CHAPTER VIII. 



SOIL AND SUBSOIL. 

 CAUSES AND PROCESS OF DIFFERENTIATION. HUMUS. 



Soil and Subsoil Ill-defined. While the general mass of rock 

 debris formed by the action of the agencies heretofore dis- 

 cussed as soil-material, may under proper conditions be- 

 come soil capable of supporting useful plant growth, universal 

 experience has long ago recognized and established the dis- 

 tinction between soil and subsoil : by which are ordinarily 

 meant, respectively, the portion of the soil-material usually 

 subjected to tillage, and what lies beneath. There can be no 

 question about the practical importance of this distinction ; but 

 the definition of the two terms, as commonly given in some 

 works of agriculture, is both incomplete and, in its application 

 to many cases, partly misleading. 



The differentiation of soil and subsoil is due partly to the 

 action of organic matter and micro-organisms, partly to 

 physico-chemical causes, now to be discussed in detail. 



THE ORGANIC AND ORGANIZED CONSTITUENTS OF SOILS. 



Humus in the Surface soil. The most obvious mark of dis- 

 tinction between soil and subsoil is, usually, the darker tint 

 of the former, due to the presence of humus or vegetable mold, 

 which becomes most apparent by darkening of the tint when 

 the soil is moistened. Thus soils having a gray tint when dry, 

 may become almost black when wetted. When no such deepen- 

 ing of color occurs in wetting, the absence or great deficiency 

 of humus may safely be inferred. The only other substance 

 whose presence may invalidate the conclusions based upon the 

 darkening of the soil tint, is ferric hydrate (iron rust), which 

 itself possesses the property of darkening on wetting, and may 

 effectually cover either the presence or the absence of humus. 



Since the formation of the humus depends upon the decom- 



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