CHAPTER III. 



THE CONSTITUTION OF THE SOIL. 



IT is well known that only the top six or eight inches of the soil is 

 suited to plant life, and that the lower part, or subsoil, plays only an 

 indirect part in plant nutrition. We shall, therefore, confine our atten- 

 tion almost exclusively to the surface layer. 



The soil was in the first instance derived from rocks, partly by dis- 

 integration and partly by decomposition. The fragments split off were 

 sooner or later carried away by water and deposited at the bottom of 

 a river or sea. There they mingled with residues of living organisms 

 which have subsequently played an important part in the history of the 

 soil as its chief source of calcium carbonate and calcium phosphate. In 

 course of time the material accumulated to considerable depths ; then, 

 as the result of some earth change, the water retreated leaving the de- 

 posited material as dry land or rock. No sooner was this exposed to 

 the air than it began once again to undergo disintegration and erosion. 

 Air, water and frost all played a part in the disintegration process ; 

 water and sometimes ice have acted as transporting agents. For im- 

 mense ages the particles have been subjected to these actions, and the 

 fact that they have survived shows them to be very resistant and prac- 

 tically unalterable during any period of time that interests us. Refer- 

 ence to Table LXIV., page 161, shows that the particles in the surface 

 soil which have been exposed to weathering ever since the soil was laid 

 down, and in some cases to cultivation for some hundreds of years, are 

 almost indistinguishable in size from those in the subsoil which have 

 been protected from all these changes. 



However, the soil particles are not wholly unalterable. The rain 

 water and its dissolved carbonic acid exert a slight solvent action, and 

 the soil water always contains small amounts of calcium and magnesium 

 compounds, silica and other substances in solution. Each individual 

 particle only loses a very minute amount of substance to the soil water, 

 and its life is extraordinarily long; nevertheless dissolution is per- 

 petually taking place. The surface soil contains less of the smallest, 

 and, therefore, most easily attacked, particles than the subsoil. 



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