CHAPTER 14 



The Soil and They That Dwell Therein 



PHYSICAL AND CHEMICAL COMPOSITION 



SOIL ORGANISMS 



THE NITROGEN C^ CLE 



THE CARBON CYCLE 



CYCLES OF OTHER ELEMENTS 



MICROBIOLOGY OF PETROLEUM 



PHYSICAL AND CHEMICAL COMPOSITION 



The upper layer of the earth's surface, varying in thickness from 

 SLX to eight inches in the case of humid soils and up to ten or 

 twenty feet in certain arid soils, possesses characteristic properties 

 which distinguish it from the underlying rocks and rock ingredients. 

 This relatively thin layer of the earth's crust is called soil. A 

 dictionary defines soil as "finely divided rock material mixed with 

 decayed vegetable or animal matter, constituting that portion of 

 the surface of the earth in which plants grow, or may grow." The 

 principal difference between soil and the sub-soil is the presence of 

 living organisms— plant and animal. It is a combination of climate 

 and living organisms which determines to a great extent the type 

 of earth in a given locality. 



Soil is such a complex substance, both chemically and biologi- 



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