ganism so small that we cannot see it with the unaided eye. Many
thousands of these organisms are contained in a bit of earth such as you
could take up on the point of a small knife blade. We have named them
_bacteria_.

Plants cannot make use of most of the substances in the soil without the
aid of these organisms. The bacteria live upon the materials of the
soil and change them into such form that plants can digest them.

Soil may be supplied with all kinds of plant food in just the right
amount and yet, if it is packed hard and is not watered, no living thing
can take root in it and grow. Plants drink their food and so we must
supply water. They also require oxygen, as do other living things. For
this reason we must leave the soil loose, so that the air can enter it
and the roots get the oxygen which it contains.

Thus we learn how wonderfully the soil is made. We learn that it
contains many things required by plants. In order that the plants may be
thrifty, there must be enough but not too much of these different
things.

CHAPTER TEN

HOW THE SOIL IS MADE

The substances which we found in the soil teach us that it was formed
from the rocks. If we could take the sand, clay, potash, soda, lime, and
iron that we found in the soil and put them together as Nature knows how
to do, we should have rock again.

But if we should take a piece of rock and crush it to a fine sand, that
would not be soil, because soil cannot be made in that way. It takes
Nature many, many years, as the rocks slowly crumble and decay, to
change the materials of which they are composed into true soil with its
swarms of bacteria and its plant food.

If we should dig down through the soft earth under our feet, we would at
last come to solid rock. This is the rough and jagged crust of the earth
on which rests the carpet of soil. In the mountains where the slopes are
steep the rocks stick up through the soil. The outer parts of this solid
rock are, however, always crumbling. Little particles, as soon as they
become loosened, either fall by their own weight or are washed away.
Some of the rock fragments collect upon the gentler slopes and finally
turn to soil. This soil is not rich and it dries out quickly, because it
is shallow. The soil in the valleys, as we have already learned from the
muddy rivulet, is deep and rich.

Nature is slowly spreading her mantle of soil over the earth. In some
parts of the earth one can travel for hundr