6. The yield of soybeans as related to the percentage of calcium saturation of two North Carolina soils. Curve z, organic soil; curve 2, White Store soil containing montmorillonoid clay.

The pH of a soil varies considerably with its water content. For acid and neutral soils, pH generally is lower for large soil-water ratios than for small. A 1:1 soil-water paste may have a pH one unit or more lower (a tenfold difference in "active acidity") than a 1:5 suspension. This is another manifestation of the suspension effect, and generally has been attributed to differences in hydrogen ion concentration in contact with the glass electrode. The correct explanation appears to be much more complicated and must await further work.

With soils that have a high pH and contain free sodium carbonate and other soluble salts, pH also increases with the water-soil ratio. That is because dilution increases the hydrolysis of sodium-clay, leading to a larger hydroxyl ion concentration in the soil solution. Hydrolysis of exchangeable sodium occurs very readily because hydrogen ions from water have a strong affinity for weakly acidic groups on clay and organic matter.

Because of the effects of soil-water ratio on measured values of soil pH, it is advisable to standardize this. A ratio of one part of soil to one part of water often is used. In studying the salted soils of dry regions, the pH of a soil saturated with water often is measured.

Some research workers have suggested measuring the pH of soil samples at field moisture contents. That is not desirable, since the high external resistance that is encountered under these conditions leads to errors.

The concentration and kind of soluble salts in the water phase profoundly affect soil pH. With acid soils this is largely an ion exchange phenomenon, with cations of the salt replacing exchangeable hydrogen and aluminum ions from the soil particles. The acidity of the solution increases because of increased hydrogen ion concentration or the partial hydrolysis of aluminum ions. As salt concentration is increased, soil pH falls rapidly at first and then becomes insensitive to further changes. Seasonal fluctuations in salt content, due to fertilization or the mineralization of organic matter, can change soil pH by 0.5 unit.

Since soil reaction varies with salt content, pH often is measured in salt solutions of definite concentration. One normal potassium chloride and 0.01 molar calcium chloride have been used. Such procedures iron out experimental fluctuations to some extent, and for that reason may be desirable.

Measurement of soil pH in dilute calcium chloride solutions appears to offer several advantages. Calcium is the most abundant basic metal cation in most soils, and addition of calcium chloride solutions does not usually change the proportions of the exchangeable cations very much.

Furthermore, there is a general relationship between the concentrations of any two ions in the water phase of a soil-water mixture which permits a description of soil reaction in a way that is independent of the soil-water ratio and of salt content. This relation, which has been called the "ratio law," is that the concentration of hydrogen ions in a soil solution divided by the square root of the concentration of calcium ions is a constant that is characteristic of the soil. The ratio law may be written as: [H+] √ [Ca++] = constant, or as pH-1/2 pCa=constant.

Since the value of pH 1/2 pCa does not depend at all on the soil-water ratio or the salt content, it is a more definite index to soil reaction than is pH.

Another advantage is that both pH and pCa are determined on a clear soil solution, and difficulties due to the suspension effect are avoided.

Considerable attention should be paid to the effect of the addition of soluble calcium salts to acid soils. When gypsum, or calcium sulfate, is added to acid soils, soil pH falls and the aluminum and manganese concentration of the soil solution increase, sometimes to levels toxic to plants.

The effect of salt concentration on the pH of neutral soils is due largely to increases in the apparent strength of acidic groups. With alkaline soils, increasing salt concentration lowers pH by reducing the hydrolysis of the exchangeable cations, largely sodium.

The pH of soils containing free carbonates of calcium and magnesium is greatly influenced by the bicarbonate concentration in the soil solution. The bicarbonate concentration is proportional to the carbon dioxide content of the air in contact with the soil. The carbon dioxide content of the atmosphere is about 0.03 percent. A soil containing free calcium carbonate has a pH of 8.3 when in equilibrium with this concentration of carbon dioxide. The carbon dioxide content of the soil air may be much larger than that of the atmosphere. Plant roots and microorganisms usually liberate carbon dioxide faster than it can escape from the soil to the atmosphere. Soil air may contain as much as 10 percent carbon dioxide, although contents of about 2 percent are more common. This carbon dioxide content would result in a pH of about 7.1.

When soil samples are taken from the field and prepared for pH measurement, they usually are allowed to equilibrate with the atmosphere. Because of the dependence of pH on carbon dioxide level, the pH measured in the laboratory may be different from that in the field.