Well-aerated soils always show a high oxygen and a low carbonic acid content. The slower the gas exchange proceeds the more the carbonic acid accumulates. Consequently, the oxygen content drops, and the floristic composition of the vegetation changes accordingly. (Braun-Blanquet 1932:224)
The activity and air content of the superficial soil layers determine the conditions of soil aeration. Moreover, there are such large fluctuations from time to time that individual tests made at random give little evidence of the actual conditions. Romell always found the highest deficit of oxygen together with the highest surplus of CO2 in soils which were so wet that water dripped from the soil sample. An almost complete lack of oxygen was found in boggy raw humus soils. In the light of recent investigations, the claim, formerly made by Gräbner, that non-boggy raw humus soils and hardpan layers are lacking in oxygen can no longer be maintained, at least not in that general form. The poorest kind of raw humus of beech woods has been found to give perfectly normal O2 and CO2 values, even after periods of rain. (Braun-Blanquet 1932:225)
The oxygen content of air in the superficial root layers approaches that of the atmosphere and is usually around 18 to 20 per cent by volume. (Braun-Blanquet 1932:225)
Carbonic Acid Content of the Soil.—Since the oxygen and carbonic acid content of the soil bear a definite ratio to each other, and vary simultaneously, only one of the factors needs to be measured. Lundegård has decided in favor of the carbonic acid. The determination of the carbonic acid content of the soil air is carried out in the same manner as that of oxygen, with the aid of the portable gas-analysis apparatus. Romell (1922) and Lundegård (1925) give details on this subject. The reports of both of these investigators are indispensable to anyone who is engaged in studying the composition of soil air. It is possible here to state only a few of the results which are most important ecologically. While the oxygen content decreases with depth of soil, the CO2 content, as a rule, rises. In deeper soil layers the percentage (by volume) of carbonic acid may reach that of oxygen or even exceed it. (Braun-Blanquet 1932:225)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.