THE MAIN TYPES OF SOIL 61 



potash and phosphates, while it also serves 

 as a source of nitrogen. But the nitrogen 

 present in humus is in the organic form, and 

 therefore cannot be utilized by the higher 

 plants until it has been converted into nitric 

 acid. The first stage in the process of 

 conversion is the formation of ammonia, and 

 this is attacked by minute living organisms 

 and worked up into nitrous acid, which, com- 

 bining with a base such as lime, forms the 

 kind of salt called a nitrite. This combina- 

 tion of nitrogen, however, is not immediately 

 important as a plant food, but it is seized 

 upon by another set of organisms, by which 

 it is induced to combine with more oxygen. 

 Thus highly oxidized, it becomes nitric acid, 

 which similarly unites with a base (generally 

 lime) to be converted into a nitrate, a salt 

 which plants can immediately utilize as 

 food. This process, which goes by the name 

 of nitrification, is dependent upon certain 

 definite conditions. In the first place, a 

 nitrifiable substance must be available, and 

 the organisms necessary to convert the 

 nitrogen into nitric acid must also be present. 

 These minute bodies (bacteria), like all 

 living organisms, demand certain conditions 

 in order that they may work satisfactorily. 

 A supply of oxygen in the soil is essential to 



