: they may be local - derived, that is, from the decomposition of the substratum; or they may be transported. As examples of the former we have sandy soils on sandstone formations and clays, not only on clays and slates, but also on chalk and limestone. In this last case the carbonate of lime, which forms the bulk of the underlying rock, may be entirely removed by the percolating action of water, the clay being merely an insoluble residue. On the Upper Chalk, it contains flints. Transported soils are of three classes- eluvial, carried by wind, such as blown sand and loess (q.v.); diluvial, carried by the ice of the Glacial Period, suoh as boulder clay and gravel, sometimes very chalky; and alluvial, carried by river-action, such as some sands and gravels and most loam or brick-earth. Soils often contain a considerable admixture of humus (q.v.), or decayed vegetable matter (leaf-mould), which in some cases constitutes what is known as black land. Soils are commonly classified as rich or poor, according to the large or small proportion of this ingredient; and as stiff and cold, if mainly clay, or light and warm, if mainly sandy or calcareous.