The key factors in equation 4.16 are summarized in Fig. 4.13 where the soil is
depicted as a simple box. Various inputs and outputs govern the box’s content
while processes within the box generate the outputs. It is important to note that
of all of these factors only time is an independent variable, the others being inex-
tricably linked. For clarity, these factors are discussed individually below, although
in nature they operate together.4.6.1 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 susceptibil-
ity 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. Ca 2 SiO 4 , Mg 2 SiO 4 , 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(H 2 O) 62 +>Mg(H 2 O) 62 +>Be(H 2 O) 62 +).
This is mirrored by experimental dissolution rates, where Ca 2 SiO 4 >Mg 2 SiO 4 >
Be 2 SiO 4 , 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 (60–2000mm) and silt (2–60mm) fractions. Although not strictly true, we will
assume that quartz is chemically inert and takes no further part in chemical reac-
tions. 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 influ-
ence 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 (CaCO 3 ) will
have a high calcium (Ca^2 +) content, like the limestone itself. It is, however, impor-
tant 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.4.6.2 Climate (cl)
Wind, rain, temperature and evaporation all play a part in soil formation. Tem-
perature is a major factor in determining the rate of chemical weathering, as heat94 Chapter Four