m CHAPTER III. 



THE COMPOSITION 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 attention almost exclusively to the 

 surface layer. 



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

 by disintegration and partly by decomposition. In most 

 cases 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 phos- 

 phate. In course of time the material accumulated to con- 

 siderable depths ; then, as the result of some earth change, the 

 water retreated leaving the deposited 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 

 immense ages the particles have been subjected to these 

 actions, and the fact that they have survived shows them to be 

 very resistant and practically unalterable during any period of 

 time that interests us. Reference to Table LXXXVII., p. 332, 

 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. 



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