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The acidity and alkalinity of soils is expressed in the pH value, which is a scale that measures the concentration of hydrogen ion held by the soil colloids (particles). In pure water, one part in 10 million is dissociated to form hydrogen ions, i.e., 10 ' and the pH is thus 7; this is a neutral state on the scale of acidity. If a strong alkali such as caustic soda is dissolved in water, the solution is marked as alkaline (pH 14). By contrast, hydrochloric acid has a pH value of 3, A neutral soil has a pl I value of about 7.2, and acid soil less than 7.2 (sometimes as low as 3). A strongly alkaline soil has a pl I value of about 8 or higher.