RELATION OF TREES TO THE SOIL. 237 



the subsoil certain nutritive salts that 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 debris will be washed 

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

 serving a growth of wood upon all 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 



