n the earth, and the greater
the number that engage in manufacturing and trade, the more important
becomes the care and cultivation of the soil. If we do not take the best
of care of the soil, there may come a time when there will not be food
enough for us all.

CHAPTER NINE

THINGS OF WHICH SOIL IS MADE

Let us take a spadeful of soft, dark earth from the garden and see if we
can find of what it is made.

We will first put the earth in a dish of water and stir it thoroughly.
We notice that the water at once becomes muddy and that little particles
of a dark substance rise to the surface. These particles appear to be
pieces of stems and leaves.

This crumbling vegetation is _peat_, a substance which fills many swamps
and, when cut into blocks and dried, is used for fuel. When scattered
through the earth peat has a very different use. As the leaves and stems
of plants die and slowly mingle with the earth, they give it the dark
color, which usually extends down for two or three feet. As this
vegetation changes, or decays, as we usually say, it furnishes a number
of substances which supply food to the roots of growing plants. One of
the most important of these is _nitrogen_, an invisible gas.

The decaying vegetation which we find mixed with the soil has other
uses. It holds water and so helps to keep the soil moist. It makes the
soil loose and more easy to cultivate. It absorbs heat from the sun and
so helps to warm the soil. This vegetable matter, when it is completely
decayed, we call _humus_. Soils that are rich in humus are usually very
fertile.

We will now turn the muddy water into another dish, pour more clear
water upon the material that remains in the bottom of the dish, and wash
it again, repeating the work until the water is no longer muddied. We
will set aside the dish containing the muddy water and examine what
remains in the bottom of the dish that once contained the earth or soil.
This is mostly sand, but with it are rough fragments of rock which can
be crumbled in the hand. The greater number of the little sand grains
are _quartz_. Some of them are clear like glass, others are reddish. In
this quartz sand are a few grains of _iron_ which the magnet picks out,
and a number of scales of yellow _mica_.

After standing a few hours the muddy water has become clear, and a
deposit of a yellowish substance has collected in the bottom of the
dish. We will carefully pour off the water and examine wh