Components, rocks, soil colloids

The components of soils and rocks have different size, shape, and quality. The particle size of organic components usually is in the colloid range (<500 nm) mineral components have different dispersity. The first classification system of soils on the basis of particle size was done by Atterberg (1905). Practically, this classification has been used until now, though some countries have their own classifications, considering their widespread soil types. The size of soil particles will also determine how the soil fraction is named (e.g., clay, sand, silt, rock, etc.). Table 1.6 provides these names along with the standard diameter of the particles for the international classification system. 

The weathering of parent rocks and minerals to form the inorganic soil components results ultimately in the formation of inorganic colloids. These colloids are repositories of water and plant nutrients, which may be made available to plants as needed. Inorganic soil colloids often absorb toxic substances in soil, thus playing a role in detoxification of substances that otherwise would harm plants. The abundance and nature of inorganic colloidal material in soil are obviously important factors in determining soil productivity. 

The major components of most soils are inorganic. These constituents are derived from the weathering of rocks and minerals or from subsequent reactions and interactions of the weathering products. During the weathering and interactions of weathering products, inorganic soil colloids are formed. 

In addition to straightforward precipitation reactions, components may dissolve and react with components already present, including atoms on colloidal surfaces. For example, phosphate may dissolve from phosphate rock and react with iron present in the soil solution or on particle surfaces to form an iron phosphate that is insoluble. 

On the surfaces of geological formations, different precipitation processes can be observed. The first one occurs when the concentration of some components reaches the value of the solubility product, the solution becomes oversaturated, and a new solid phase is precipitated (Section 1.2.3). The quantity of the precipitate depends only on the concentration of the solution. The precipitation takes place in a solution without the necessary presence of a solid surface. When, however, a solid phase, rocks, or soil is originally present, the precipitate is formed on it, and thus the total composition of the solid phase changes. When the precipitation forms colloidal particles, especially in diluted solutions, they can be adsorbed on the solid, if it is present. This process is governed by the so-called theory of colloid adsorption (Derjaguin and Landau 1941 Verwey and G. Overbeek 1948). 

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