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 tem-
peratures speed up reactions, but mainly because the consistent supply of heavy
rainfall allows rapid flushing and removal of all but the most insoluble com-
pounds, for example oxides of aluminium and iron (Section 4.7). Flushing con-
stantly 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, chemi-
cal 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).
4.6.3 Relief (r)
Relief describes the form and gradient of slopes and is thus a component of topog-
raphy (Section 4.7). Slope steepness governs the extent of water infiltration into
soils. Where slopes are steep water infiltration is generally lower, although over-
land-flow and through-flow of water is greater. Thus, the potential for mass flow
The Chemistry of Continental Solids 97Fig. 4.15Highly 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.