Strongly acid soils with aluminium toxicity (pH < 5)

Strong soil acidity considerably effects physical, chemical and biological properties of soils.  The low calcium concentrations in the soil solution of acid soils restrict biological activity and soil structure stability.  The major production constraint on strongly acid mineral soils is, however, the possible occurrence of high aluminium concentrations in the soil solution which are toxic to plants.  Especially many tropical soils which are extremely weathered contain high amounts of exchangeable aluminium due to advanced soil formation processes.
The first effects of aluminium toxicity to be observed are a shortening and thickening of the roots.  Roots become brown in colour and branching is reduced.  The symptoms of aluminium toxicity are similar to phosphorus or calcium deficiency.  If aluminium toxicity further progresses various plant metabolic processes are affected such as uptake and translocation of nutrients (especially immobilization of phosphorus in the roots), cell division, respiration, nitrogen mobilization and glucose phosphorylation of plants.
The tolerable aluminium concentration varies from plant species to plant species.  But there are few crops such as tea which may accumulate greater amounts of aluminium without negative effects on plant growth.  Tolerance of plants may depend on different factors such as differences in root morphology, ability to increase the pH of the root zone, mechanisms to reduce translocation of aluminium from the roots to the shoots, accumulation of aluminium in the shoots without affecting plant metabolism and mechanisms for not inhibiting nutrient uptake (Ca, Mg, P) despite the presence of aluminium.
Lime application is a common means for increasing soil pH and consequently reducing the amount of aluminium in the soil solution.  However, in many tropical areas lime is in short supply.  Therefore the breeding of aluminium tolerant crop varieties is of specific importance for tropical agriculture.  There are tolerant varieties of various crops available.  Especially varieties of upland rice, cassava, mango, cashew, citrus, pineapple and cowpeas may be suitable for arable production on these soils.  Also various grasses (Brachiaria decumbens, Paspalum plicatulum, Pueraria lobata, Melinis minutiflora, Hyparrhenia rufa) and legumes (Stylosanthes ssp., Desmodium ssp, Centrosema ssp.) can be productively used on these soils

Reference Soil Groups which may contain acid soils with aluminium toxicity are:

• Acrisols and Ferralsols
• Podzols