48 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
no error more commonly made than the indiscriminate distribution 
of the different kinds. To this error much of the unhealthiness 
of some plantations is due, and then the young trees soon 
become diseased and die off rapidly. When the natural 
peculiarities of the plants and the. causes of disease are 
properly understood, the plantations will be healthier and more 
profitable than if formed merely in a hap-hazard fashion. But 
almost equally important is the planting of trees that will 
produce timber saleable in the locality; and the planter 
should consider beforehand at what age the crop can prob- 
ably be most profitably cleared, in order that he may adopt 
a system of management well suited to the circumstances. 
Deep heathy soil is usually best suited for the Scots pine, and 
low-lying and moist situations for spruce and other firs. 
Probably the Douglas fir will be considered the most valuable 
of these. It is certainly a free grower, and forms a large quantity 
of timber in a short time. It will thrive on a great variety of 
soil, and should generally form one of the trees in a mixed fir 
plantation. 
Many kinds of pine may be planted with confidence on soil not 
sufficiently good for the production of valuable deciduous trees. 
In Britain it has often been found difficult to form plantations on 
ground that has previously grown a timber-crop, and especially 
when old stumps afford breeding-places for injurious insects. 
In such cases it is better to defer planting for some years after 
the ground has been cleared. 
In a well-mixed plantation the trees comprising it do not all 
extract their food from the same distance below the surface of the 
soil. As is well known, the roots of some trees penetrate deeply, 
while others have their roots near the surface; and by a proper 
selection the soil is thus more completely utilised. Deciduous 
trees gain by having evergreen conifers intermixed with them, 
the shelter thus produced being beneficial in protecting the soil, 
while the side-shade tends to prevent any extensive formation of 
side-branches, and has thus the direct effect of rendering pruning 
unnecessary, and of producing long, clean, valuable timber. 
As the influence of different altitudes is always evident in 
the growth of trees, the situation must be considered as well as 
the soil. The oak will generally succeed best on deep and 
somewhat stiff clay soil. Ash will grow most freely on a deep 
loamy soil, on steep declivities, or in deep ravines. A dry and 
