Soils differ in nature mainly due to the type of bedrock
(lithology) and the climatic conditions. The nature of soils must be understood
in order to determine the influence of pollutants in the soil. For example, the
impact of acid rain on acidic soils with heavy metal pollution in southern
China will be different from the impact on sodic soils – salty/alkaline soils
with heavy metal pollution in northern China.
Soils can be classified in many different ways, according to
their colour, texture, chemical properties, percentage of clay, etc. One of the
easiest and common way of classifying soils are according to their colour. A
soil Munsell colour system is usually used to determine the redness/yellowness
of the soil, and this categorisation gives us a basic idea on the acidity/alkalinity
of the soil, and thus its climatic or paleoclimatic conditions.
The percentage of clay is also usually examined in order understand
the extent of sorption which may take place. In soils, clay colloids are very
important as they are where the chemical process of sorption takes place, which
usually refers to the exchange of metal ions in the soil solution with clay
colloids (usually negatively charged, variable). This affects the toxicity of
metals in soils as it is the free metal concentration in soil solutions which
is polluting and affects plants and humans.
Other factors such as soil structure and detailed chemical
composition also affects the impact of pollutants on soils and their toxicity
to the biosphere, hydrosphere and atmosphere.
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