Soil Acidification: Fundamental Concepts
Natural soil acidification processes have been recognized and studied for decades or perhaps centuries. An understanding of these processes is essential to an understanding of soils and of natural and agricultural ecosystems. One of the most important characteristics of soils is the cation-exchange complex. These are negative charges, either on clay minerals or on soil organic matter. In the case of the clay minerals, these charges usually arise from isomorphic substitution within the mineral lattice of a cation of lower positive charge for one of higher charge. In the case of organic matter, the charges arise mainly from the ionization of H+ from carboxyl, phenol, and enol groups (Coleman and Thomas 1967). In alkaline or neutral soils, the negatively charged exchange complex is dominated by basic cations (i.e., Ca2+, Mg2+, K+, and Na+). In acid mineral soils this complex is usually dominated by aluminum species [i.e., Al3+, A1(OH)2+, and A1(OH) 2 + ] formed by the dissolution of soil minerals in acid systems. In acid organic soils, H+ may be the dominant exchangeable cation. The acidity of a soil is thus determined by the relationship between the amounts of the basic cations and the acid aluminum species on the exchange complex. Processes that would tend to acidify a soil include those that tend to increase the number of negative charges, such as organic matter accumulation or clay formation, or those that remove basic cations, such as leaching of bases in association with an acid anion. Processes that would tend to make a soil more basic would add basic cations, either from outside sources or from the weathering of soil minerals, or reduce negative charge, such as might occur during the destruction of organic matter by fire.
KeywordsSoil Solution Soil Mineral Basic Cation Acid Deposition Exchange Acidity
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