HOW VEGETATION HOLDS THE SOIL

A walk up the mountains on a rainy day is not a pleasant one. There are mud and water under our feet, and overhead are the dripping branches which, if touched, send down a shower of drops. But if we keep our eyes open we shall learn something which will be of great value to us. We shall learn how it is that Nature holds the soil on the slopes the wonderful soil which it takes her so long a time to make and which is the source of all our wealth.

Our way up the mountains is by a winding road. We first pass the foothills upon which there are scattered oaks. The rain is steadily pouring down and rivulets loaded with mud are eating little gullies all over the slopes. Along the roadside, where they have united, the rivulets form a torrent which is making a deep ditch that threatens to render the road impassable.

These slopes were once covered with grass and the rivulets ran down them without doing any harm. But so many sheep were pastured here that the grass was killed. The roots, which once formed a thick protecting sod, are now decaying. How quickly the rivulets have taken advantage of the unprotected slopes!

The road leads still upward until it brings us to where there were once pine forests. The lumbermen cut off all the trees, and then fire came and burned the decaying vegetation which once lay spread over the ground. Now all that remains is bare earth and blackened stumps.

What are the raindrops doing here? They gather in rivulets just as they do on the once grassy hillside; but because there are so many roots still remaining in the ground they have not done much work. They are not loitering, however, and by and by, when the roots have rotted, they will seize their chance and begin tearing away the soil from the mountain side.

But this is not the end of the road. Farther up we come to the primeval forests, where the giant trees stand just as they did before men came. Here we can see how the slopes are protected, for in making the road the workmen cut deep into the hillside. They first removed a layer of pine needles and decaying branches. Then they cut through a layer of soil about two feet thick which was completely filled with little roots of trees and bushes. Below this they came to the soft subsoil, which contained only a few roots, and at the bottom they reached the solid rock.

The layer of roots and soil at the top of the bank, you can see from the picture, now overhangs the road, because the raindrops which beat against the bank have washed away all that they could reach of the unprotected earth at the bottom. How plainly we can see the network of roots. What a hard task it must be for the water to get at the soil in which these roots are growing.

We will now leave the road and, although it is still raining hard, we will walk a distance through the forest and see if there is anything more that we can learn. We are soon in the deep woods where, perhaps, no one has ever been before. Around us are trees of all ages and sizes, from little seedlings to great giants six feet through. Among them are the crumbling stumps of trees long dead. Their trunks lie on the ground, and many are so soft and rotten that we can kick them to pieces with our feet.

As we walk our feet never touch the real earth. It is always on the soft, yielding leaves and crumbling branches that we step. These leaves and branches form a thick layer completely hiding the soil. But the strangest thing is that, although the rain is still falling, we can discover no rivulets. What, then, becomes of the water? The soft, decaying vegetation on which we are walking and the rotting stumps and logs act like a great sponge. As long as this sponge can take up the falling drops, none have a chance to run away. If it rains a very long time and the sponge becomes saturated, the drops that creep away and finally unite in rivulets in the hollows do no harm to the soil, for they cannot get at it.

Long after the storm has passed, the earth underneath the trees remains wet, while the ground out in the open has become dry. A part of the water held by the decaying vegetation evaporates. Another part creeps down through the earth to the crevices in the rocks and feeds the springs.

Let us now put aside our storm clothes and journey, in imagination, far away to where it seldom rains to that land which we call the desert. Here the bare rocks of the mountain slopes are burned brown by the hot sun. Here there is little soil and only a few little bushes that somehow manage to live. Why does not the soil gather over the rocks as it does in other places? The rocks are surely crumbling, for we can crush some of the pieces in our hands.

Once in a long time it rains in this desert. Then the drops descend furiously. The water gathers in rivulets and these turn to torrents which sweep down the slopes. They carry away the particles of sand and clay which would in time, if there were plant roots to hold them, turn to soil.

The winds also help keep the desert rocks bare and free of soil. Have you ever been in a dust storm or have you read of caravans caught in such storms in the Sahara Desert? The fierce wind picks up the particles of sand and clay from the bare earth and sweeps them along as it does the snow in winter, or it whirls them in clouds high in the air. The dust clouds are often so dense that they hide the sun and all landmarks by which the traveler can guide his way. But have any of us ever seen the winds pick up much dust from the green fields where the vegetation protects the surface?

If we turn now to a very wet country, such as that upon our northwest coast, where often nearly eight feet of rain falls in a year, we shall find the vegetation so dense that it hides both soil and rocks. Here water can do little in wearing away the soil, even upon the steepest slopes, while the wind cannot get a peep at the earth.

Does it not seem strange that where little rain falls the earth washes a great deal faster than where it rains very heavily? The reason is that the more it rains the more dense becomes the carpet of vegetation. If we wish to preserve the soil, we must preserve the natural growth on the hillsides.