Soil acidity is associated with the presence of hydrogen and aluminum in exchangeable form. Since soil acidity is a condition that results from prolonged leaching of soluble salts, soils in humid areas are usually acidic. The concept of acidity was developed in connection with the behavior of aqueous solutions, which are said to be acid when the activity of hydrogen ions exceeds that of hydroxyl ions. The same criterion may be applied to soil. Most soils in the humid regions are acid or “sour” as a result of losses by leaching and crop removal of such basic elements as calcium, magnesium, and potassium. In arid or desert regions, soils are usually alkaline or “sweet”. The degree of acidity or alkalinity of a soil is conveniently expressed in terms of pH values. The pH scale is divided into 14 divisions or pH units numbered from 1 to 14. Soils with a pH value of 7 are neutral. Soils with pH values below 7 are acid or “sour” and soils with pH values above 7 are alkaline or “sweet”. A pH of 5 is ten times more acid than a pH of 6 and a pH of 4 is ten times more acid than a pH of 5. Thus, a soil with a pH of 4 is 100 times more acid than a soil with a pH of 6 (Black 1957; National Plant Food Institute 1962).
KeywordsClay Phosphorus Magnesium Corn Hydroxyl
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