Many elements are found within the earth's crust, and most of them are in soil as well. However, a few elements predominate. These are hydrogen, carbon, oxygen, nitrogen, phosphorus, sulfur, aluminum, silicon, and alkali and alkaline earth metals. Various trace elements or micronutrients are also biologically important as enzyme co-factors, and include iron, cobalt, nickel, copper, magnesium, manganese, molybdenum, and zinc.
A more functional and esthetically pleasing approach is to define soil as predominantly a sand-silt-clay matrix, containing living (biomass) and dead (necromass) organic matter, with varying amounts of gases and liquids within the matrix. In fact, the interactions of geological, hydrological, and atmospheric (Fig. 1.4) facets overlap with those of the biosphere, leading to the union of all, overlapping in part in the pedo-sphere. Soils, in addition to the three geometric dimensions, are also greatly influenced by the fourth dimension of time, over which the physicochemical and biological processes occur.
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