FACTORS OF FOREST PRODUCTION. 119 



in preference to low-crowned and branched trees, 

 and to a slight extent the structure of the annual 

 ring, can be directed; but so far the wood of nature's 

 production and that of man's are very nearly if not 

 quite the same, and forms which are better adapted 

 to chmatic or soil conditions have not been bred by 

 man. The short cycle of development in agricultural 

 crops and the long cycle in forest crops explain this 

 difference. The forester can improve upon nature 

 mainly by making it produce a larger quantity of ma- 

 terial of useful form and of useful species per acre. 



But the greatest and radical difference between 

 the two industries, one of the highest national 

 economic importance, is the difference in the use 

 of the soil. 



Agriculture is engaged in producing starch and 

 sugar, proteids and albuminoids, in short, the com- 

 pounds which are directly food materials ; and this 

 production relies largely on the fertility, the min- 

 erals of the soil, especially the rarer phosphorus, 

 sulphur, potash, nitrogen. With the harvest all 

 these are removed from the soil, and must be 

 replaced by manures or through rotation of crops, 

 or else the soil is sooner or later exhausted and 

 becomes infertile. 



Forestry is engaged mainly in the production of 

 cellulose and its derivatives, carbohydrates,^ which 

 contain a minimum of these rarer elements. 



^ T'ne composition of wood is approximately 50 per cent C, 6 per 

 cent H, 42 per cent O, i per cent N, i per cent mineral ash. 



