78 PLANTING 



3. The Demands of Trees as to Depth of Soil. 



With reference to the depth of soil required by trees, the 

 condition of the subsoil or the rock underneath the surface- 

 soil is often one of the most important factors to be taken 

 into consideration. If the subsoil consist of a disintegrated 

 rock, a shallower surface-soil will suffice than would be the 

 case if the rock were unbroken. So also, if the stratification 

 of the rock be vertical, a shallower surface-soil will suffice 

 than if it were horizontal. 



Deep soils are always more beneficial than shallow soils, 

 even if the trees be shallow rooted species. For the roots of 

 trees will have a greater space in which to find the food they 

 require, and there will usually be a more constant supply of 

 moisture. 



The trees requiring the greatest depth of soil in a 

 finely divided state, are : Oak, Spanish Chestnut, Ash, Black 

 Walnut, Acacia, Silver Fir, Douglas Fir, Cupressus macro- 

 carpa, Thuya gigantea, Weymouth Pine, and Sitka Spruce. 



Whereas, on very shallow soils of about 15 inches in 

 depth, only Birch and Aspen Poplar will grow if the subsoil 

 be impenetrable. Spruce would grow on such a shallow soil if 

 there were sufficient moisture in the soil and air, but in this 

 country such would rarely be the case. 



There are, however, some trees which naturally require 

 rather a deep soil, but which will grow on shallow soils, if 

 the subsoil rock be disintegrated. Thus Larch, Beech, 

 Scots Pine, English Elm, Sycamore, and Norway Maple 

 will grow on shallow clay soils overlying oolitic limestone. 



Beech will thrive well on thin soils overlying the chalk. 



Scots Pine, which naturally requires a deep dry soil, 

 is very accommodating, and will succeed well on many quite 

 shallow soils. 



Trees naturally requiring a deep soil, will usually, if 

 grown on soil without sufficient depth, fall off greatly in 

 height, and never reach maturity ; and any timber which may 

 be grown will be of poor quality. 



On shallow soils, it is often possible to grow poles 30 



