CHAPTER XII 

 BOIL AIB 



The soil is a porous mass of material of which only about 

 one-half is solid matter. The pore space that results is occu- 

 pied by water and by air in a constantly varying proportion. 

 When a soil is in good condition for crop growth, the air 

 space rarely makes up more than from 20 to 25 per cent, of 

 its volume. The texture of the soil and the amount of mois- 

 ture are obviously the main controls. The individual air 

 spaces of the soil are more or less continuous and seem to 

 maintain a fairly complete communication between the vari- 

 ous horizons. The better the granulation of the soil and 

 the greater the number of cracks and burrows, the easier and 

 quicker is this communication. The air of the soil is either 

 directly in contact with the roots and the soil bacteria or 

 separated from them by only a thin layer of moisture or col- 

 loidal material. 



The air of the soil is not merely a continuation of the atmo- 

 spheric air into the interstitial spaces. As it is enclosed by the 

 soil complexes and by the soil-moisture movement does not 

 take place readily. Hence it is greatly influenced by its local 

 surroundings. This leads to important differences between 

 the atmospheric air and the soil air, the character of the latter 

 depending on a variety of conditions in which the physical, 

 chemical and biological properties of the soil play a large 

 part. 



129. Composition of soil air. — The air of the soil differs 

 from that of the outside atmosphere in that it contains more 

 water-vapor, a much larger proportion of carbon dioxide, a 



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