. 89 



rate of diffusion was approximately proportional to the square of the 

 porosity .... the escape of carbonic acid from the soil and 

 its replacement by oxygen take place by diffusion, and are determined 

 by the conditions which affect diffusion, and are sensibly independent 

 of the variations of the outside barometric pressure." 



In the upper, better ventilated, moist, neutral or alkaline layers of 

 vegetable debris decomposition is brought about mainly by the agency 

 of fungi ; but in the deeper, poorly ventilated acid layers, lacking oxy- 

 gen, bacteria are the active agents (cf. Transeau, '05, '06). The 

 higher the temperature the more rapid the circulation, and on this ac- 

 count ventilation in the open is relatively more rapid than in the cooler 

 woodlands. The black soil prairies are thus favorable to a higher tem- 

 perature and better ventilation. Dry soil, according to Hilgard 

 ('06: 279) contains from 35 to 50 per cent, its volume of air, and in 

 moist or wet soils this space is replaced by water. Thus the condi- 

 tions which influence the amount of water present have a very im- 

 portant influence upon aeration. As water is drained from the soil, air 

 takes its place ; so drainage and the flow of water through the soil facil- 

 itate ventilation. The part of the soil containing air is thus above the 

 water-table ; and as this level fluctuates with the season and from year 

 to year the lower boundary of this stratum is migratory. Hilgard 

 states that cultivated garden soil contains much more air than uncul- 

 tivated forest soil. Warming ('09:43) says that the "production of 

 acid humus in the forest leads to an exclusion of the air." If lime is 

 present, such an acid condition can not arise. 



While the source of oxygen in the soil is the air, the reverse is the 

 case with carbon dioxide. The surface layers of the soil, among 

 dense vegetation, constitute an area of concentration of carbon 

 dioxide. Because this is more soluble than other gases, it is found 

 in rain water, according to Geikie, in a proportion 30 to 40 times 

 greater than in the air. Rains thus assist in the concentration of 

 carbon dioxide in the soil. This concentration is well shown by the 

 following table by Baussungault and Lewy (Van Hise, '04:474). 



