Parent bedrock material p

Parent material is the material from which soils are derived. The main constituent of most soil (excluding peat soils) is inorganic mineral material. Crustal rocks are the main source of these mineral components, and the rate of rock weathering is strongly dependent on the solubility and stability of the constituent minerals. The mineral fraction in soils is dominated by silicate minerals, and their susceptibility to weathering follows a sequence which is roughly the reverse of the original crystallization order or 'Bowen's reaction series' (Fig. 4.14). High-temperature silicates such as olivine and calcium feldspar are furthest from stability at Earth surface temperatures (and pressures) and are easily weathered, whereas lower temperature minerals, such as quartz, are quite resistant.

There is experimental evidence that dissolution rates of specific monomer silicates (e.g. Ca2SiO4, Mg2SiO4, etc.) are proportional to the rate of reaction between the divalent cation and soil water molecules during hydration (see Section 5.2). The rate of reaction between water molecules and alkaline earth ions (see Fig. 2.2) is related to ionic size (Ca(H2O)2+ > Mg(H2O)2+ > Be(H2O)2+). This is mirrored by experimental dissolution rates, where Ca2SiO4 > Mg2SiO4 > Be2SiO4, and is controlled by the relative strength of the cation-oxygen bond.

From a global perspective, the weathering of average upper-crustal granodi-orite will produce two types of solid product. Quartz being quite resistant to weathering (Fig. 4.14) will be released into the soil as a major component of the sand (60-2000 mm) and silt (2-60 mm) fractions. Although not strictly true, we will assume that quartz is chemically inert and takes no further part in chemical reactions. Feldspars, however, are weatherable (Fig. 4.15) and break down to form clay minerals (Section 4.5) as part of the soil clay (<2 mm) fraction. The relative proportions of sand, silt and clay size classes are important because they influence the water-holding capacity of soils. Sandy soils tend to be free draining and dry, whereas clay soils are usually poorly drained and wetter.

In general, the mineralogical and elemental composition of a soil will reflect that of the parent rock. For example, a soil forming on limestone (CaCO3) will have a high calcium (Ca2+) content, like the limestone itself. It is, however, important to note that both solids and solutes are transported to some degree during rock weathering. As a result, soil compositions may not directly match those of the rocks beneath them.

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