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July 2002

Mountain soils

by Freddy Nachtergaele
Land and Plant Nutrition Management Service
FAO Land and Water Development Division

See also "Mountain climates"

Soils in mountainous zones are often strongly influenced by the relatively steep slopes on which they occur resulting in soil losses superior to the rate of soil formation. This process leads to relatively shallow soils in which a surface horizon, more or less rich in organic matter and of varying thickness, rests immediately on rock or parent material.

These eroding uplands are marked by the occurrence of unstable rocky slopes and outcrops of bedrock. In these landscapes Leptosols and Regosols dominate. Both are characterized by a very limited soil forming processes, due to constant wind and water erosion. Another soil type quite typical for volcanic landscapes are Andosols.

Leptosols are characterized by their shallow depth (less than 30 cm of soil over hard rock or ironpan) or by their very high gravel content. Their limited soil volume make them subject to drought, but also to waterlogging and run-off. It has been estimated that about 545 Million hectares of these soils occur in mountainous areas. Their global distribution is illustrated at:

Most Leptosols remain under natural vegetation, and as such have a potential for grazing and forestry. If exploited be it for agriculture or for tourism, their fragility and susceptibility to erosion necessitate protective management.

Subtypes of Leptosols of depend on the parent material. In the mediterranean zone these soils often occur over calcareous materials these are called Rendzic Leptosols or Rendzina's (Figure 1) In more humid climates they are often more acid and a relatively thick topsoil horizon rests on non-calcareaous rocks, called Umbric Leptosols or Rankers (Figure 2).

Regosols in mountainous areas are characterized by their little profile development and reflect simply the weathering of the parent material in which they have developed. They are generally deeper than Leptosols, and their topsoil horizons are often thin and poor in organic matter. Most Regosols remain under their natural vegetation.

Andosols these soils typically developed in volcanic materials are a very special case of mountain soils. The rapid weathering of the porous parent material results in an accumulation of amorphous complexes such as allophane and immogolite. Generally Andosols have a fluffy consistence and a dark colour. These soils are further characterized by their high porosity, high permeability and high aggregate stability. They have generally a large moisture storage capacity and are rich in nutrients. Many Andosols have a very large specific surface with aluminium hydroxides which have a strong affinity for phosphate ions and consequently they are often P-deficient. The total extent of Andosols is estimated at about 110 million hectares concentrated in the circum - Pacific region corresponding with areas where earthquakes and volcanoes are common.

The global distribution is given at:

Most Andosols would have a high potential for agricultural production but for the problem of pohosphate fixation, which can be corrected, and for their occurrence on steep slopes which is more difficult to correct. Depending on the climate, Andosols are used for a wide variety of crops and remain under forest where slopes are steep.

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