THE NITROGEN CYCLE 445 



have passed through the metabolic processes of the organ- 

 ism; (2) those that have been formed because of the 

 removal of certain atoms by bacteria or enzymes from 

 compounds, thus necessitating a readjustment of the 

 remaining atoms and the consequent formation of a new 

 compound. 



Putrefaction is carried on by a large number of forms 

 of bacteria, the resulting product depending on the sub- 

 stance in process of decomposition and on the bacteria 

 involved. Some of the characteristic, although not con- 

 stant, products formed in the putrefaction of albumin 

 and proteins are albumoses, peptones, and amino acids, 

 followed by the formation of cadaverine, putrescine, ska- 

 tol, and indol. Where an abundant supply of oxygen is 

 present, or where a sufficient supply of carbohydrates 

 exist, these substances are not formed. There are many 

 other products of putrefaction, including a number of 

 gases, as carbon dioxide, hydrogen sulfide, marsh gas, 

 phosphine, hydrogen, nitrogen, and the like. 



It will be noticed that these changes, like those occur- 

 ring in the non-nitrogenous organic matter, involve a 

 breaking-down of the more complex compounds and the 

 formation of simpler ones ; and that a very large number 

 of bacteria are concerned in the various steps, while even 

 the same substances may be decomposed and the same 

 resulting compounds formed by a number of different 

 species of bacteria. 



Present-day knowledge of the subject does not make 

 it possible to present a list of the bacteria concerned in 

 each step, or to name all the intermediate products 

 formed; but for the student of the soil the principal 

 consideration is a knowledge of the circumstances under 

 which the nitrogen is made available to plants, and the 



