MICROORGANISMS AND HUMAN WELFARE 21 



bacteria; they cause it to decay, and thus decompose it 

 into simpler chemical compounds that soak into the earth and 

 may then be used in the nutrition of the higher plants. And 

 since plants are constantly taking 

 from the soil the food materials that 

 they need, this soil would tend to be- 

 come less and less fertile were it not 

 for the work of the bacteria that cause 

 decomposition. This is the reason 

 why rotting manure adds to the fer- 

 tility of soil. 



Again, it has been proved that 

 certain kinds of bacteria directly in- 

 crease the amount of nitrogen com- 

 pounds that are so essential for plant 

 growth. It has long been known that 

 corn and other crops will grow better 

 in soil that has just borne a crop of 

 peas, beans, clover, or other members 

 of the pea family. Within recent 

 years an explanation of this fact has 

 been found. When the roots of these 

 pod bearing plants are examined, 

 small swellings are seen (Fig. 9). 

 These contain multitudes of bacteria 

 that are able to take the free nitrogen 

 from the air, where it exists in such 

 abundance, and store it away in the 

 form of nitrates, which are very important mineral matters 

 needed by all crops. Since these bacteria can be put into 

 soils that do not have them, it may be possible in the near 

 future to restore much of the fertility that has been lost 

 (Fig. 10). 



FIG. 9. Roots of horse 

 bean, with tubercles. 



