CHAPTER V 



iV 



\ I SOIL AND SITUATION IN RELATION TO 



WOODLAND GROWTH 



7 



I. Soil in Relation to Woodland Growth. 



,/ BY Soil is understood the product of the decomposition of 

 the rocks forming the crust of the earth. Soil is formed 

 mechanically and physically by rains, frosts, rivers, volcanoes, 

 &c., or chemically and organically by oxidation, deokidation, 

 and changes effected by carbonic acid. Through its permea- 

 bility and its other physical factors, the soil enables trees 

 to develop the root-systems necessary alike for their support 

 mechanically and physiologically. By far the greatest portion 

 \J of the food of all plants possessing chlorophyll is obtained 

 from the carbonic acid of the atmosphere ; but certain neces- 

 sary substances (nutrient salts) are only obtainable from the 

 soil when held there in solution, so that they may be imbibed 

 by the rootlets. A distinction is made between the soil or 

 surface-soil and the subsoil. The former consists of the 

 usually more thoroughly decomposed earth forming the upper 

 layer permeated by the root-system ; whilst the latter may, or 

 may not, be of similar composition to the superposed layer. 

 Even when such patent signs of shallowness of soil as stony 

 outcrops are not visible, the want of depth may easily be 

 noted by the stunted appearance of the timber-crops, in which, 

 owing to the deficiency in soil-moisture and food-supplies, an 



