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CHAPTEE III. 

 THE SOIL. 



SOIL is the layer of more or less disintegrated rock which 

 covers a large portion of the surface of the earth and which is 

 fitted, under proper conditions of climate, to support the 

 growth of plants. 



In addition to the mineral ingredients, which usually con- 

 stitute the largest portion, all soils contain certain quantities 

 of organic matter resulting from the decay of previous vege- 

 table growths. Soils also contain living organisms and 

 varying quantities of water and gases. The depth of the 

 layer of soil varies, but it is generally from 9 to 12 inches. It 

 rests upon the subsoil, which differs from it in being less 

 weathered, not so rich in organic matter, and often lighter in 

 colour ; the difference in the last respect being due partly to 

 the subsoil being poorer in the dark brown organic matter 

 humus and partly to the metals iron, &c., being in a lower 

 state of oxidation in the subsoil. Since soil consists largely of 

 disintegrated rock, it is evident that a knowledge of the com- 

 position of the rock beneath and of its constituent minerals is 

 of considerable help in judging of the probable composition 

 and character of a soil. An acquaintance, therefore, with 

 geology and mineralogy is useful to the scientific agriculturist. 

 Any detailed account of rocks, their origin, and characteristics 

 would be out of place here ; but a few words may fitly be said 

 about the chemical composition and characteristics of a few 

 typical soil-yielding rocks. 



If we accept the nebular theory of the earth's origin, it is 

 evident that all rocks must have been formed out of the 

 original intensely heated matter which ages ago represented 

 the earth. The greater portion of the rocks at present forming 

 the crust has probably been through a succession of changes, 

 at one time forming hard igneous rock, then broken down by 

 weathering into debris, which in course of time was again 



