Environmental Engineering Reference
In-Depth Information
Figure 19.5 Effects of climate, altitude and relief on the
distribution of soils on the uplands of northern England.
LEACHING, DECALCIFICATION, CALCIFICATION
The process of leaching is caused by the continual washing of the soil with rainwater.
Rainwater has a natural pH of about 5ยท5, owing to dissolved carbon dioxide, making it a
weak hydrocarbonic acid (H 2 CO 3 ). In some regions atmospheric pollution by sulphur
dioxide (SO 2 ) and nitrogen oxides produces an even more acid leachate, 'acid rain'. Also,
as it passes through the surface organic horizon, it dissolves organic acids from
decomposing plant residues. It is thus able to dissolve and decompose minerals and carry
away cations and anions dissolved in the soil solution. The basic cations (bases) held on
the soil colloids (calcium, magnesium, potassium, sodium) are released from the colloid
surface and replaced by hydrogen or aluminium ions. This leads to a lower pH and
percentage base saturation as leaching progresses. Thus soil pH is a good general
indicator of the intensity of leaching, which is related to the amount of annual rainfall and
the chemistry and texture of the soil parent material.
The leaching of bases is an important process ecologically and agriculturally as the
ions are moved downwards out of the rooting zone of plants. In natural vegetation it is
important that deeply rooting species (e.g., grasses, deciduous trees in the temperate
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