Acidity and alkalinity of an aqueous system (solution) are described (measured) by the term pH. pH is the negative logarithm of the hydrogen ion [H⁺] concentration. Hydrogen is a component of all acids. When hydrogen ionizes to form ions [H⁺], its concentration determines the acid properties of soil solutions.

Pure water, free of all minerals such as calcium, magnesium, etc., and free of carbon dioxide [CO₂], has a pH of 7.0, which is called neutral. Water [HOH] ionizes to form hydrogen ions [H⁺] and hydroxyl [OH^-] ions according to the following equation:

  HOH <--> H⁺ + OH^–          (1)

If carbon dioxide is dissolved in water in the absence of other compounds or minerals, the water becomes slightly to moderately acid. Equation 2 shows the effect of distilled water open to CO2 in the atmosphere:

   HOH + CO₂ <--> H⁺ + HCO₃^– (Carbonic acid)          (2)

There is now more H⁺ than OH^-; thus, the pH of distilled water that has absorbed CO₂ from the atmosphere may range from 4.0 to 7.0, depending on temperature and the amount of CO₂ in the atmosphere. The reaction in Equation 2 is probably responsible for seasonal fluctuation of 0.2 to 0.5 units in soil pH.