EFFECT OF SOIL ON FOREST VEGETATION. 89 



pine, which is satisfied \Yith about one-fourth of those wanted 

 by beech. 



(4.) For the production of wood alone (excUuhng the leaves) 

 forest trees require much smaller quantities, than field crops. 

 Thus beech takes only one-ninth, Scotch pine about jV^h, and 

 the six species enumerated on an average about J^th of the 

 quantity required by field crops. Of the rarer substances, 

 potash and phosphoric acid, trees take, on an average, only 

 about ij\y^h of the quantity necessary for field crops. 



(5.) Almost any soil can furnish a sufiicient quantity of 

 mineral substances for the production of a crop of trees, 

 provided the leaf mould (humus) is not removed, and good 

 soils will continue to do so, even if a certain portion of the 

 humus is taken away. If, however, the removal of litter is 

 carried on annually and for a long period, any but really 

 fertile soils are likely to become exhausted, just as lands, on 

 which fields crops are grown, cannot as a rule go on for ever 

 without manuring. 



(6.) Poor soils, which are not capable of producing a crop 

 of broad leaved trees, may yet be able to yield a fair return if 

 planted with less exacting conifers, especially Scotch pine. 



The above conclusions agree with the results of practical 

 experience. It has been found, that tbe quantity of wood 

 production is not directly proportionate to the quantity of 

 mineral nourishing substances in the soil. Again, woods 

 thrive equally well on soils of the most different geological 

 origin, while great differences exist in the development of one 

 and the same species if grown on soils of the same geological 

 origin. These phenomena are explained by the great im^Dort- 

 ance of the physical properties of the soil, depth, porosity and 

 a proper degree of moisture. To ensure a favourable condition 

 of the physical properties should, therefore, be the forester's 

 chief aim, and this he can do best by preserving the humus, 

 especially on middling and poor soils. Humus increases the 

 depth of the soil, absorbs from the atmosphere considerable 

 quantities of aqueous vapour, carbon dioxide and ammonia ; 



