EDAPHIC OR SOIL FACTORS: PHYSICAL 225 



clogged with water, aeration falls to about one ten-thousandth normal 

 value. 



The activity and air content of the superficial soil layers determine 

 the conditions of soil aeration. Moreover, there are such large fluctua- 

 tions 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 Griibner, 

 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. 



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. 



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. Lunde- 

 gardh 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 Lundegardh (1925) give details on 

 this subject. The reports of both of these investigators are indispen- 

 sable 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. 



Fodor (1875) measured, under a meadow sod, in Cluj, the CO2 and 

 O2 quantities shown in Table 26 on page 226. 



Fodor records 14..3 per cent as the maximal CO2 value at a depth of 

 4 m., while Russell and Appleyard (1915) have found a maximal value 

 of 9.1 per cent of CO2 in a wet moor sod near Rothamsted, England, 

 at a depth of only 15 cm. 



Romell (1922) gives comparative data on the formation of carbonic 

 acid at different depths under natural plant communities in Sweden. 

 In a beech wood with Vaccinium myrtillus ground cover he found the 

 following CO 2 content of the soil: 

 CO2 at 15 cm. 0.2 per cent; at 30 cm. 0.4 per cent; at 60 cm. 0.4 per cent. 



