Roles of clays in soils

Abstract

In soil science, the term clay refers to all particles less than 2 μm in diameter. Thus it includes layer silicates, oxides and other minerals. Clays are the source of many of the chemical and physical properties of soils that make them a useful medium for the growth of plants and for the less common uses such as a medium for the disposal of wastes. Clays add much of the diversity found in soils. The minerals in soil clays frequently differ from their counterparts in commercial deposits. Also, the behavior of soil clays is influenced by the associated minerals in the coarser fractions. Organic matter is an important reactant with clays of some soils, but it is beyond the scope of this review. The cation exchange properties of clays are among their most important properties in retaining plant nutrient ions (e.g., NH 4 +, K +, Ca 2+, Mg 2+, etc.). Cation selectivity of clays influences soils as a plant growth medium and as a disposal medium for wastes (e.g., radioactive and toxic metal ions). Native K in layer silicates of soils is the most important element provided to plants by illites and other micas. Clays contribute to the formation of soil structure by undergoing seasonal shrinking and swelling. Also, they are transported and form clay films that coat natural aggregates that characterize many friable soils. The dispersion and flocculation of clays are important reactions in the physical behavior of soils which in turn influence friability, water infiltration rate, erodibility and other behavioral properties. Vermiculite and smectite in soils frequently have Al 3+ or polymeric Al on the cation exchange sites. Thus the behavior of these minerals is different from structurally similar minerals in natural deposits. The Fe oxides in soils occur largely as goethite and hematite. Yet they contain Al substituted in their structures, consequently the crystals are smaller and less soluble than their ideal counterparts. Iron oxides contribute to the color, aggregation, and adsorptive properties of soils. Manganese oxides in soils contribute to the retention of trace metals (Co, Zn, Ba, Ni, etc.) and to the oxidation of Fe. Lithiophorite forms in acid soils thus marking another group of minerals that occurs in soils and that is influenced by Al in the structure or in interlayer positions as a result of weathering.