SOIL PHYSICS AND 

 MANAGEMENT 



CHAPTER I 

 SOIL MATERIAL AND ITS ORIGIN 



Definition of Soil. The land surface of the earth is covered 

 almost everywhere with a layer of unconsolidated material de- 

 rived from rocks by the processes of weathering. This stratum 

 varies in thickness from a few inches to hundreds of feet and may 

 even be absent from small areas, not because it was never formed 

 there, but because it has been carried away. The agencies of trans- 

 portation have done so. much work that in many instances much 

 or all of the loose material covering the rocks was not derived 

 from those beneath, but from rocks at some distance, even hundreds 

 of miles away. This material varies in composition with the rock 

 from which it was derived and the agencies producing it. It cannot 

 be termed a soil until organisms have worked upon it, modifying 

 it to a greater or loss extent. The depth of the layer upon which 

 the organisms have acted is only a few feet. 



From its origin, a soil may be defined as disintegrated and 

 decomposed rock mixed with more or less organic matter, while 

 from its use it is defined as that part of the ea rib's surface adapted 

 to the- mechanical support and nourishment of plants. 



Elements of the Earth's Crust. The earth has been studied 

 by various means and the composition determined to a depth of 

 approximately twenty miles. Of the elements known, comparatively 

 few occur in any large quantities in this stratum. Eight constitute 

 about 08.. 5 per cent. The following table shows the relative abun- 

 dance of these elements. 



Soil Forming Minerals. Aside from oxygen and nitrogen as 

 air, and carbon as graphite or diamond, these elements rarely ever 

 exist in a free state, but are found in combinations as minerals. 

 These are natural substances, possessing definite physical charac- 

 teristics as, specific gravity, hardness, brittleness. color, cleavage, 

 and sometimes crystalline form and having a more or less definite 

 chemical composition. 



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