NATURE AND FORMATION OF SOILS. 31 



of dissolving- many minerals not soluble in rain- 

 water. 



Since plants can derive their food from much 

 simpler elements than can animals, many scien- 

 tists believe that the first forms of life were 

 those of a very low t\ pe of vegetation. The 

 only organisms which could exist upon a bare 

 rock must be those which could subsist upon a 

 purely mineral food obtained from the rock 

 itself, and from the water and gases of the at- 

 mosphere. It has been discovered that even the 

 denuded rocks of ver\- hiorh mountains are cov- 

 ered by a layer of organic matter, evidently 

 formed by microscopic vegetation. These micro- 

 organisms have even been discovered at con- 

 siderable distance in the interior of these rocks. 

 They begin the formation of humus, J and make 

 it possible for other low forms of plant life to 

 creep in, which, in turn, help to prepare the soil 

 for the sustenance of chlorophyll-bearing, or 

 green, plants. 



Bacteria. — The micro-organisms which are 

 of most importance to agriculture are the bac- 

 teria which (i) oxidize nitrogenous substances, 

 thereby forming nitric acid, and (2) those which 

 reduce nitric acid to ammonia or to free ni- 

 trogen. 



In the processes of nitrification, ammonia is 

 one of the first products formed from ferment- 

 ing organic matter by one species of bacteria. 



