RELATION OF TREES TO THE SOIL. 237 



the subsoil certain nutritive salts tliat enter into the sub- 

 stance of all parts of the tree. This is restored to the sur- 

 face by every tree or branch that falls and moulders upon 

 it, and the leaves increase its bulk still more by their 

 annual decay. According to Vaupell, " the carbonic acid 

 given out by decaying leaves, when taken -up by water, 

 serves to dissolve the mineral constituents of the soil, and 

 it is particularly active in disintegrating feldspar "and the 

 clay derived from its decomposition." These facts ex- 

 plain why the surface soil in a forest may constantly 

 increase in bulk, without communication with any for- 

 eign sources of supply. 



If a wood be situated in a valley or on a level plain, it 

 retains all these substances for its own benefit. But if it 

 stand upon a declivity, a part of the dibris will be washed 

 down by iloods into the fields below. Hence, by pre- 

 serving a growth of wood upon aU. barren slopes and 

 elevations, the farmer derives benefit from it, both as a 

 fertilizer and as a source of irrigation to the lower part 

 of the slopes or the base of the hill. For some days after 

 a rain, thousands of little rills are constantly oozing from 

 the spongy bed of the wood, that cannot immediately be- 

 come dry like an open surface. Hills, when either very 

 barren or steep, are unprofitable alike for tillage or pas- 

 ture. They require more manure than other grounds, and 

 more labor in its distribution. Hence, if divested of wood, 

 as I have often repeated, they are almost useless ; while, 

 if densely wooded, they fertilize and irrigate the lands be- 

 low, protect them from winds, and afford a certain annual 

 amount of fuel. 



When I am journeying through the country and behold 

 the rocky hills, sometimes for miles in extent, entirely 

 bare of trees, and affording too little sustenance to sup- 

 port even a crop of whortleberry-bushes, where an acre 

 would hardly pasture a single sheep, I am informed by 



