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Acid soils

Acid sulphate soil

Acid soils are those that have a pH value of less than 5.5 for most of the year. They are associated with a number of toxicities (Aluminum) as well as deficiencies (Molybdenum) and other plant restricting conditions. Many of the acid soils belong to Acrisols, Alisols, Podzols and Dystric subgroups of other soils. An extreme case of an acid soil is the acid sulphate soil (Thionic Fluvisols and Thionic Cambisols).

There are two main belts of acid soils:

  • in the humid northern temperate zone, which is covered mainly by coniferous forests; and
  • in the humid tropics, which is covered by savannah and tropical rain forest.

Acid sulphate soils are usually left under natural vegetation or used for mangrove forestry. If water is managed well they can support oil palm and rice. Some other crops grown on acid soils around the world include: rice, cassava, mango, cashew, citrus, pineapple, cowpeas, blueberries and certain grasses.

An integrated approach to acid soil management comprises a spatially variable liming strategy, the use of acid-tolerant species, efficient use of fertilizers, suitable crop rotations and crop diversification. Soil testing needs to be carried out every two to three years to determine the lime requirements of the field. The buffering capacity needs to be assessed to work out the amount of lime needed to neutralize soil acidity to the desired level. The negative effects of soil acidity on physical and chemical soil conditions can be partly compensated by ensuring high organic matter content.

Acid sulphate soil management is more delicate and has to be based on cautious water management in order to prevent oxidization processes of pyrite:

  • The first strategy is to drain and completely oxidize the soil, and then flush the acidity formed out of the soil. This strategy solves the problem once and for all but has severe disadvantages: it is expensive, poses a threat to the environment (acid drain water!) and depletes the soil of useful elements together with the undesirable ones Liming of drainage water has been applied to reclaim Acid Sulfate soils in Australia.
  • The second strategy is to try to limit pyrite oxidation by maintaining a high groundwater table. A precondition is the availability of sufficient water. This method also requires substantial investments in water management, while the potential danger of acidification remains present. This strategy is widely followed, both in temperate regions and in the tropics, often with ingenious adaptations to suit local conditions and practices.

Incorporation of lime or dolomite into the upper cultivable soil layer is an effective method for amelioration of acid soils.  Banding or pelleting lime onto the seed at sowing is also a common practice used to aid with the establishment of temperate pasture legumes. Lime can also be applied as a preventative treatment for soil infertility, and to supply calcium and magnesium to deficient soils. Liming raises the pH of acid soil, thus the action of nitrogen fixing bacteria becomes uninhibited and nitrogen fixation increases. Nitrogen mineralization from plant residues and organic matter has been reported to increases when lime is applied to acid soil. Although lime is primarily applied to raise soil pH and amend toxicities associated with acid soil, liming has also been used to improve soil structure.