CHAPTER XXIV 

 SOIL 



Introduction. Soil to most people simply means a 

 layer of dirt covering the surface of the earth; some- 

 thing necessary perhaps, yet objectionable because of 

 the dust that it produces in drought, and mud, in rainy 

 weather. Yet this same dirt is one of the substances 

 indispensable to all life. If a sample of soil be taken 

 from almost any place on the surface of the earth and 

 examined carefully, it will be found to be largely a 

 layer of sand and clay mixed with more or less decayed 

 animal and vegetable matter. This surface layer is 

 less than one foot in depth over the greater portion of 

 the earth's surface, yet in places it extends to a depth 

 of several feet. Beneath the soil is a layer of more com- 

 pact earth containing less of the decayed organic mat- 

 ter and usually lighter in color. This layer is called 

 sub-soil and grades down gradually into the solid rock, 

 called the bed rock, from which all soil originally came. 



Origin. From 60% to 95% of the weight of ordinary 

 soil consists of rock fragments. The fragments have 

 come either from the layer of rock bed just below the 

 sub-soil, in which case it is called residuary or seden- 



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