94 Studies in Forestry [CHAP. v. 



exhaust the land to anything like the same extent as agricultural 

 crops, which require manuring in order to supply deficiencies, 

 and to stimulate the upper soil to increased decomposition. And 

 when the dead foliage is retained on the ground and decomposes 

 to form humus, this in turn exerts a very beneficial hygroscopic 

 influence, stimulates the soil to more rapid decomposition, 

 renders the mineral food-materials more easily soluble, and, in 

 short, improves the productive capacity of the land, instead of 

 exhausting it. This shows the importance of maintaining a close 

 leaf-canopy, and of adopting all practical measures in order to 

 obviate insolation of the soil, and prevent the free play of winds 

 that may remove the foliage and exhaust the surface-moisture. 



But the mineral constitution of the soil is not the only factor 

 determining its productive capacity sylviculturally ; for various 

 physical factors are of equal, and often greater, importance 

 e. g. depth, porosity (aeration), consistency, &c. So far, however, 

 we may be guided by practical experience as to assert that 

 Elm, Maple, Sycamore, and Ash can only be grown satisfac- 

 torily on fertile soils ; whilst Birch and Pines are least exacting 

 as to mineral richness. Between these extremes, Oak, Beech, 

 Lime, and Silver Fir make somewhat greater demands than 

 Sweet Chestnut, Larch, Hornbeam, Spruce, Alder, Aspen, and 

 Willow. When planted out on soils not naturally favourable 

 to their growth and development, Scots Pine and Birch show 

 a decidedly greater capability of accommodating themselves 

 to any given conditions than Oak, Beech, Spruce, and Silver 

 Fir ; whilst Ash, Oak, Elm, Maple, and Sycamore those 

 genera whose demands on fertility are greatest possess the 

 least power of accommodation. 



Soils are largely made up of Salts, with Oxides and some- 

 times Chlorides. The Salts are mostly Silicates (often com- 

 bined with water), Carbonates, and Sulphates; whilst the 

 Phosphates, so important for plant-life, only occur occasion- 

 ally. In the formation of these Salts, the most important of 

 the Acids are Silica (Si O 2 ), Carbonic acid (CO 2 ), Sulphuric 



