Acrisols and Ferralsols

• Definition

• Further description

• General environment

• Global extent and location

• Land use

• Main production constraints

• Summary

Definition

Acrisols are characterized by a clay accumulation horizon, the argic horizon, in combination with the occurrence of low activity clays (cation exchange capacity of <24cmolckg^-1) and a low base saturation (i.e. <50%).

Ferralsols are soils that have a ferralic horizon at some depth between 30 and 200 cm from the soil surface.  A ferralic horizon is a fine textured horizon which has been formed by strong weathering and leaching over a long period of time resulting in the accumulation of stable sesquioxides of iron and aluminium.

Further description

Acrisols and Ferralsols are tropical and subtropical soils of old landscapes under high rainfall conditions. They are usually very deep, yellowish to reddish coloured, extremely weathered and leached. Most of the soils are characterized by a high percentage of low-activity clay minerals such as kaolinite and sesquioxides (goethite, gibbsite, haematite). Hydrolysis of the silicate minerals, combined with rapid removal of weathering products leads to a low pH and low concentrations of weathered products in the soil solution.

General environment

Acrisols and Ferralsols are most common in old land surfaces in humid tropical climates.  They occur on gently undulating Tertiary and early Pleistocene erosion surfaces upon which repeated cycles of weathering, erosion and deposition have taken place.

Global extent and location

Acrisols cover about 1 billion hectares (see table), mainly in the equatorial tropics on old deeply weathered land surfaces in Southeast Asia, West Africa and the central part of South America (see figure).  Ferralsols occur almost on 745 million hectares worldwide (see table), concentrated in the humid tropics.  They are mainly situated on the continental shields of South America and Central Africa (see figure).  Outside the continental shields, Ferralsols are restricted to regions with easily weatherable basic rock and a hot and humid climate, e.g. in Southeast Asia and on some Pacific Islands.

Land use

Acrisols and Ferralsols are naturally covered by tropical rainforests.  For agriculture, Acrisols and Ferralsols can be used as pasture or arable land but their sustainable use highly depends on appropriate land management.  Large areas of Acrisols and Ferralsols are used for subsistence farming, partly under shifting cultivation.

Main production constraints

Acrisols and Ferralsols are extremely nutrient deficient and acid, often with toxic levels of exchangeable aluminium.  The high amounts of exchangeable aluminium may act as firm barrier to deep rooting of the plants, leading to physiological drought of crops at dry spells within the rainy season.  Acrisols and Ferralsols are also characterized by low availability of phosphorus due to a high capacity for phosphorus fixation.  Thus lime, macro-nutrients and often micro-nutrients (e.g. molybdenum) have to be added for crop production.  The exchange capacity of the organic matter in these soils is very important because of the low cation-exchange capacity of the clay minerals;  however the organic matter is rapidly mineralized under the hot tropical conditions.

Summary

Acrisols and Ferralsols are tropical and subtropical soils which are extremely weathered and leached.  These soils are generally of low fertility because of both macro- and micro-nutrient deficiencies often combined with aluminium toxicity and soil acidity.  In addition they are highly susceptible to erosion if used for arable cultivation.  For sustainable agricultural use a careful soil fertility management in combination with conservation practices have to be applied.