THE EFFECT OF CARBON DIOXIDE ON SOIL REACTION

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Abstract
Small changes in CO2 pressure cause comparatively large changes in pH within the range of low CO2 pressures comparable to those normally found in the soil. The pH is approx. a straight-line function of the log of the CO2 pressure in the pressure range from about 0.0003 to 1 atmosphere of CO2 at constant moisture. At constant CO2 pressure the pH of soil suspen-sions tends to drop slightly with dilution. The rise in pH of soils frequently observed with increasing water content probably is due primarily to dilution of the CO2 absorbed in the soil sample. The curves showing the effect of varia-tions in CO2 pressure on pH of the soils studied were similar in shape to curves for CaCO3 but were affected more by dilution than were the curves for CaCO3 or MgCOs. Curve positions are changed appreciably on the pH scale by the presence of Ca, Mg, or Na carbonates. The curves tend to group themselves into families depending upon the presence or the absence of 1 or more of these compounds. Expressing the pH of soils as variable functions of CO2 pres-sure would give a better indication of the probable pH range in the field in the presence of plant roots and decaying organic matter than could be obtained from single pH measurements. Since the pH is apparently a straight-line function of the log of the CO2 pressure, 2 points would be sufficient to determine a curve at constant moisture. Whether single detns. or curves are to be used in expressing pH values, it is evidently necessary to make measurements under equilibrium conditions with controlled CO2 pressure if the results are to be reproducible. Controlling CO2 pres-sure by using distilled water treated to remove CO2 in making pH detns. has the disadvantage of producing a CO2 pressure lower than would likely be found in the field and, therefore, giving pH values higher than the maximum under field conditions.