THE PI.ANT AND THE ATMOSPHERE 51 



tion, may give an arid or semi-arid climate. When the average 

 annual rainfall is 20 inches or below, it is generally assumed that 

 crops cannot be grown without irrigation. 



Soil Atmosphere. The gases which occupy the pores of soils 

 differ in composition from the atmosphere, chiefly in the fact that 

 they contain much more carbon dioxide and less oxygen. 



The oxygen of the soil atmosphere performs the following 

 functions : 



(1) It oxidizes the organic matter, producing carbon dioxide. 

 Bacteria play an important role in this change. In the absence 

 of air, putrefaction takes place, with production of acid or bad 

 smelling bodies. 



(2) It is necessary for the oxidation of organic nitrogen or 

 ammonia to nitrates. 



(3) It is necessary for the roots. 



(4) It oxidizes partly oxidized mineral compounds, such as 

 ferrous or manganous salts. 



The nitrogen of the soil atmosphere is fixed by legumes, in 

 co-operation with certain bacteria, producing organic nitrogenous 

 compounds. 



The carbon dioxide of the soil atmosphere is formed by pro- 

 cesses of oxidation, and it is also evolved by the roots of plants. 

 If not removed with sufficient rapidity, it excludes oxygen, and 

 thereby interferes with the normal processes of the soil. The 

 carbon dioxide of the soil increases the solvent action of the soil 

 water, thereby aiding in the disintegration of the minerals of 

 which the soil is composed, and in the solution of plant food. 



Processes of Soil Ventilation. The exchange of gases between 

 the atmosphere and the soil atmosphere depends to a consid- 

 erable extent upon the character of the soil. A coarse-grained, 

 open soil is much more easily ventilated than heavy, stiff soils. 

 Indeed, it is possible that processes of oxidation take place too 

 rapidly in some porous soils, resulting in the rapid loss of nitro- 

 genous material and consequent poverty of the soil. 



The particles of air in the soil tend to move out into the atmos- 



