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 1 , 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. Koot ofan Kastcrn ( Wisconsin i Fruit Tree (Photograph \iv I'mf I' II King \ 



ready and minute penetration of rvui 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 of roots ; when a 



