218 



oxygen and hydrogen in the proportion of water, it is no wonder that the humidity of the 

 soil is of so much importance for timber growth, and its supply may be held as the chief 

 ■office of the same. 



Another factor of the soil has been considered in the light of a plant nourisher, and 

 lihrough its chemical influence favourable to tree growth. This is the humus, which forms 

 the covering of all good forest soils, and is produced by the decay of the yearly fallen 

 foliage, twigs, etc., and other decaying vegetable matter, and consists of the combination 

 of neutral salts. The acids which are formed in some humus soils, according to Liebig, 

 are not components of a fertile humus, but belong to that of peaty and marshy soils, 

 which are not favourable to tree growth. Carbon, hydrogen, and oxygen are the main 

 •components of humus. It has been asserted that the humus ought to be considered as 

 furnishing the supply of carbon, which forms the largest part in the composition of the 

 woody fibre. For any one who has seen the forests of large extent along the dunes of 

 southern France and the sea sand of the north German plain, lacking all traces of humus, 

 nay, containing so little carbon that after heating it will not leave a trace of black colour- 

 ing, it needs not to cite Liebig's proof of the insufficiency of the humus or any part of the 

 soil, to provide the amount of carbon necessary for the building up of the tree and a for- 

 est. Besides, who could reasonably accept, as logic would compel us, the creation of de- 

 cayed organic matter previous, and as a condition of following plant life. 



Yet that there is a chemical influence of the humus on forest growth cannot be denied. 

 Not only does the decaying vegetable matter develop a considerable amount of ammonia, 

 which imparted to the atmosphere enriches it with the needed nitrogen, but also of carbonic 

 acid, which contributes largely to the disintegration of the rocks and increases the solubility 

 of the carbonate and phosphate of lime. This influence will be readily admitted as im- 

 portant, when we remember that in ten thousand parts of pure water only one part of 

 •carbonate of lime is soluble, whilst in the same quantity of water acidulated with carbonic 

 •acid, ten parts of that salt will dissolve. 



But the greatest significance of the humus lies in its physical influence, which is the 

 more important, where the other factors of " soilbonity, " depth, looseness, humidity are 

 lacking. A considerable layer of humus increases depth ; as a bad conductor of heat it 

 counteracts the drying effects of the sun, which, added to its capacity of absorbing easily 

 and retaining long the meteoric precipitations, makes it a very desirable covering of the 

 soil. The humus being of medium looseness tends to diminish the extremes of the 

 physical properties of the soil. 



We may here sum up the influence of the soil on forest growth by stating that its 

 •chemical composition is only of minor importance, almost all soils furnishing sufficient 

 inorganic bases of the description which is needed by forest growth ; that its main influence 

 consists in its physical properties, represented by its depth, looseness and, depending on 

 these, the capacity of absorbing and retaining moisture, which properties may be increased 

 or even compensated for by a sufficient layer of humus. The existence of these properties 

 in their highest perfection in due proportion are conducive to the prosperity of any species, 

 yet the necessity of their existence is a relative one with regard to the different species. 



Seeing then that the soil, though a contributor, does not form the bulk of those 

 elements which form the fibre of the tree, we must look for another source of supply. 

 By a simple mathematical calculation we find that the 1846 kilo of carbon, which are repre- 

 sented by the yearly accretion of one hectare of pine forest cannot be supplied by the soil. 

 There is then only the atmosphere left as a source of this component as well as of the 

 small quantity of nitrogen required. 



We need not go into any proof that the quantity of carbon present in the atmospheric 

 •uir is sufficient to grow wood on the entire area of our globe, nor need we apprehend any 

 •danger from overproduction of carbonic acid to the detriment of vegetation. In short 

 we may conclude, that on the whole we need not apprehend any danger of exhaustion of 

 the sources of food, which our forests require, such as we see possible in our coal mines. 

 But it is incumbent on us to utilize this inexhaustible source of plant food by providing 

 the proper means for its conversion into marketable values, that is, by promoting and 

 •directing the growths of forests. 



Yet tree-growth as well as all other vegetation is confined in locality. Even the 



