14 PRACTICAL FORESTRY 
ence, but the ends attained are practically the same. 
The farmer carefully tills the soil to pulverize and 
loosen it, and to prevent the growth of weeds. A 
layer of well-tilled soil on the surface and a mulch 
or layer of humus have practically the same effects. 
Instead of tillmg the soil, the forester increases its 
moisture content and porosity by encouraging the 
formation of humus. The roots penetrate to deep 
layers of the soil for food material, much of which 
is deposited on the forest floor in the leaves, twigs, 
and wood. This material rots, and in rotting forms 
food for the roots. In this way the fertility of the 
forest soil is maintained, and, in fact, increased with- 
out the application of manure. Weeds are kept in 
check by regulating the quantity of light which filters 
through the forest canopy, and although there are 
weeds which endure shade, these are usually not 
aggressive. The growth of trees which have been 
planted in rows may be increased surprisingly by 
cultivation and application of manure, as with corn, 
especially in youth. In fact, in the tight soil of the 
Western plains cultivation is necessary until the 
trees reach such a size that their canopy prevents the 
growth of grass and protects the soil. By the 
“plains” is here meant the territory east of the 
one-hundredth meridian. West of this line, extend- 
ing to the Rocky Mountains, are the “ prairies.” 
