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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