30 TREE GROWTH IN RELATION TO 



of minerals than a field crop. The fact that agricultural 

 crops are annually removed totally, except in some cases the 

 roots, means the relatively rapid exhaustion of the mineral 

 matters in the soil. This does not apply in the case of a wood, 

 where each year a considerable quantity is returned to the 

 soil in the fallen leaves and only a very small proportion is 

 retained in the timber, to be finally removed when the timber 

 is felled. As a consequence a soil can support a continuous 

 succession of forest crops without any necessity for manuring, 

 and, as another consequence, we find that soils which are too 

 poor for agricultural crops can, as a rule, be profitably em- 

 ployed for afforestation. Conifers require smaller quantities of 

 mineral matters than broad-leaved trees, and should therefore 

 be preferred for planting up minerally poor soils. 



Turning now from the mineral character to the physical 

 properties of the soil we find these are of far greater import- 

 ance, and that a soil which has a sufficient depth, a suitable 

 degree of porosity, and a proper amount of moisture, may be 

 accepted as a good forest soil whatever the mineral composi- 

 tion. 



With regard to depth we may say that four feet of soil, 

 including the subsoil so far as it is well broken up, is sufficient 

 for the good growth of all our forest trees. A deep soil is 

 always more favourable than a shallow one, as in it the roots 

 can obtain a good hold, and have a larger space in which to 

 search out and obtain supplies of water and mineral matters. 

 Moreover, a deep soil is less likely to become dry in dry 

 weather or to become swampy after a heavy rainfall. A deep 

 soil has a great effect upon the height growth of trees, so 

 much so that a fair estimate of the depth of the soil may be 

 made by looking -upwards to see the height of the trees. 



The different species are content with varying depths of 

 soil according to the nature of their root system ; the spruce, 

 for instance, having roots which spread out laterally and 

 remain close to the surface, will grow to a good size on 



