FUNCTION OF THE SOIL AIR 437 



The movement of carbon dioxid from the soil depends 

 chiefly upon diffusion into the outside atmosphere. 

 The conditions governing diffusion, which will be dis- 

 cussed later (page 439), therefore largely determine 

 the rate of loss of carbon dioxid from the soil. 



302. Effect of roots upon composition. The absorp- 

 tion of oxygen and excretion of carbon dioxid by roots 

 has a real, but as yet unmeasured influence upon the 

 composition of the soil air. It is worthy of note, however, 

 that the carbon dioxid thus excreted is in a position 

 where its aqueous solution can be of the greatest benefit 

 to the plant in its solvent action upon the soil, as it is 

 in direct contact with the absorbing portion of the 

 roots. 



III. FUNCTIONS OF THE SOIL AIR 



Both carbon dioxid and oxygen as they exist in the 

 air of the soil have important relations to the processes 

 by which the soil is maintained in a habitable condition 

 for the roots of plants. Deprived of these gases, the soil 

 would soon reach a sterile condition. 



303. Oxygen. An all-important process in the soil 

 is that of oxidation, because by it the organic matter 

 that would soon accumulate to the exclusion of higher 

 plant life is disposed of, and the plant-food materials 

 are brought into a condition in which they may be 

 absorbed by plant-roots. The presence of oxygen is 

 essential to the life of the decomposing organisms and 

 to the complete decay of organic matter. Through this 

 process, roots of past crops, as well as other organic matter 

 that has been plowed under, are removed from the. soil. 



