204 COLLEGE BOTANY 



sometimes poisonous to green plants and in most cases are 

 changed to nitrates before they are used. 

 Soil nitrates and ammonia are derived : 



( 1 ) By the decay of organic materials of various kinds. 



(2) By bacteria (Pseudomonas radicicola) which cause and 

 live in the tubercles 011 the roots of leguminous plants. 



(3) By certain bacteria and fungi which live free in the 

 soil and are able to use nitrogen from the air. 



(4) By ammonia carried to the soil by rainfall. 



(5) By nitrous and nitric acid produced by lightning and 

 carried down by rainfall. 



Ammonification. Dead plants and animals undergo decom- 

 position (by means of bacteria and fungi) by which their ni- 

 trogenous compounds are broken into ammonia, carbon dioxide 

 and other products. This is known as ammonification. The 

 carbon dioxide returns to the air, but the greater part of the 

 ammonia usually unites with soil acids or salts. 



Nitrification. The ammonification is followed by (1) the 

 oxidation of the ammonium salts into nitrites and (2) the 

 oxidation of the nitrites into nitrates. 



Nitrifying Organisms. It is now well known that certain 

 bacterial and fungous organisms are capable of fixing the free 

 atmospheric nitrogen which reaches the soil in the ways pre- 

 viously described into nitrates which can be used by plants. 

 Some of these bacteria live free in the soil, but one of the most 

 important of these is the bacterium Pseudomonas radicicola, 

 which attacks the roots of leguminous plants, causing character- 

 istic nodules (Fig. 104- ), in which the bacteria live and work. 

 These organisms! obtain their carbon, minerals and water 

 from the leguminous plant and give in return the combined 



