at remains.
This fine soft mud we call _clay_. As it dries and becomes hard it
shrinks and cracks, and thus breaks up into little pieces. Clay forms a
greater or lesser part of all soil. Clay soil is very sticky when it is
wet, as you will be sure to remember if you have tried to walk over it.
When soil is formed largely of clay we speak of it as a _heavy soil_. In
the West it is called _adobe_ and is sometimes used in making houses.
When adobe soil dries, great cracks form in it. These cracks are
sometimes large enough for small animals to fall into. When there is a
large amount of sand, we speak of the soil as _light_ or _sandy_. A soil
composed of sand and clay is sometimes called _loam_. If it is nearly
all clay it is a _clay loam_; if there is much sand it is a _sandy
loam_.

Soils found in low, swampy places are sometimes formed almost wholly of
decaying vegetable matter. Such soils are known as _peat soils_. They
are usually very fertile.

We have now learned about three things that the soil contains that are
bulky and easy to discover: decaying vegetation, sand, and clay. These
are, however, far from being all that compose the soil. There are still
many other things, some of which are invisible to the unaided eye and
difficult to find.

We will next take the clear water that remained after the mud settled.
We will pour it into a dish, place the dish over a fire, and let the
water boil slowly until it has all evaporated. There will remain in the
bottom of the dish a thin white coating. Moisten this with a drop of
vinegar or other weak acid and it will disappear in a mass of little
bubbles. Such behavior teaches us that the white substance is probably a
mixture of _lime_ and _soda_. Besides these there are tiny particles of
_potash_ and _phosphorus_, which we cannot distinguish by the means we
have used.

Some soils contain a great deal of lime, and because they have been
formed from limestone, are called _limestone soils_. Plants need a
little soda, but when there is much in the soil it will kill them. Soils
rich in soda are known as _alkali soils_. They were formed in the bottom
of lakes the waters of which contained soda. Salt is another harmful
thing found in the soil. You can sometimes see faint whitish deposits of
soda and other salts on the soil in flower pots.

There is one more thing that the soil contains that we must not forget,
for it is one of the most important of them all. This is a living
or