CHAPTEK 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. The 

 thickness of this layer varies greatly, being mainly determined by the 

 relative rates at which weathering (i.e., disintegration of the rock under 

 climatic influences) and denudation (i.e., removal of the debris pro- 

 duced) go on under the local conditions. 



In countries of temperate climate and moderate rainfall, with 

 many rocks, these processes are so proportioned that the depth of 

 debris over-lying the hard rock is a few feet, but obviously this de- 

 pends largely upon the nature of the rock. In other places, denuda- 

 tion is restricted, either by the contour of the surface, the character of 

 the rainfall or other circumstances and the layer of debris attains a 

 great thickness. For example, in many districts in the Transvaal, the 

 material available for the formation oif soil is 30 or 40 ft. in depth. 

 On the other hand, in mountainous districts with heavy rainfall, 

 denudation keeps pace with weathering and little material is left from 

 which soil can be formed. In such places the layer of soil may be 

 only a few inches in thickness, and may rest upon hard, undisturbed 

 rock. In limestone or chalk districts, too, where denudation consists 

 largely in the removal of calcium carbonate in solution, the soil -forming 

 material is often mainly confined to the siliceous or argillaceous matter 

 present and is, therefore, small. 



In addition to the mineral ingredients, which usually constitute 

 the largest portion, all soils contain certain quantities of organic matter 

 resulting from the decay of previous vegetable growths. Soils also 

 contain living organisms and varying quantities of water and gases. 

 The depth of the soil proper varies, but on cultivated land, is gener- 

 ally from 9 to 12 in. It rests upon the subsoil, which differs from it 

 in being less oxidised, not so rich in organic matter, and often lighter in 

 colour, the difference in the last respect being due partly to the sub- 

 soil being poorer in the dark-brown organic matter humus and 

 partly to the iron 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 composition of the rock beneath and of its constitu- 

 ent minerals is of considerable help in judging of the probable com- 

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

 geology and mineralogy is useful to the scientific agriculturist. 



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