34 TREE GROWTH IN RELATION TO 



many ways. It acts as a manure, returning to the soil much 

 of the mineral matter taken out of it, and, in addition, provides 

 a large amount of vegetable matter which on decomposing 

 forms carbon dioxide gas, ammonia, and humic acid, and thus 

 the fertility of the soil is improved. The acid is dissolved in 

 rain-water, and acts on the minerals in the soil, making them 

 soluble and therefore useful to the trees. Humus also acts 

 like a sponge, retaining moisture, which gradually finds its 

 way into the soil and is available for the roots of the trees. 

 It also improves the degree of porosity, making a stiff soil 

 looser and a loose soil heavier. It adds in the course of years 

 a good deal to the depth. Finally it acts like a blanket and 

 keeps the soil warmer in cold weather and cooler in hot 

 weather than a bare soil. 



The humus of all species, however, is not equally good. 

 During youth, while in the thicket stage, all species form 

 a good humus and improve the soil, but when the woods 

 begin to open out, between the twentieth and thirtieth years, 

 certain species have only a light cover and their leaves do not 

 form sufficient humus. As a general rule shade-bearing species 

 are soil improvers, while under a cover of light-demanding 

 trees the soil gradually deteriorates, owing to the fact that 

 the cover is broken and sun and air get into the ground and 

 the humus disappears. 



Beech is the greatest soil improver, because it has a dense 

 cover and throws a heavy crop of leaves which decay slowly 

 and form the best kind of humus. Hornbeam, silver fir, 

 spruce, and Douglas fir are also good soil improvers ; chest- 

 nut, sycamore, maple, and hazel are moderately good ; while 

 oak, ash, larch, birch, poplars, willows, and elm are all species 

 under which, when grown pure, the soil will gradually 

 deteriorate. Of the pines, Weymouth pine gives the best 

 humus, whilst Scotch, Austrian, and Corsican pines improve 

 the soil for some thirty or forty years ; after that age they 

 usually open out, and the soil begins to deteriorate. 



