Geoscience Reference
In-Depth Information
KEY POINTS
1
Soils are formed by a series of processes within the top one or two metres of the soil surface. Many
processes will be going on simultaneously, but the soil formed will mostly reflect the dominant one. High
rainfall plus a permeable parent material favour leaching. Acid vegetation and parent material promote
podzolization. Intense weathering and leaching give rise to laterization. Poor drainage gives gley soils,
which, in arid and sem-arid climates, become saline and alkaline soils.
2
The soil formed under the prevailing climate and vegetation conditions is the zonal soil. This corresponds
to the climax vegetation and will be found on flat, well drained sites. In the real world, however, this zonal
profile will be considerably modified by local factors (drainage, limestone rocks), by topography (giving
soil hydrologic sequences or soil catenas) and by the age factor (the length of time particular processes
have been operating).
3
The concept of soil zonality was developed in the late nineteenth century but is still relevant today as
it fits neatly into ecological concepts of biomes and ecosystems. An understanding of soil-forming
processes is vital for a thorough understanding of the potential of the world's soils for agriculture and
forestry.
4
The profile is formed by vertical movements of water and materials, both downwards and upwards.
Processes of leaching, decalcification, clay translocation, podzolization and laterization are in a
predominantly downward direction by gravity. Processes of salinization, alkalization and calcification
involve precipitation of chemicals in situ, or by upward movement by capillarity. The processes of
rubefaction and gleying give distinctive colours to soil.
5
Whilst the soil profile is the prime sampling unit for studying soils in the field, soils are linked in the real
landscape by movements of water, chemicals (including nutrients), and solid particles, all of which follow
the influence of gravity down slopes. Thus the slope sequence or catena of soils is the fundamental unit
of study throughout the world, from cold arctic slopes to hot and humid tropical topography. In the soil
catena, the hydrologic cycle, nutrient cycles and hill-slope mass movements are all interconnected. In
temperate climates, soil changes along slopes are largely conditioned by hydrology. In subtropical and
tropical climates soil changes along a catena are also greatly influenced by the movement of mineral
particles by rainwash erosion.
6
In regions beyond the limits of Quaternary glaciations, it is necessary to be aware of geomorphology and
landscape history, for polycyclic soils are common features of older, polycyclic landscapes.
FURTHER READING
Avery, B. W. (1990) Soils of the British Isles, Wallingford: CAB International. This topic is the definitive account of the
distribution, formation and properties of British soils. A good source of examples and illustrations.
Ellis, S. and Mellor, A. (1995) Soils and Environment, London: Routledge. Comprehensive coverage of soil formation, soil
profiles and soil properties.
Soil Survey of England and Wales (1983) Soil Map of England and Wales, Harpenden: Soil Survey of England and Wales.
This work comprises six soil bulletins for the regions of England and Wales, together with accompanying maps at the
scale of 1: 250,000. The many photographs and block diagrams are extremely helpful in showing and explaining soils
in the landscape.
