THE SOIL AIR 481 



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 ab- 

 sorbed by plant roots. The presence of oxygen is essen- 

 tial to the life of the decomposing organisms and to the 

 complete decay of organic matter. Through this pro- 

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

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

 The process of decay gives rise to products, chiefly car- 

 bon dioxide, that are solvents of mineral matter, and leaves 

 the nitrogen and ash constituents more or less available 

 for plant use. 



Oxygen is also necessary for the germination of seeds 

 and the growth of plant roots. These phenomena, al- 

 though not involving the removal of large quantities 

 of oxygen, are yet entirely dependent on its presence in 

 considerable amounts. 



399. Carbon dioxide. — The solvent action of carbon 

 dioxide is its most important function in the soil. By 

 this action it prepares for absorption by plant roots most 

 of the mineral substances found in the soil. Although 

 a weak acid when dissolved in water, its universal pres- 

 ence and continuous formation during the growing season 

 results in a large total effect. 



Carbonic acid dissolves from the soil more or less of 

 all the nutrients required by plants. The amounts so 

 dissolved are appreciably greater than those dissolved 

 in pure water. The constant formation of -carbon dioxide 

 by decomposition of organic matter keeps this solvent 

 continually in contact with the soil. 



Carbon dioxide serves a useful purpose in combining 

 with certain bases to form compounds beneficial to the 

 soil. Particularly is this the case with calcium carbonate, 

 2i 



