Acid soils

In the context of agricultural problem soils, acid soils are soils in which acidity dominates the problems related to agricultural land use.  They are characterized by a pH which is strongly (5.5-4.5) to extremely acid (<4.5), a low cation-exchange capacity and a low base saturation. In the World Reference Base (WRB) soil classification system acid soils (as agricultural problem soils) may mainly occur in the following Reference Soil Groups:  Acrisols, Arenosols, Cambisols, Histosols, Ferralsols, Luvisols, Planosols, Podzols and Fluvisols.  However, the Reference Soil Groups with the widest distribution of acid soils are Acrisols, Ferralsols and Podzols.

Acid soils occur in the tropics and subtropics as well as in moderate climates.  Their formation depends on specific conditions of climate, topography, vegetation, parent material and time for soil formation.  Acrisols and Ferralsols are most common in old land surfaces in humid tropical climates.  Acid sulphate soils occur in the tropics, in low-lying coastal land formerly occupied by mangrove swamps.  Podzols are typical soils of the northern coniferous forests but may occur in the tropics, too.

The types of acid soils vary considerably due to different factors in soil formation, especially differences in climate, parent material and vegetation.  Acid soils place major difficulties for agricultural use but can be very productive if lime and nutrients are constantly applied and appropriate soil management is practised.

Special management implications are caused by the occurrence of aluminium toxicity. In general soils with pH (H2O)  <5 and >60% Al saturation of the cation exchange capacity (CEC) may suffer from Al toxicity.  High concentrations of Al affect root growth, uptake and translocation of nutrients (especially immobilization of phosphorus in the roots), cell division, respiration, nitrogen mobilization and glucose phosphorylation of plants.  Therefore, in the context of agricultural problem soils, it is necessary to make a distinction between acid soils with Al toxicity and soils without Al toxicity because of the differing management implications for these soils.

• Strongly acid soils with aluminium toxicity (pH <5)

• Moderately acid soils without aluminium toxicity 
  (pH >5 and <6.5)

A special case of acid soils with aluminium toxicity are Thionic Fluvisols (acid sulphate soils) which have a sulfuric horizon in the upper soil horizons.  If these soils are drained sulphuric acid is produced by oxidisation processes resulting in extremely acid soil conditions.

• Acid Sulphate Soils