that
comes from the soil they furnish our fuel, our ships, our cars, our
furniture, and countless other things. Our clothing is made from the
cotton or flax which grows from the soil, the wool from the sheep that
feed on the pastures, or from the silk-worms that feed on leaves.

So it is to the earth that we turn for every need, and Mother Nature
supplies it. But it is of the soil as it gives us our food supply that
we shall speak in this chapter, and we must first learn the nature of
the soil, and the process of its making, in order to understand the need
of extraordinary care in its management, and also how to use it so that
it will not wear out, or become exhausted, but will increase in value
for years and even centuries, as it will if properly cared for.

The earth's surface is constantly being renewed. Although the great
formative movements occurred ages ago, yet earthquakes, volcanic action,
wind, frost and water are working continual changes. Hills and mountains
have been thrown up, and nature has gone to work at once to shave down
the mountains and fill up the valleys. The whole earth is as carefully
adjusted and balanced as the wheels of a watch, but these adjustments
take place in long periods of time. In a lifetime, or even a century,
the changes of the earth's surface seem few and small, but they are none
the less sure.

The soil or humus, that is, the upper layer of the earth's crust which
is used in farming, has an average depth of about four feet, and has
been formed by decay, first and most important of all by rock decay
which is constantly going on under the surface of the earth and in
exposed places everywhere, and is caused by the action of air and water.
This process is very slow. In places where the rock is already partly
ground up, or, disintegrated, as we sometimes say, it is more rapid, but
the average growth of the soil from beneath by rock decay is scarcely
more than a foot in ten thousand years.

Some waste of this upper layer is constantly taking place from above,
caused by wind and floods, and considerable additions are made to it by
the decay of animal and vegetable matter, but in order to keep the soil
at its best, the average soil waste should not amount to more than an
inch every thousand years.

When this humus is once exhausted there is no way to repair the damage
but to wait for the slow rock-decay. In the river valleys there is no
immediate danger of exhausting the entire body o