Soil Acidity

one of the most important properties of many soils caused by the presence of hydrogen ions in the soil solution and of exchangeable hydrogen and aluminum ions in the soil adsorbing complex. High soil acidity has an adverse effect on plant development and on many beneficial soil microorganisms. Two forms of soil acidity are distinguished: actual, or active, that is, acidity of the soil solution, soil suspension, or water extract from soil; and potential, or passive or “latent,” that is, acidity of the solid phase of soil.

Actual soil acidity is caused by the presence of hydrogen ions. It is expressed by conventional pH value (negative logarithm of the hydrogen-ion concentration); at pH 7 the reaction of the soil solution is neutral and below 7 it is acid; the lower the numerical value of the pH, the higher the soil acidity.

Potential soil acidity is divided into exchange and hydrolytic. Exchange soil acidity causes marked acidification of the soil solution when the soil interacts with a neutral salt, which occurs when physiologically acid fertilizers (potassium chloride, ammonium sulfate) are added to the soil. According to the Russian scientist K. K. Gedroits and some other investigators, exchange soil acidity is caused by the presence in the solid phase of soil of hydrogen ions capable of exchanging with the cations of neutral salts and passing into the soil solution. Other scientists, however, believe that exchange soil acidity may be due to the presence in an adsorbed condition of both hydrogen and aluminum ions in different proportions. Hydrolytic soil acidity is caused by the presence in the soil of hydrogen ions not displaced by neutral salts from the adsorbing complex but capable of substituting for (exchanging with) other cations when the soil is treated with solutions of alkalies or of hydrolytically alkaline salts (for example, with a sodium acetate solution, which is also used to determine hydrolytic acidity).

The degree of soil acidity must be taken into account when selecting mineral fertilizers and preparing them before they are added to the soil. The principal means of controlling high soil acidity is liming the soil.