Climate cl

Wind, rain, temperature and evaporation all play a part in soil formation. Temperature is a major factor in determining the rate of chemical weathering, as heat

Influences

React with

Vegetation*

Parent material

Biomass development

Weathering

Energy Water

CO2, O2

Minerals and sediment

Soil r\

processing

Energy

Water

Nutrients

Outputs

Radiated heat

Drains to groundwater

New biomas (evapotranspiration)

Leached to groundwater

Fig. 4.13 Flow diagram summarizing soil-forming processes, inputs and outputs. * Note that vegetation is influenced by parent material as well as climate.

Ferromagnesian series

Felsic series

Olivine

(monomer silicate)

Calcium feldspar (framework silicate)

Pyroxene (chain silicate)

Amphibole (double chain silicate)

Sodium feldspar (framework silicate)

Potassium feldspar (framework silicate)

Biotite mica (sheet silicate)

Muscovite mica (sheet silicate)

Increasing temperature of mineral crystallization in magma

Quartz

(framework silicate)

Increasing susceptibility to weathering

Fig. 4.14 Common silicate minerals ranked in Bowen's reaction series. Note that minerals formed under high temperatures with more ionic bonding are more susceptible to weathering. Ferromagnesian—minerals which contain essential iron and magnesium. Felsic—a rock containing feldspars and quartz.

speeds up chemical reactions by supplying energy (Box 4.8). For most reactions a 10°C rise in temperature causes at least a doubling of reaction rate. This suggests that weathering in the tropics, where mean annual temperatures are around 20°C, will be about double the rate of weathering in temperate regions where mean annual temperatures are around 12°C.

The effect of temperature is linked to the availability of water. The dry air of hot, arid environments is an ineffective weathering agent. Vegetation and hence soil organic matter are sparse, and this reduces the concentration of organic acids. Moreover, close contact between rock particles and acid is prevented by the lack of water. Short-lived rainfall events may move surface salts into the soil, but the

Fig. 4.15 Highly weathered granite, showing residual materials—mainly kalolinized feldspar and quartz. The granite has broken down principally due to the weathering of feldspars. Note the 'corestone' of less weathered granite by the figure. Two Bridges Quarry, Dartmoor, UK. Photograph courtesy of J. Andrews.

Fig. 4.15 Highly weathered granite, showing residual materials—mainly kalolinized feldspar and quartz. The granite has broken down principally due to the weathering of feldspars. Note the 'corestone' of less weathered granite by the figure. Two Bridges Quarry, Dartmoor, UK. Photograph courtesy of J. Andrews.

general dominance of evaporation over rainfall means that soluble salts tend to precipitate on the land surface, forming crusts of gypsum, calcium carbonate and other salts.

In humid, tropical climates, weathering is rapid, partly because the high temperatures speed up reactions, but mainly because the consistent supply of heavy rainfall allows rapid flushing and removal of all but the most insoluble compounds, for example oxides of aluminium and iron (Section 4.7). Flushing constantly removes (leaches) soluble components and is particularly important in the undersaturated zone of soils (Box 4.9).

The presence of water is critically important to most of the physical, chemical and biological processes that occur within soils and is expressed as a soil 'water balance'. Water balance expresses the difference between water use and water need, i.e. the water stored in soil that affects soil (micro)biology and plant growth (Section 4.6.4).

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