On the sandy plains, where the winds are apt to blow the sand, shifting
it hither and thither, a forest belt to the windward is the only means
to keep the farm protected.
In the mountain and hill country the farms are apt to suffer from
heavy rains washing away the soil. Where the tops and slopes are
bared of their forest cover, the litter of the forest floor burnt up,
the soil trampled and compacted by cattle and by the patter of the
raindrops, the water can not penetrate the soil readily, but is earned
off superficially, especially when the soil is of, day and naturally
compact. As a result the waters, rushing over the surface down the
hill, run together in rivulets and streams and acquire such a force as
to be able to move loose particles and even stones; the ground becomes
furrowed with gullies and runs; the fertile soil is washed away; the
fields below are covered with silt; the roads are damaged; the water
courses tear their banks, and later run dry because the waters that
should feed them by subterranean channels have been carried away in the
flood.
The forest cover on the hilltops and steep hillsides which are not fit
for cultivation prevents this erosive action of the waters by the same
influence by which it increases available water supplies. The important
effects of a forest cover, then, are retention of larger quantities of
water and carrying them off under ground and giving them up gradually,
thus extending the time of their usefulness and preventing their
destructive action.
In order to be thoroughly effective, the forest growth must be
dense, and, especially, the forest floor must not be robbed of its
accumulations of foliage, surface mulch and litter, or its underbrush
by fire, nor must it be compacted by the trampling of cattle.
On the gentler slopes, which are devoted to cultivation, methods of
underdraining, such as horizontal ditches partly filled with stones and
covered with soil, terracing, and contour plowing, deep cultivation,
sodding, and proper rotation of crops, must be employed to prevent
damage from surface waters.