SOIL AND SUBSOIL. 163 



those of the surface soil. In the farmer's parlance, the sub- 

 soil is " raw " as compared with the surface soil ; it is not so 

 suitable for plant-nutrition, and therefore must not be brought 

 to the surface to form the seed-bed, or be incorporated with 

 the surface soil to any considerable extent at any one time, if 

 crop-nutrition is to be normal. It is only in the course of time, 

 by exposure to atmospheric action as well as to that of the 

 humus, and of plant roots, that it becomes properly adapted to 

 perform the functions of the surface soil. 



Soils and Subsoils in the Arid Region. But however pro- 

 nounced and important are these distinctions and differences in 

 the humid region, they are found to be profoundly modified in 

 the arid ; where, as before stated, the formation of colloidal 

 clay is very much diminished, so that most soils formed under 

 arid conditions are of a sandy or pulverulent type. There is 

 then little or no clay to be washed down into the subsoil, hence 

 there is no compacting of the latter; the air consequently cir- 

 culates freely down to the depth of many feet. 



Thus one of the most important distinctions between soil 

 and subsoil is to a great extent practically non-existent in the 

 arid region, at least within the depths to which tillage can be 

 made to reach ; so that the limitations attached to subsoil-plow- 

 ing in the countries of summer rains do not apply to the 

 characteristic soils of the arid regions. 



Even the distinction in regard to humus is here largely ob- 

 literated by the circumstance, already alluded to, that most of 

 that substance must, in the arid regions, be derived from the 

 decay of roots, which moreover reach to much greater depth in 

 these soils. Hence even in the uplands of the arid region it is 

 common to find no change of tint from the surface down to 

 three feet, and even more. This, like the free circulation of the 

 air in consequence of porosity, tends to render the distinction 

 of soil and subsoil practically useless ; since it disposes of the 

 objection to " subsoiling " based upon the inert condition of the 

 subsoil, which in humid climates so effectually interferes with 

 the welfare of crops unless subsoiling is restricted to a fraction 

 of an inch at a time. 



These fundamental differences in the soils of the two regions 

 are illustrated schematically in the subjoined diagram, which 

 shows on the left the contrast between clay or clay loam soils. 



