BACTERIA AS NITROGEN GATHERERS 187 



depend upon them for partial sustenance, but the ultimate 

 source of all animal food is plant life, since meat-producing 

 animals live upon plants. 



Nitrogen is absolutely necessary to plant life and growth. 

 Since a vast store of nitrogen exists in the air, it would 

 seem that plants should never lack for this food. But most 

 plants are unable to make use of the boundless store of atmos- 

 pheric nitrogen, because they do not possess the power of ab- 

 i stracting nitrogen from the air. For this reason, they have to 

 I depend solely upon nitrogenous compounds which are present 

 ! in the soil and are soluble in water. These soluble nitrogenous 

 soil compounds are absorbed by the roots of the plant. 



The poverty of the soil. Plant roots are constantly taking 

 j nitrogen and its compounds from the soil ; the soil becomes 

 \ poorer in nitrogen and finally possesses too little to support 

 j vigorous and healthy plant life. The fertility of the soil can 

 be restored if we add to it a fertilizer containing nitrogen com- 

 pounds which are soluble in water. Decayed vegetable matter 

 contains large quantities of nitrogen compounds, and if de- 

 cayed vegetation is placed upon soil it acts as a fertilizer, 

 returning to the soil what was taken from it. Since man and 

 all other animals subsist upon plants, their bodies likewise 

 contain nitrogenous substances, and manure and waste animal 

 matter is valuable as a fertilizer or soil restorer. 



Bacteria as nitrogen gatherers. Soil from which crops are 

 removed year after year usually becomes less fertile, but the 

 soil from which crops of clover, peas, beans, or alfalfa have been 

 removed becomes richer in nitrogen rather than poorer. This is 

 because the roots of these plants have on them tiny swellings, 

 or tubercles, in which millions of bacteria live and multiply 

 (Fig. 66). These bacteria have the remarkable power of taking 

 free nitrogen from the air in the soil and of combining it with 

 other substances to form compounds which plants can use. The 



