E. THE SOIL AIR 



I. FACTORS DETERMINING VOLUME 



The amount of air that soils contain varies with 

 different soils, and in any one soil it varies with cer- 

 tain changes to which it is subject from time to time. 

 The factors affecting the volume of air in soils are: 

 (1) The texture. (2) The structure. (3) The organic 

 matter. (4) The moisture content. 



295. Texture. The size of the soil particles affect 

 the air capacity of the soil in exactly the same way 

 as it does the pore-space (see page 92), since the 

 two are identical. A fine-textured soil in a dry condi- 

 tion would, therefore, contain as large a volume of 

 air as a coarse-textured one, provided the particles 

 were spherical and all of the same size. 



Under the conditions actually existing in the field, 

 those soils composed of small particles generally 

 possess the larger air-space. 



296. Structure. The volume of air in a water-free 

 soil being identical with the pore space, the formation 

 of aggregates of particles is favorable to a large air 

 volume. The volume of air in any soil, therefore, 

 changes from time to time; and particularly is this true 

 of a fine-grained soil, in which the changes in structure 

 are greater than in a soil with large particles. A change 

 in soil structure may greatly alter the volume of air con- 

 tained, by altering the pore space, thereby influencing 

 the productiveness. Clay is most affected in this way. 



(432) 



