48 THE SOIL. III. 



which is always found in large proportion in the 

 air of a soil. These acids have powerful solvent 

 properties. 



5. Bacteria. As will be discussed hereafter, ordinary vege- 

 tation requires the presence of nitrogenous organic 

 matter, " humus," in the soil, and since the main source 

 of this organic matter is the remains of previous plants, 

 the question naturally suggests itself What is the first 

 source of organic matter in the mineral debris resulting 

 from the disintegration of rocks? According to the 

 observations of Miintz* the bare surfaces of the rocks 

 even near the summits of mountains yield large num- 

 bers of nitrifying and other organisms which are able to 

 withstand the lowest temperatures. Direct experiments 

 showed that these organisms, with no other food than 

 the mineral matter of the rocks and small quantities of 

 ammonia and alcohol vapour in the moist atmosphere 

 around, were able to live and produce nitrates, also to 

 accumulate carbon. According to Winogradsky I these 

 organisms by the energy liberated by the oxidation of 

 ammonia to nitric acid are able to obtain carbon from 

 mineral carbonates. It is evident, if these conclusions 

 are correct, that bacteria must play an immensely 

 important part in initiating the formation of the indis- 

 pensable humus in soils. They are equally important 

 in carrying on the functions of a fertile soil (r. Chap. 

 IV.). 



It is also to be noted that certain low forms of vege- 

 tation, e.g., lichens and algae, are apparently able to 

 grow on a purely inorganic soil ; they must therefore 

 obtain the nitrogen they require from the air, possibly 

 by the aid of micro-organisms associated with them. 

 Such vegetable growth furnishes humus to a soil and 

 renders it suitable for maintaining the life of higher 

 plants. 



* Ann. Chim. Phys. [61, 11, 136; Jour. Chem. Hoc. 1887, abst. 1135. 

 t Ann. Agron. 16, 273 ; Jour. Chem. Soc. 1890, abst. 1180. 



