Relation of Bacteria to Fertility. 179 



the protein substances, and 3rd. the fats. The first are 

 composed of carbon, hydrogen, and oxygen and include 

 such substances as starch, sugars, cellulose, and woody 

 fiber; the third is made up of the same elements, and is 

 represented by the animal and vegetable fats and oils. 

 When these substances are completely decomposed, the 

 elements appear as water, and carbon dioxid (C0 2 ), both 

 of which can be used by the plant. 



Protein material contains in addition to the above ele- 

 ments nitrogen, sulphur, and phosphorus. The end 

 products of their decomposition are carbon dioxid, water, 

 ammonia, free nitrogen, hydrogen sulfide, and some com- 

 pound of phosphorous. The ammonia, hydrogen sul- 

 fide, and phosphorous compounds must be further 

 changed by certain kinds of bacteria before the green 

 plant can use the nitrogen, the sulphur or the phos- 

 phorus. 



The simple end products, carbon dioxid and water, of 

 the bacterial decomposition of organic matter do not in- 

 terest us further. The compounds that are formed dur- 

 ing the stages of decomposition are of more importance 

 because of their influence on the soil. 



The decomposition is carried on by both aerobic and 

 anaerobic forms of bacteria. The great distinction be- 

 tween their work is that the work of the first is complete ; 

 the most simple products resulting, such as carbon dioxid, 

 water, and ammonia. The material acted upon dis- 

 appears as completely as though it had been burned. 

 With the anaerobic bacteria the work is not wholly fin- 

 ished, but a part of the material remains in the soil, and 

 forms what is known as humus. 



Because of the fact that in cultivated soil, conditions 

 are favorable for the aerobic bacteria, humus does not 



