CLASSIFICATION OF SOILS. 31 



Where a sufficient depth of soil is not available, the direct 

 assistance of the subsoil is called in, especially its degree of 

 permeability. The depth depends chiefly upon the nature of 

 the rock and soil, the stratification of the rock, the situation, 

 and the general shape of the surface of the ground. The 

 nature of the I'ock governs the rate at which it is disinte- 

 grated, and the rock may be more or less permeable. An 

 impermeable subsoil is all the more injurious the nearer it is 

 to the surface, because it makes the surface soil too wet and 

 cold at one time, and too dry and hot at others, apart from 

 the fact that the roots may not find sufficient room for 

 spreading, or that the stability of trees may be endangered. 



A vertical stratification and a much crumpled state of the 

 rock act most favourably upon the movements of water in the 

 soil and the penetration of the roots ; a horizontal stratification, 

 if unaccompanied by crumpling, is generally the least favour- 

 able form. Low lands have ordinarily deeper soils than high 

 lands. On ridges and steep slopes the soil is liable to be 

 washed away, while it is collected in depressions. In the 

 former places the covering of dead leaves is lialjle to be 

 blown away by wind. 



All physical properties are of special importance through 

 their action upon moisture. In this respect the chemical 

 composition of the soil is of less importance than the admixture 

 of humus and the degree of division of the particles, whether 

 the latter are fine or coarse grained. It is for this reason 

 that sand and clay represent, ordinarily, the extremes. 



6. Classi/icatioii of Soils. 



For the purposes of sylviculture soils may be classified, 

 either according to their chemical composition, or according 

 to one or other of their physical properties. 



a. Classififutioii according to Chemical Coui/iosi/iu/i. 



It is, of course, out of the question to attempt a classifica- 

 tion according to all component substances, nor is it necessary, 



