32 SYLVICULTURE. 



(1) Depth is the extent to which soil is decomposed before a practically 

 unaffected subsoil is reached, which may, or may not, be of a different 

 kind from the soil above it. As regards depth, a soil may be classed as 

 deep, medium, and shallow. 



Depth of soil is very important for trees like Oak, Sweet -Chestnut, 

 Larch, and Pines, which form a deep-going tap-root, because the growth 

 in height soon falls off if the root-system cannot develop normally. Trees 

 with fairly deep root-systems, like Beech, Ash, Elm, Maple, and Sycamore, 

 Silver Fir and Douglas Fir, and even shallow-rooting kinds like Birch, 

 Aspen, and Spruce, all thrive better on deep than on shallow soil. The 

 disadvantages of a shallow soil are all the greater when the subsoil is a 

 stiff clay or impermeable, and tending to make the soil-water stagnate. 

 Deep soil produces a long, clean bole, shallow soil a short stem with a big, 

 branching crown. 



(2) Stiffness, the resistance offered to separation or disintegration of 

 the soil particles, is important in relation to air, moisture, and warmth, 

 and on it also depends the resistance to be overcome by roots in penetrat- 

 ing and ramifying throughout the soil. 



Clay soil is the stiffest, sand the loosest or lightest ; lime is more like 

 clay, and loam more like sand. An admixture of humus or leaf -mould 

 makes clay and lime less tenacious, and stiffens loam and sand, besides 

 yielding ammonia-compounds and assisting in the beneficial processes of 

 nitrification and deuitrification. The liability of soil to expand after 

 rainfall and to shrink during drought is practically proportional to its 

 stiffness. Soil may be distinguished as heavy or stiff (clay and clayey 

 loam, lime, and marl) ; mild (loam, sandy loam, and loamy lime) ; light 

 (loamy sand and sandy marl) ; loose (the poorer sandy soil) ; shifting 

 (sand-drifts and dunes). A light and fairly moist soil produces most 

 rootlets and foliage, and consequently most timber. 



(3) Soil -moisture is essential for tree -growth, because only soluble 

 salts can be imbibed by the suction-roots, and neither transpiration nor 

 assimilation could possibly take place without it. It helps to regulate 

 soil- temperature, and to prevent sandy soils heating or cooling too rapidly. 

 But too much soil-moisture leads to the formation of injurious acids and 

 of marshes, interferes with aeration of the soil, and both retards vegeta- 

 tion and increases danger from frost. A soil may be wet, moist, fresh, 

 dry, .or arid. 



Most trees thrive best on a fresh soil, though Willow, Poplar, Ash, Elm, 

 and Hornbeam prefer a moist, and the Alder even a wet, soil ; but stag- 

 nating moisture is never favourable to tree-growth. A dry soil is not 

 demanded by any of our trees ; but Birch, Rowan, Aspen, Black Pines, 

 Scots Pine in general, and Beech and White Alder on limy soil, can there 

 best accommodate themselves. 



