ABSTRACT
Acid soils occupy 3.95 billion ha (30%) of the world’s ice-free land area (Baligar et al., 1998), comprising both the tropical and temperate belts. The distribution of acid soil in selected regions in the world is shown in Table 1. Depending on the degree of weathering and soil acidity, acid soil can be classified into 10 groups. Oxisols and Ultisols are the major acid soils in the tropical region and occupy 22% (846 m ha) and 18% (727 m ha), respectively, of the acid soil area in the world. Inceptisols including the sulfate soils of many tropical river deltas are very acidic owing to acid formed upon oxidation of sulfides. The level of acidification of acid soil generally reflects the degree of weathering and leaching it has experienced (Baligar et al., 1998). In addition to the natural factors that affect weathering, agricultural farming processes such as the excessive supply of inorganic fertilizers or removal of cations by harvest lower the pH. Furthermore, the acidity of soils is gradually increased owing to environmental pollution and acid rain. Acid soils are infertile because they lack the basic nutrients, such as Ca2+, Mg2+, and K+. Acid soils are characterized by high content of toxic elements such as Al, Mn, and Fe or deficiency of Ca2+, Mg2+, K+, N, and P. Most acid soils have low cation exchange capacity, leading to loss of essential minerals and to poor crop production.
