SOIL 



roots of plants congenial to them, causing the 

 formation of tubercles. It is an association of 

 organisms for mutual advantage — ^the plants 

 supplying nourishment for the bacteria, and the 

 bacteria supplying plants with nitrogenous com- 

 pounds from the soil-air. Other plants are de- 

 pendent for their nitrogen upon the nitric acid 

 and ammonia in the soil. A large part of the 

 nitric acid in the soil is a final product of the 

 life processes carried on in the soil in the decom- 

 position of manures and other organic matter. 

 Ammonia, the odor of which is present during 

 the fermentation of manures, is produced from 

 the compounds of nitrogen in plant-issues and 

 excretions of animals through action of certain 

 bacteria. The same process takes place in the 

 soil when organic matter decays. The ammonia 

 is seized upon by nitrous ferments reducing it 

 to nitrous acid, and the nitrous acid is seized 

 upon by other bacteria converting it into nitric 

 acid, in which form it is used by plants. When 

 plants are used as food by animals, and animals 

 by man, the circuit is extended. 



The transfer of plant-food in solution is due 

 to the capillary movement of the surface film of 

 water around the soil-grains. The capillary 

 water in contact with the soil-grain dissolves the 

 surface molecules of mineral matter and nitrog- 

 enous compounds. The solution continues un- 



165 



