192 GEOLOGY AND CHEMISTRY OF SOILS. 



agency, the soils in a few years would become exhausted of 

 all their alkalies, the vegetable matter would not decay, and 

 hence no food in the soil would be provided for the plant. 

 Absolute barrenness must therefore succeed. For without 

 alkalies or alkaline earths and geine, no plants can grow. 



Depth of soil. The influence of the agents above de- 

 scribed, has not extended to an average depth of more than 

 15 feet ; although in some places, the soil is actually more 

 than a hundred feet in depth. This is but a small por- 

 tion of the whole mass of the earth, whose mean diameter is 

 7,911 miles; hence "the soil would be less in proportion to 

 the whole earth, than the slightest tarnish of rust on an iron 

 globe 100 feet in diameter compared with its mass." But a 

 small part of this constitutes what is properly denominated 

 the soil. That part only of the surface, varying from 3 to 20 

 inches in depth, which has become mingled with vegetable 

 and animal matters, constitutes the true soil, and it is most- 

 ly this part, which concerns the farmer, and which is pre- 

 sented for our investigation, classification, description and 

 improvement. 



CHAPTER V. 



SOILS AND THEIR RELATIONS TO VEGETATION. THEIR ANAL- 

 YSIS, COMPOSITION, MUTUAL ACTION OF THEIR ELEMENTS, 

 GEOLOGICAL AND CHEMICAL CLASSIFICATION AND DESCRIP- 

 TION. 



The relation, which the soil sustains to vegetation, has 

 been pointed out in a general way in the first chapter, p. 81, 

 where it was shown to be one of the essential conditions to 

 the action of the vital power in those vegetables which were 

 cultivated for the use of man ; furnishing support for the roots, 



