Soil Acidity
Soil Acidity quality or state of a soil of being acid. It is developed due to acidification and a process of generating hydrogen ions in an ecosystem. As a component of an ecosystem, soil acidification/acidity depends on the nature of the parent materials from which soils are derived and also on the environmental conditions where the soils are developed. Soil acidification is a natural process, which starts when rock surfaces are first colonised by algae and lichen. Acids largely derived from the carbon and nitrogen cycles are involved in the dissolution of soil and rock minerals during the processes of soil development. In natural ecosystems, soils usually become gradually more acidic than younger soils.
In general, superimposing agricultural production on an ecosystem results in faster rates of soil acidification. Two adsorbed cations, namely hydrogen and aluminium are largely responsible for soil acidity. Hydrogen ions may be added or removed from an ecosystem, or they may be produced or consumed in reactions within the ecosystem.
Soil acidification can be decreased by addition of calcium carbonate or increased by addition of sulphuric acid. Acid soils occur in humid regions because basic cations are leached downward. Soil acidification may result from the transformation of applied acid forming fertilisers. Application of urea initially causes a slight increase in soil pH, but at the later stage of transformation, ammonium, the hydrolysis product of urea, is oxidised to nitrate with the concomitant formation of hydrogen ions, which exerts influence on soil acidification.
The impact of acidification is expressed in terms of pH values. Soils can be classified on the basis of the degree of acidification. The following classification is used to express the extent of acidification: a) very strong acidic (pH values < 4.5), b) strongly acidic (pH value ranges 4.5 to 5.5), c) moderately acidic (pH value ranges 5.6 to 6.5), and d) neutral (pH value ranges 6.6 to 7.3).
Soil acidification has great influence on important chemical properties like nutrient availability, metal toxicity and percent base saturation of soil colloids. Among the physical properties, the development of soil structure and as a consequence the water-air relationship of the soil is influenced by soil acidification. Proliferation of different types of micro-organisms and thus the transformations of soil organic matter and other soil components are also influenced by soil acidification. In Bangladesh, soil acidification has been slightly intensified in agricultural soils which is evident in the calcareous soils. Most alluvium in Bangladesh is near neutral to moderately alkaline in reaction when first deposited. However, unless sedimentation continues the topsoil in non-calcareous deposits usually becomes moderately acid within about 50 years and topsoils in calcareous deposits usually become partially or totally decalcified; they may even become acid in reaction. On somewhat older floodplains, on both Gangetic and non-Gangetic alluvium, topsoil pH values of 4.5 to 5.5 are general. The pH values referred to above are found only in the dry season. In most seasonally flooded floodplain soils it is only the topsoil that becomes acid, and lower layers remain neutral or alkaline. In acid basin clays the subsoil is very strongly acid (pH<5.0) up to 50-100 cm or more below the soil surface. Even Calcareous Dark Grey Floodplain Soils in basin centres can be strongly or very strongly acid to a depth of 30-50 cm, becoming calcareous only at a greater depth. [Aminul Islam and Sirajul Hoque]
