. Soils, their properties and management . ost cases begreatly benefited by some form of drainage. 60. Soil and subsoil. — A common distinction is madebetween the surface soil and that which is some distancebelow the surface. This is natural, as the forces of soilformation have served to bring about certain distinctions,especially in humid regions, which are of importance inany consideration of soil fertility and crop growth. Cli-matic agencies acting on soil after it has been formed haveserved to intensify these distinctions. The term soilis used to designate the top layer of earth, which usu
. Soils, their properties and management . ost cases begreatly benefited by some form of drainage. 60. Soil and subsoil. — A common distinction is madebetween the surface soil and that which is some distancebelow the surface. This is natural, as the forces of soilformation have served to bring about certain distinctions,especially in humid regions, which are of importance inany consideration of soil fertility and crop growth. Cli-matic agencies acting on soil after it has been formed haveserved to intensify these distinctions. The term soilis used to designate the top layer of earth, which usuallyextends to the plow line or even deeper. The soil belowis spoken of as the subsoil, and may be rather variable in itsdepth (Fig. 10). Often the subsoil is divided into the upper 1 Hiigard, E. W. Soils, pp. 283-285. New York. 1906. 80 SOILS: PROPERTIES AND MANAGEMENT and the lower subsoil, the upper subsoil being consideredto extend to about the depth of three feet below the sur-face. I^sually, especially in humid regions, there is a. Fig. 10. — Soil and subsoil, (a), Surface soil with many plant roots;(b), subsoil; (c), country rock. sharp line of demarcation between the soil and the sub-soil, due to differences in the humus content. As organicmatter accumulates faster at the surface, the soil theretends to assume a darker color. Whether the land hasbeen tilled or not, this line of separation is fairly markedand can usually be located with little difficulty. Ontilled land, where the surface soil extends to about thedepth of plowing, the plow line marks the separation ofsurface and subsoil. Where soil samples are being takenfor soil survey or soil analysis, some arbitrary depth,depending on circumstances, is usually established forthe surface soil. This depth varies from six to Soil and subsoil of humid regions. — In humid cli- CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 81 mates there are usually certain well-defined differences be-tween the surface soil a
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