CHAPTER IX 



THE SOIL: ITS PHYSICAL AND CHEMICAL PROPERTIES 



THE green plant derives its carbon, in the form of carbonic 

 acid gas, from the air. All the other materials required 

 for growth are obtained from the soil. If we burn a 

 plant, the organic matter is burnt off, and an ash is left 

 the materials of which were obtained by the plant, during 

 life, from the soil. 



The soil, in which the plant is fixed, and from which it 

 draws its inorganic food, is derived in situ from the break- 

 ing up, or " weathering," of the rock-masses upon which 

 it rests ; in other cases the soil has been deposited in its 

 present position by the sea (marine deposits), by lakes 

 (lacustrine deposits), rivers (fluviatile deposits, alluvium), 

 or glaciers (till, boulder-clay), or it may have been blown 

 there by the wind, as in the case of dunes. Mingled with 

 the rock-particles forming the soil are the rotting remains 

 of animals and plants, which form a constituent part of all 

 soils supporting vegetation. 



Agents of Denudation. 



The most important natural agencies which bring about 

 the fragmentation of rocks into soil are : 



1. Water, which acts in two ways : 



(a) Chemically. The water which falls on and pene- 

 trates into the rocks is not chemically pure, but contains 

 gases in solution. The most important of these gases 

 carbonic acid dissolves out certain of the mineral con- 

 . stituents, thereby causing the rest of the rock to crumble. 

 Clay is formed in this way from granite. Granite is a 

 rock composed principally of three minerals quartz, 

 felspar, and mica. Quartz is quite insoluble in water. 



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