1 68 



SOILS. 



Root-system in the Humid Region. It is quite otherwise 

 where a dense subsoil not only obstructs mechanically the deep 

 penetration of any but the strongest roots, but at the same 

 time is itself too inert to provide sufficiently abundant nourish- 

 ment apart from the surface soil, which is there the portion con- 

 taining, alongside of humus, the bulk of the available plant- 

 food, and in which alone the processes of absorption and nu- 

 trition find the proper conditions; such as access of air and the 



FIG. 28. Root of an Lastern (Wisconsin) Fruit Tree. (Photograph by Prof F. H. King.) 



ready and minute penetration of even the most delicate rootlets 

 and root-hairs. The largest and most active portion of the 

 root-system being thus accumulated in the surface soil, it fol- 

 lows that unless the latter is constantly kept in a fair condition 

 of moistness, the plant must suffer material injury very quickly ; 

 hence the often fatal effects of even a few weeks' drought. 

 The same occurs in the arid region when often-repeated 

 shallow' plowing has resulted in the formation of a " plow- 

 sole " which prevents the deep penetration cf roots; when a 



