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 hundreds of miles and see no rocks. One might think that in time Nature’s work would be finished. But before the mountains in one place have crumbled and been washed away, she raises up new ones somewhere else so that the tearing-down work begins again.

Let us, in imagination, sit down by the side of a rock, prepared to stay there many years, that we may learn just how Nature makes the soil. It will be a long, long time before we can see any change in the rock. Each bright day the sun warms the cold rock and makes it expand a very little. At night the rock grows cold and shrinks. In this way minute crevices are finally formed between the grains of the different minerals that make up the rock.

When it rains, water creeps into the tiny crevices. The water carries with it a little carbonic acid which the raindrops took from the air. This substance aids in dissolving some of the rock materials. If the nights are very cold, the water in the crevices freezes and opens them a little wider, for ice, as you know, takes up a little more room than it did when it was water.

Plants also aid in breaking the rock. Often seeds are dropped by the wind, and the rootlets of some of these seeds, when they sprout, may find a crevice large enough and deep enough for them to push their way into the rock. In these crevices they find a little food and slowly grow larger and stronger. By and by some of the roots are strong enough to push apart large pieces of rock.

If the rock which we are studying is granite, we shall after a time be able to pick out the different minerals of which it is composed. We can tell the grains of quartz, because they look glassy and remain very hard. Other grains, which we call feldspar, soften and change into clay, which makes the water muddy as it runs over the rocks. We see also little scales of yellow mica, sometimes called “fool’s gold,” and a few grains of iron. There are tiny quantities of other things which we shall not be able to see, for the rainwater dissolves them and carries them away.

As the rock slowly crumbles to sand and clay, the bacteria begin to make their home in it. Hardy plants, that are not particular about what they grow in, get a foothold, and when they die their stems and leaves decay and mix with the rock particles until at last this material begins to look like soil. It has become dark in color and rich in plant food. Then, many other plants that require a good soil take root there. The rock has at last completely disappeared under the layer of soil and its carpet of vegetation.

Suppose, now, that we dig down and find how deep the soil is and what lies below it. When we have gone down two feet the soil is harder and of a lighter color, for there are fewer plant remains in it. This poorer, lighter-colored soil we call subsoil. If we dig a little deeper, we shall find pieces of rock in the subsoil. Below these we come to soft, crumbling rock and last of all the solid rock.

The soil that is found resting on the rocks from which it was formed is known as residual soil. This name is given to such soil, because it is what remains after long years of rock decay during which the rains have washed away a part of the finer material.

What has become of the soft earth that the water washed away? The muddy rivulet has already told us its interesting story. We have learned that a part of this earth (or soil) is borne to the distant ocean. There it is forever lost unless the sea bottom should some day become dry land. Stranger things than that have happened on this ancient earth of ours. The part of the soil which the water carried away to form the rich valley lands and deltas is known as alluvial soil.

Long ago the northern part of our country was covered with a sheet of ice. This ice crept slowly southward, and as it moved along it tore off all the soil and loose rocks on the surface of the earth over which it passed. When it melted it left them spread roughly over the country. Such material forms glacial soil. It is often deep but not very rich.

There is another kind of soil, formed by the wind. If you have ever been in a dust storm you have seen the fine, powdery substance that settles over everything and creeps into the smallest cracks. In some countries where there are strong winds and not much rain there is little vegetation on the surface to hold the soil. Year after year the winds pick up particles of the dusty soil, whirl them high in the air, and do not let them down again until they have been carried many miles. In some far-off land where the winds go down the dust particles settle again to the earth. After a long, long time, enough dust collects to form a thick layer of the richest soil. This is called aeolian soil, from the word AEolus, meaning the “wind.”

There is one more kind of soil which we ought to know about; that is peat soil. It is found in marshy or swampy lowlands and is formed largely of plant remains. When lands with such soil are drained, they prove very rich.