BACTERIA IN NATURAL PROCESSES. 



99 



this important element. It should be stated also 

 that there are other compounds in the soil which 



PRODUCTS OF ANIMAL LIFE 



c 



FREE\C 

 NITROGE'N ^ 



TUBERCLE BACTERIA 

 AND LEGUMES 



A 

 SOIL - NITRATES 



FIG. 25. Diagram illustrating Nature's food cycle. 

 Explained in the text. 



furnish plants with part of their food com- 

 pounds containing potassium, phosphorus, and 

 some other elements. For simplicity's sake, 

 however, these will be left out of consideration. 

 Beginning at the bottom of the cycle (Fig. 25 A), 

 plant life seizes the gases from the air and these 

 foods from the soil, and by means of the energy 

 furnished it by the sun's rays builds these simple 

 chemical compounds into more complex ones. 

 This gives us the second step, as shown in Fig. 

 25 B, the products of plant life. These products 



