Geochemical parent rocks-soils

Most soils contain approximately 95% of their dry mass as mineral material and 5% as organic material. This chapter is largely concerned with the mineral material, and organic matter is discussed only so far as it influences the fate of the inorganic elements. The mineral material is derived from the break down of rocks. Embryonic soils are little more than a few millimetres of rock debris colonised by lichens while at the other extreme, in the tropics, tens of metres of weathered material may overlie unaltered rock. In all cases, however, the chemical nature of the soil will be influenced by the geochemical nature of the parent rock. In turn, plant contents tend to reflect soil contents, albeit indirectly. 

The elements of the 1st group-Co, Mo, Pb, Cd are characterized by an increased content in the soil relative to claike, which indicates the accumulation of these substances, but not higher than the MAC level. The coefficient of the chemical substance concentration (Cs) is from 1.61 units - Co to 3.42 units - Cd. The increased content of these elements may be due to geochemical characteristics of the parent rock materials. 

The soil inherits from the parent material a stock of trace elements which, in turn, is controlled by the geochemical nature of the parent material (West, 1981). In global terms this relationship can be illustrated by comparing the commonly accepted mean contents of soil elemental constituents with crustal rock abundances. Reasonably reliable data are available for 67 elements. 

In the rural situation, the total trace-element content of the soil may be virtually the same as that of the rocks from which the soil parent material was derived by weathering. The geochemical principles controlling the trace-element content of uncontaminated soils have been reviewed and reported by Mitchell [5, 80] who stated that it is possible to assess, with some degree of accuracy, the approximate total content of trace elements in a soil, when the nature of the parent material is known. 

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