Page 66 - Soil and water contamination, 2nd edition

Environmental compartments 53

other marks of biological activity. For purposes of classification, the lower boundary of soil is
arbitrarily set at 200 cm.
Apart from this extensive definition of soil, there are many other definitions. From the
viewpoint of pollution issues, it is opportune to also include all unconsolidated, granular
mineral material down to the water table in the definition of soil, instead of restricting the
definition to the top 2 metres. The unsaturated zone may extend from zero to several tens of
metres thickness.
The soil is made up from porous, unconsolidated material consisting of autochthonous
(i.e. native), weathered bedrock, called regolith, or of allochthonous (i.e. alien) sediment . It is
characterised by intense activity of biota, including plant roots, fungi, bacteria, earthworms,
nematodes, snails, beetles, spiders, and soil macrofauna (e.g. moles, voles), particularly in
the top few decimetres. Biological activity is one of the soil-forming factors responsible for
the physical and chemical nature of the soil by decomposing organic matter and freeing
nutrients for reuse by plants. The other soil-forming factors are parent material, climate
(mainly precipitation and temperature), topography (elevation, slope, aspect), and time. The
interplay of these factors results in the development of a soil profile through gains, losses,
transformations, and the translocation of soil material made up of inorganic minerals of
various grain sizes and organic matter from dead plant material. Gains of soil material occur
as a result of inputs of, for example, water from precipitation or irrigation, organic litter from
plants and animals, nitrogen from microbial fixation, sediment from water or wind, salts
from groundwater, and fertilisers from agriculture. Losses arise from evapotranspiration and
percolation of water, leaching of substances soluble in percolate water, uptake of nutrients
and other chemicals by harvested or grazed plants, oxidation of organic carbon to CO , and
2
denitrification of nitrate . Transformations involve a wide range of biochemical reactions: for
example, the decomposition of organic matter, alteration or dissolution of soil minerals, and
the precipitation of newly formed minerals. Translocation takes place through the burrowing
activity of animals and plant roots (bioturbation), the expansion and shrinkage of soil
materials due to freezing–thawing or wetting–drying cycles, or the dissolution, mobilisation,
and subsequent precipitation or deposition of clays, carbonates or iron oxides deeper in the
profile. Soil formation is inherent in weathering , including the physical disintegration of
particles to smaller sizes (physical weathering) and the dissolution or chemical alteration of
minerals (chemical weathering).
Variation in the above soil-forming factors and process leads to the formation of distinct
soil profile s that consist of different soil horizons as mentioned above. Based on, amongst
others, the presence, thickness, and composition of the horizons, more than 15 000 different
soils have been described throughout the world. However, there are many general features
present in this broad range of different soil profiles. Figure 3.2 shows a generalised soil profile
typical in a loamy soil in temperate humid regions. The uppermost topsoil layer is composed
of fresh organic material, primarily from leaf litter and is called the O horizon . Below this is
the thin A horizon that consists of mineral material rich in dark organic matter (humus). The
zone below the A horizon, the E horizon, is often leached by water that has removed most
of the organic matter and some clays. This leached material accumulates in the B horizon,
below which the slightly altered parent material is present. Note that this is a generalisation
of a soil profile in the temperate humid climate regions. There are many other possible soil
profiles: for example, organic soils (peats and mucks) that are formed where stagnant water
impedes the decomposition of organic matter, so that only the partially decomposed plant
remains accumulate in the soil profile. In tropical regions, the abundant excess precipitation
and high temperatures cause an intense weathering and leaching of the soil profile.
Conversely, in arid and semi-arid regions, evaporation and the consequent predominately
upward movement of water results in the accumulation of soluble salts in the upper part
of the soil profile. The study of soils is an entire field of science itself and for further details

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