CHAP, xii.] Protection of the Soil 273 



being robbed of their soluble salts by weeds and useless 

 growth. 



VI. The Manner in which the Depth of the Soil may 

 be affected by the Nature of the Crop, and the 

 Method adopted in its Treatment. 



So far as sylvicultural operations are concerned, but little can 

 practically be done with a view of directly increasing the depth 

 of the earthy soil throughout woodland tracts. The natural 

 processes of the disintegration and decomposition of rocks, 

 stones, &c. are gradual and slow. Much can certainly be done 

 to improve the productive capacity, as has already been men- 

 tioned with reference to the planting up of poor limy soils ; 

 and there is no doubt that the gradual humification of the dead 

 foliage not only adds to the depth of the upper layer of soil, 

 but also favours the processes of further decomposition, which 

 again lead to increase in depth. In the majority of cases, how- 

 ever, the existing depth of the soil is taken as one of the 

 concrete factors, to which the choice of the crop and of the 

 method of treatment must accommodate themselves. 



Wherever the sylviculturist has to deal with shallow soils, 

 he must naturally select species of trees for cultivation that 

 are able to develop normally within the depth available for the 

 ramification of the root-system. To attempt, for instance, to 

 grow high forest of Oak on a soil of only a couple of feet 

 in depth, with a subsoil of an impenetrable nature for the tap- 

 root and the main side-roots, could not reasonably be expected 

 to prove a success, however fertile the land might be. If the 

 land were good enough, and there were reason to anticipate 

 better returns from such than from any other crop, then Oak- 

 coppice might be cultivated for the sake of tanning-bark. 

 Wherever the soil under timber is shallow, any measures must 

 be avoided that may probably lead to interference with the 

 amount of moisture contained in it, or with the normal process 



T 



