SOIL FORMATION 3 



In the formation of soil from an exposed rock mass, 

 the first stage, in a temperate chmate such as ours, is the 

 work of frost. Water accumulating in minute crevices of 

 the rock surface or soaking into joints and fissures expands 

 with enormous force on freezing, with the result that greater 

 or smaller fragments are loosened and the disintegration 

 of the rock begins. Even this early stage is assisted by 

 plants. Lichens and simple algae can live on bare rock 

 surfaces ; on chalk and limestone especially lichens live 

 actually in the rock substance. In the former case thin 

 fixing hyphse, the colourless filaments of the fungus con- 

 stituent, in the latter a considerable portion of the vege- 

 tative body, penetrate the rock, chiefly by the solvent 

 action of water rich in carbon dioxide. The rock surface 

 is thus eroded by the plant and at the same time a 

 further entrance for water which will freeze in winter is 

 gained. 



With this initial breaking down of the rock the solvent 

 action of water containing carbon dioxide in solution 

 becomes more marked. The extent of this action depends, 

 of course, on the minerals of which the rock is composed ; 

 quartz sand grains are unafiected and mica is very slowly 

 attacked ; the felspars give up their alkali metal constituents 

 and are reduced to the aluminium silicates which form the 

 basis of clay ; the calcium carbonate in limestone and chalk 

 is rapidly dissolved. 



As the rock mass begins to disintegrate it off"ers a foot- 

 hold to an exiguous vegetation of modest requirements ; the 

 lichens and algae persist, mosses come in, and with them 

 hardy higher plants. The action of sych a vegetation is 

 manifold. Roots and rhizoids penetrate rock crevices, 

 enlarging them by solvent action and even helping to split 

 considerable masses by the force of their growth expansion. 

 The aerial parts of the vegetation arrest blown dust, which 

 gets washed down and adds fine material to the substratum. 

 As roots die off and leaves wither and fall they are partly 

 decomposed by the action of bacteria, and the products 

 become incorporated — the beginning of humus formation. 



