676 PROTECTION AGAINST DISEASES. 



a. Unsuitable Soils. 



The kinds of soil which induce red rot in the roots of trees, 

 and chiefly in their deeper-lying roots, are generally peaty 

 humus, calcareous soils, soils containing pans or impenetrable 

 substrata of ochrous iron ore, lignite, clay or loam, also very 

 fine sand, not infrequently found in the lias formations in 

 Germany. Such subsoils interfere with aeration of the surface 

 soil, the oxygen so necessary for the roots of trees being unable 

 to reach them in sufficient quantity. This is due to the fact 

 that the air in spils too compact or waterlogged is gradually 

 deprived of its oxygen by the roots of the plants -growing on 

 it, and by the decomposition of the litter ; this loss of oxygen 

 is not sufficiently replaced by the admission of fresh air to the 

 soil. The denser the wood, the faster the evil progresses ; fungi 

 also accelerate the disease. 



In the North-west German loamy heather tract, where pan 

 is very prevalent, more than 75 per cent, of the Scots pine 

 woods suffer terribly from red rot. Spruce, however, thrives 

 there. 



This form of red rot is more prevalent with larch and some- 

 times with Scots pine than with spruce, for the roots of the 

 larch as well as those of Scots pine penetrate more deeply 

 into the soil, and therefore rot more readily than those of 

 spruce that spread in all directions in the upper layers of 

 the soil. 



Scots pine, however, when grown on shallow soils, can pro- 

 duce superficial roots like those of spruce, whilst experience in 

 Windsor Forest shows that larch growing on a gravelly soil 

 above a pan always gets red rot, and this is confirmed by 

 A. D. Webster,* who states that larch always gets red rot 

 when grown on gravelly soils. 



b. External Injuries. 



Trees are frequently wounded during the felling, conversion, 

 and transport of timber. Wounds also arise owing to forest 

 pasture, game, mice, insects, from pruning green branches, or 

 from meteoric influences, frost-crack, bark-scorching, windbreak 



* " Practical Forestry." William Rider & Son, London, 2nd edition, 1895. 



