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 what 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 organism 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.