SOIL AND SURFACE CONDITIONS 123 



average condition of the first three or four feet downwards. 

 For tree growth to be profitable, this depth of soil, sub- 

 soil, surface stratum, or whatever one likes to call it, must 

 be porous, and capable of being easily penetrated by the 

 roots of trees, and also contain a sufficient quantity of 

 moisture in which nutrient salts are in solution to supply 

 the needs of trees at all seasons. Subsoils almost imper- 

 vious to air and water, whether solid rock or stiff adhesive 

 clays, are not suitable for many species, although much 

 depends upon the climate. Spruce and larch sometimes 

 do well on a foot or so of soil resting on rock or clay ; but 

 in the former case the summer rainfall must be frequent 

 and heavy, or in the latter a comparatively steep slope 

 must exist to carry off water which would otherwise stag- 

 nate. It is seldom that big timber can be found on either 

 of these classes, although the clay soil may be more valu- 

 able for pasture than many soils on which timber does 

 well. Similarly, very fine sand, owing to lack of aeration, 

 is not favourable for many species, except, perhaps, for oak, 

 chestnut, etc., and even larch rarely produces sound timber 

 upon it. Coarse rubble, again, which allows rain to pass 

 through like a sieve, will not support tree growth, 

 although it will sometimes be found in the form of ' talus ' 

 or ' detritus ' with loam, clay, or gravel within a foot or two 

 from the surface, in which case the trees, of course, otow on 

 the lower stratum. Very coarse gravel in a dry climate is 

 not suitable for the same reason, and many other instances 

 might be given of thin soils resting on very open or porous 

 subsoils which fail to give satisfactory results through lack 

 of moisture during dry weather. 



But whatever mechanical condition the soil or subsoil 

 may be in, the finer particles which compose the soil of 

 agricultural or garden land must also be present in some 

 form or other, although they need nob necessarily be in 

 the form of a solid layer of surface. In all timber-growing 



