382 The Journal of Forestry. 



even more suggestive to the tree grower than a knowledge of its geo- 

 logical age is likely to be. 



The constituents of a soil, physically speaking, are usually stones, 

 gravel, sand, clay, organic matter, and moisture, and on the propor- 

 tion in which tliese are present many of the physical properties of 

 the soil depend. Their proportion is no less important in a chemical 

 point of view, the mechanical condition of the materials of a soil 

 largely affecting the availability of its chemical ingredients. 



The chief physical properties of the soil are depth, hygroscopicity, 

 compactness, and colour ; of which the first two are by far the most 

 important. 



Of these, depth needs no definition ; but I may here remark that 

 the subsoil is comparatively unimportant, the whole upper layer of 

 the earth, whatever its composition, which is capable of being pene- 

 trated by the roots of the plants, being known as the topsail. That 

 portion of the topsoil which is formed by organic debris in a state 

 of decomposition is known as humus or vegetable mould. 



The hygroscopicity of a soil is the facility Avith which it absorbs and 

 retains water, and is in proportion to its compactness. 



The compactness of a soil is also of importance as determining its 

 permeability to the roots of trees. 



On the colour of the soil, as I mentioned in my last article, depends 

 its power of absorbing heat. 



The best forest soil is deep, dark-coloured, and moderately compact. 



Compact clays, or fine sand, constitute Iteavy or stiff soils, which 

 are also said to be cold on account of the large amount of water they 

 contain ; while limestone, and more or less gritty sand, on the other 

 hand, constitute light, vjarm soils. 



A soil containing much vegetable mould is rich ; one containing 

 little, 2JOor. 



With regard to the quantity of water they contain, soils may be 

 classified as marshy when covered with stagnant water, and thus 

 unfit for trees ; aquatic, those which are so saturated with water tliat 

 it appears under the pressure of the foot, but can drain away. The 

 elm, ash, poplar, willows, and alder will grow on such soil ; damp, 

 where water does not appear, but which never dries up in any season. 

 Qiierciis pcdunculata, spruce fir, elm, ash, alder, and others grow 

 here ; moist, which do not dry up to a greater depth than four or six 

 inches, in which almost every species will grow ; and dry, on which 

 grow the birch, Scotch fir, Austrian, cluster (P. pinaster), and other 

 pines. 



Soils are composed chemically of siliceous, argillaceous, and cal- 

 careous matter, variously mixed, — that is of sand, clay, and limestone. 

 Sand, when very fine, has the same compactness and hygroscopicity 



