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Soil pH is probably the single most informative measurement that can be made to determine soil characteristics. Soil pH values in the neighborhood of two to three indicate the presence of free mineral acid, usually H2SO4. The ratio of water to soil in the suspension has the effect of increasing pH as the ratio increases. In soils with higher pH values, hydrolysis of basic cations tends to maintain a stable pH with dilution. A major factor influencing pH of soils is the salt content of the soil solution. Colorimetric methods for pH determination are based on weak acids or weak bases, the colors of which change with undissociated or dissociated forms. Glass used in the H+ -sensitive electrode is characterized by low electrical resistance, low melting point, and high Na content. In addition, the glass must have the tendency to hydrate easily if it is to be predictably related to the activity of H+.