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9.4. Soil

The soil: formation and contamination

The soil is a thin natural layer located in the superficial part of the earth's crust, made up of fragments from the decomposition of the bedrock.

Atmospheric agents (temperature, rain, wind, ice, etc.) and the action of living beings contribute to the formation of the soil, which will later be the sustenance of many animal and plant species.

Soil materials are distributed in a series of layers with different characteristics called horizons:

  • Horizon 0 is the upper layer, made up of plant and animal remains. Its color is dark because it has a lot of humus.
  • Horizon A (from washing  or  leaching).  Its color is clear because the infiltrated water dissolves the mineral salts and carries them further down.
  • Horizon B (of  precipitation).  In this horizon the precipitation of the mineral salts washed in the horizon A occurs.
  • Horizon CIt has fragments and remains of the bedrock.
  • Bedrock. Undisturbed rock, splitting which, when weathered, the soil was generated.
The formation of the soil begins when, from a bare rock mother, the occurs weathering by action of geological and climatic agents  that make develop a small layer of loose materials. Later will come the weathering by living beings. Some plant species will begin to take root and alter rocks with them. Later, other animals will arrive that will continue with the weathering. The layer that is being created on the rock will get bigger and bigger and the horizons will be differentiated.

Soil degradation

The main environmental problems that affect soils are two:
  • The pollution of soil. Alteration of the natural composition of the soil.
  • The degradation of the soil. Loss of the natural properties of the soil. The main mechanisms of soil degradation are:
    • Soil erosion. When humans carry out some activities (public works, abandonment of agricultural fields, removal of trees, etc.), the vegetation cover is lost and the slope is modified. Then, the action of atmospheric agents (rain, wind, etc.) causes the destruction of the soil structure, losing fertility.
    • Soil salinization. In addition to occurring in areas near the sea, if the soil is poorly permeable or irrigation is not carried out correctly, salts can be deposited on the surface of the soil. A hard crust is generated that damages the vegetation and facilitates its erosion.


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