Relation of Bacteria to Fertility. 181 



The green -plant gets its supply of carbon from the 

 carbon dioxide of the air. Only a small amount is pres- 

 ent in the air at any one time, so little in fact that if the 

 supply were not renewed it would soon be exhausted. 

 This renewal comes from the decomposition of Organic 

 matter by bacteria. The respiration of plants and ani- 

 mals also produces carbon dioxide. The burning of fuel 

 also frees the carbon as carbon dioxide. There is thus 

 a constant passing of the carbon from the air to the plant 

 and back again to the air through the action of plants, 

 animals and especially the bacteria of soil. 



