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 type 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 very high 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, 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. 



