120 ECONOMICS OF FORESTRY. 



The air furnishes one-half the constituents, 

 namely, the carbon, which the chlorophyll cells 

 of the leaves assimilate under the influence of 

 the sunlight, and almost the entire other half is 

 furnished by the water of the soil. Not that tree 

 life and wood production can entirely dispense 

 with the presence of these minerals, but it requires 

 them in smallest amounts, and the final product, 

 which the forester harvests, is practically devoid 

 of them. Moreover, those parts of the tree which 

 in its life processes accumulate the largest amounts 

 of these elements, namely, the foliage and small 

 branchlets, do not usually form part of this har- 

 vest, but are returned to the soil, so that, in fact, 

 not only does the soil not lose any of its fertility, 

 but, on the contrary, it is enriched at its surface 

 by the decay of the litter, not only through the 

 vegetable humus and the nitrogen-condensing bac- 

 teria formed in the same'_(see Appendix), but through 

 mineral constituents in soluble form, which the tree 

 has brought up from greater depths. Hence the 

 well-known fertility of virgin woodland soil ; while 

 agriculture exhausts soils, forestry enriches them.^ 

 From the soil the forest crop derives mainly the 



1 A field of potatoes, for instance, uses of phosphoric acid three 

 times as much as a beech forest, five times as much as a spruce 

 forest, and nine times as much as a pine forest, and of potash nine, 

 thirteen, and seventeen times as much as the three tree species 

 respectively, while of nitrogen virood requires 10 to 13 pounds per 

 acre as against 60 to 90 pounds in potatoes, the conifers generally 

 requiring less than the deciduous-leaved trees. 



