6 A MANUAL OF FORESTRY 



.ape and the depth to which they reach; but even shallow- 

 rooting kinds derive advantage when the soil over which they 

 izrow i- deep, owin^ to the greater fertility within easy reach 

 of their roots. Some species, like the oak, have a strong tap 

 root; other-. -uch as beech and birch, develop strong side 

 roots, but no tap root; and still others, like spruce, have a 

 pronounced shallow root system. Spruce and birch require 

 least depth of soil, oak most. Whether the soil is loose or 

 binding is a matter of great importance for tree growth. As 

 a rule, the broadleaf trees do better than the conifers on the 

 stifiVr classes of land, although soils of average tenacity are on 

 the whole most suitable for all kinds of trees. The chief con- 

 stituents of soil are clay, lime, and sand; and as clay yields the 

 most valuable materials for plants, the qualities of soil are often 

 determinate to some extent by the quantity of clay in them. 

 Clay soils are hard and interfere with the movement of moisture; 

 sands are too porous; limes too easily heated; loamy soil, there- 

 fore, is usually best. 



The chief chemical constituents of wood are carbon, hydrogen, 

 n. and nitrogen, but the ashes of wood also contain sulphur, 

 phosphorus, chlorine, silicon, potassium, sodium, calcium, mag- 

 nr-ium. and iron. The carbon is obtained solely from the car- 

 b< mil arid of the air; the hydrogen and oxygen are derived chiefly 

 from the water in the soil; and the nitrogen from the ammonia 

 oi the -oil formed by nitrifying bacteria from organic decaying 

 matter. These other chemicals taken from the soil in salts and 

 oxide- are contained in very small quantities. Thus iron, 

 though very important for the formation of chlorophyll in the 

 leaves, is present only in very small quantities, so the old remedy 

 ot driving nail- into a dying tree was of no value. 



The percentage of pure ash in the dry leaves of trees varies 

 from 2.3 per cent in white pine to 7.6 per cent in the ash tree, 

 and dry leaves have a higher percentage of minerals than wood. 

 Thi- indicates the well-known fact that conifers are less demand- 

 ing as regards chemicals than broad leafs. For this reason the 

 s>il improves under conitVrou- crops if a good canopy is main- 



