42 



AERATION AND AIR-CONTENT. 



As no plants were growing in the soil, this effect was due to the living 

 soil organisms. 



Howard and Howard (1920) give in table 11 the results obtained 

 by Mukherjee in determining the amount of C0 2 in three plots with 

 different treatment. 



TABLE 11. 



Summary. The amount of carbon dioxid regularly increases with 

 the depth of the soil, and the amount of oxygen decreases corre- 

 spondingly. Since the carbon dioxid is derived from respiration and 

 decomposition in the soil, it is most abundant in soils and at depths 

 where its escape into the air is most difficult. On the other hand, the 

 supply of soil-oxygen is obtained from the air, and the amount de- 

 creases with the distance from the source and the increasing tribute 

 taken by the roots. The complementary nature of the two gases in 

 the soil is a direct result of the respiration ratio. Pettenkoffer (1871) 

 was the first to determine that the amount of carbon dioxid was 

 greater with the depth, and that in midsummer it was 2 or 3 times 

 as much at 4 meters as at 1.5 meters. Similar results were obtained 

 by Fleck (1872), Risler (1872), Salger (1882), Schloessing (1889), 

 Ebermeyer (1890), Lau (1906), and Leather (1915), the results of 

 the last two being especially detailed and complete. Fleck showed 

 that the reverse relation existed in sand, and this probably furnishes 

 the explanation of Bentzen's view that carbon dioxid was more 

 abundant in the upper layers. 



Seasonal changes, especially temperature and rainfall, have a 

 marked influence upon the composition of the soil-air, though part 

 of this is often to be ascribed to the effect of vegetation. Petten- 

 koffer found that the carbon dioxid was least abundant in January 

 and most abundant in August, the ratio being about 1:4. Risler 

 obtained the maximum amount late in June, while in Lau's studies 

 it fell in July and August and the minimum in February. The lat- 



