vii.] FORMATION OF HUMUS 195 



especially when the reaction of the medium is at all 

 acid : the organisms are most numerous in the surface 

 layer, though they are still to be found in the deepest 

 subsoils. Below a certain depth they must disappear, 

 because deep well water often comes to the surface in 

 an absolutely sterile condition. The changes which 

 organic materials undergo in the soil may be roughly 

 grouped into two classes ; according as there is free 

 access of oxygen or not, either decay (eremacausis) 

 with eventual resolution into the simplest inorganic 

 oxidised compounds, or " humification " will set in. 

 These changes can be illustrated by the fate of a 

 dead branch when it falls either upon the ground, or 

 into a pond or swamp where it becomes buried in the 

 mud at the bottom. In the latter case the fermentation 

 changes cause the wood to darken even to blackness; 

 gases like carbonic acid and marsh gas are split off, so 

 that the material becomes proportionally richer in 

 carbon and poorer in oxygen. Eventually, however, the 

 process slackens, the losses practically cease, and a 

 large proportion of the original material persists. On 

 the other hand, the branch exposed to the air, without 

 darkening very much, becomes slowly resolved by the 

 action of fungi and bacteria into carbonic acid and 

 water, ammonia, nitrogen gas, and mineral salts, with 

 much the same final result as though it had been placed 

 in a furnace. In soil, both these types of change may 

 go on, and the conditions of the soil as regards aera- 

 tion, drainage, temperature, and cultivation, determine 

 which will predominate. 



Practically, the whole group of aerobic bacteria, i.e., 

 those which require free oxygen for their development, 

 and fungi are capable of bringing about the oxidation 

 changes which result in the production of carbonic 

 acid, the combustion of some carbohydrate being 



