532 TEXTBOOK OF BOTANY 



The occurrence of nitrogen in a form usable by green plants is due 

 almost entirely to the action of certain bacteria. Such bacteria may be 

 referred to as nitrogen bacteria, and several groups are known to play 

 an important part in the nitrogen cycle in nature. The importance of 

 such microorganisms is recognized when we recall that every time a 

 crop is harvested, the nitrogen within the plant is removed from the 

 field. In the course of a few years this practice, along with the natural 

 leaching by water, would seriously deplete the nitrogen content of the 

 soil. To maintain soil fertility the nitrogen compounds lost in this way 

 must be constantly renewed. The nitrogen bacteria are very important 

 in this renewal. 



The three groups of bacteria that form nitrogen compounds usable 

 by green plants are the ammonifi/ing bacteria, the nitrifying bacteria, 

 and the nitrogen-fixing bacteria. A fourth group, the denitrifying bac- 

 teria, by releasing nitrogen from these compounds, have the opposite 

 effect and decrease the nitrogen content of soils. 



Ammonifying and nitrifying bacteria. Although millions of dollars are 

 spent every year in adding commercial fertilizers containing nitrogen 

 compounds to soils, our immediate interest is how the combined ni- 

 trogen in natural organic compounds is again made usable to green 

 plants. These compounds include the undecaved remains of plants and 

 animals. The ammonifying and nitrifying bacteria bring about a chain 

 of chemical and physical processes by which the nitrogenous substances 

 are transformed into simpler and more soluble compounds. These are 

 really processes of decay, and the nitrogen bacteria are among the 

 saprophytes that bring this about. 



Ammonification. The disintegration and digestion of the large protein 

 molecules of organic bodies in the soil are activated by 'proteolytic 

 (protein-dissolving) enzymes. The resulting products, largely urea, 

 peptones, and amino acids, undergo additional chemical changes by 

 which ammonia (NH.s) is released. In so far as nitrogen is involved, 

 this is a reduction process known as ammonification, and it takes place 

 only through the agency of microorganisms. Some of the chemical re- 

 actions may be represented as follows: 



Urea + Water > Ammonium carbonate 



CO(NH2)2+2H20 > (NH4)2C03 



Ammonium carbonate > Carbon dioxide + Ammonia -f- Water 



(NH4)2C03 > COo -f^ 2NH3 + H2O 



In addition to the investigators named above, Berthelot, Hellriegel, Wilfarth, Beijerinck, 

 and Winogradski should be mentioned. 



