CHEMICAL COMPOSITION OF SOILS

In chemical composition of soils, it is said that soil is the collection of natural bodies which are present on the earth's surface, in places which are modified or even made by man consisting of earthy materials, containing living matter and supporting or capable of supporting plants which are located out-of-doors. At its margins, it grades to deep on water or to barren areas of rock or ice. Its lower limit to the not soil beneath the earth surface is perhaps the most difficult to ascertain. Soil also includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through series of time, of climate, living organisms, parent materials, as well as relief. In places where it contains thin cemented horizons that are impermeable to roots, the soil is then as deep as the deepest horizon. More commonly soils are grades from their lower margin to hard rock or to earthy materials which are virtually devoid of roots, animals, or marks of other biological activity. The lower limit of soil, therefore, is normally and proportionate to the lower limit of biological activity, which generally coincides with the common rooting depth of native perennial plants.