THE PHYSICAL PROPERTIES OF THE SOIL/ 



Dr. EWALD WOLLNY. 



PAET 3. 

 IV. THE BEHAVIOR OF THE SOIL TOWARD AIR AND GASES. 



THE AIR CAPACITY (OR POROSITY) OF THE SOIL. 



This is expressed by tlie ratio of the volume of the air spaces to the 

 total volume of the soil. The volume of the x)ores or intergranular 

 spaces varies in natural soils, as we have already seen (p. 764) within 

 wide limits — in the dry state from 20 to 85.3 per cent. In one and the 

 same soil the air capacity depends principally on structural conditions. 

 In the dry state the volume of the air spaces is greater the smaller the 

 particles. Stirring and flocc^lation increase the volume of the pores 

 generally to a considerable extent, while the pressing together of the 

 soil produces the opposite result. It may be mentioned further that the 

 quantity of air inclosed in a soil decreases as the proportion of stones 

 increases. 



With the increase in the quantity of water present the volume of the 

 pores of course decreases, but to a varying extent according to the 

 water capacity. In the moist state clay and humus contain the small- 

 est quantity of air of any of the soil constituents; in fact the air may 

 be entirely absent when these substances are saturated with water. 

 The quantity of air in the wet mass always increases with the quantity 

 of coarse-grained constituents. The influence of water on the volume 

 of the pores is less marked in crumbly than in powdery soils, because 

 in the former the noncapillary spaces are not filled with water. The 

 decrease in the quantity of air due to an increase in the density of the 

 mass is greater in the moist than in the dry condition. 



From Avhat has been said regarding the influence of the quantity of 

 water in a soil on the volume of the pores it is plain that the quantity 

 of air in the soil increases during drought and decreases correspond- 

 ingly with an increased precipitation. The extent of the influence of the 

 soil in this particular is determined by its mechanical structure."^ 



The air inclosed in the soil is seldom stagnant, but is generally in 

 constant motion, causing an exchange of air between the atmosijhere 

 and the soil. This motion is due to the variations in atmospheric pres- 

 sure, temperature, and wind movement. 



■Continued from p. 863. »E. Wollny, Forsch. Geb. agr. Phys., 8, p. 371. 



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