Acid Soil Type of soil in which the quantity of free H+ ions is higher than that of alkali or alkaline earth cations. The pH is therefore less than 7.0. However, the term ‘neutral’ is generally used for a soil with pH between 6.6 and 7.3. Acid soils therefore have a pH less than 6.6. This means that it is more acidic, which results from the presence of exchangeable hydrogen and aluminium ions. The salt contents of this soil are composed predominantly of aluminium sulphate and ferrous sulphate. The sources of hydrogen cations in soil solution could be from acidic parent materials, plant roots, humus, carbon dioxide, alumino-silicate, iron pyrites, use of ammonium fertilisers and sulphur too.
The low soil pH is associated with a number of soil chemical and biological characteristics that manifest themselves as the components of the problem acid soil syndrome. The following specific problems are associated with problem acid soils:
• Aluminium toxicity; • Manganese toxicity; • Molybdenum deficiency; • Legume nodulation failures; • Increase in plant disease and • Calcium and magnesium deficiency.
Hydrogen ion toxicity, decreased phosphorus availability and toxicities of some other trace elements and heavy metals have also been reported. The acid soil problem is associated with poor establishment and persistence of lucerne and phalaris, reduced barley and wheat yields and the thinning out of sub clover and other pasture legumes leading to a substantial decline in carrying capacity.
Most of the soils of Bangladesh are low to medium acid in reaction, due to the predominance of high rainfall areas and leaching. There are mainly three groups of acid soils found in Bangladesh, such as: acid basin clay, acid sulphate soil and brown hill soil.
[Md Khurshid Alam]