TRANSFORMATIONS OF CARBON AND NITROGEN. 79 



two quite different conditions. Part of it has become partially 

 broken down, forming urea, while another part remains in the 

 higher condition of proteid, gelatin, or other complex organic 

 compounds. Neither is within the reach of plants and further 

 decomposition agencies are needed to make them available 

 again for life processes. 



DESTRUCTION OF NITROGENOUS COMPOUNDS. 



Decomposition Bacteria. We now come to one of the 

 most important functions of the soil bacteria. The destruction 

 of nitrogenous compounds, urea, proteids, gelatins or other 

 bodies, is brought about by several agencies, but the chief one 

 is undoubtedly that of microorganisms. A small amount of 

 the proteid appears to be decomposed in plant tissue without 

 the aid of bacteria, and considerable is disposed of by yeasts, 

 molds and other fungi. But it is to the bacteria that this 

 decomposition is chiefly due and to that class of bacteria prop- 

 erly called the decomposition bacteria. 



This term is a very broad one and includes a very large 

 number of different species of bacteria and a great variety of 

 types of decomposition. There are commonly recognized two 

 types of decomposition under the names of decay and putrefac- 

 tion. These two terms are not commonly very clearly dis- 

 tinguished and are frequently used indiscriminately to refer to 

 the decomposition of organic substances under the influence of 

 bacteria. There is a distinction between them, however, which 

 may be properly drawn and which is commonly recognized 

 when they are separated. Decay is that type of decomposition 

 which takes place in the presence of plenty of oxygen. It is 

 produced by aerobic bacteria and results in a very complete 

 chemical disintegration of the organic substances, the carbon 

 mostly becoming CO 2 , the hydrogen mostly H 2 O, etc. Putre- 

 faction, on the other hand, is the decomposition which occurs 

 in the absence of an abundance of oxygen. It is produced by 



