32 Studies in Forestry [CHAP. n. 



these debris is stored up a large portion of the mineral sub- 

 stances extracted from the soil. But the work of decomposing 

 and dissolving the mineral nutrients is indirectly aided by the 

 leafy canopy of the woodlands. This both prevents insolation 

 of the soil, and also safeguards the retention of a certain 

 amount of soil-moisture requisite for rendering the mineral 

 nutrients soluble ; because it is only in the form of soluble 

 salts that food can be imbibed by the rootlets. By ramifying 

 throughout the soil, the root-systems help to cleave and fissure 

 the subsoil previous to other changes taking place ; whilst 

 shrubs, grasses, and mosses, thriving under the shade of the 

 trees, also die off, decompose, and are transformed into forest 

 mould, which, from its strongly hygroscopic nature, absorbs 

 and retains the atmospheric moisture and precipitations, and 

 assists in regulating the flow of water within the soil. 



When the leaf-canopy overhead is much interrupted or 

 broken, the soil is apt to be overrun with a rank growth of 

 noxious grasses and weeds, which consume the nutrients un- 

 profitably ; whilst at the same time it is exposed to the inimical 

 influences of sun and wind, whose action is apt to reduce the 

 quantity of moisture in the soil below that which is most 

 favourable for tree-growth. 



For the retention of the soil-moisture, and the protection of 

 the soil generally, those species of trees are the most favourable 

 crops, which have a dense crown of foliage and maintain them- 

 selves in close canopy until their maturity qualities with which 

 the different kinds of trees are very unequally endowed. 

 Among broad-leaved genera the Beech ranks highest in this 

 respect, its canopy being close and its foliage dense, whilst the 

 dead leaves are rich in potash and form mould of excellent 

 quality and strongly hygroscopic power. The leaves of Oak 

 do not form good humus, as a considerable amount of tannic 

 acid is evolved during decomposition. Hornbeam foliage 

 yields good mould, but the crop of leaves is lighter in every 

 way than in the case of the Beech; Lime shades the soil 



