Plants As Soil Builders

Broken rock alone, however, does not make a fertile soil, as the farmer defines fertility. There are plants that thrive on bare rock, but the plants that are grown as farm crops are of a higher order and cannot rough it like this. A fertile soil — one that will grow large crops of the higher plants, either wild or cultivated — must contain a considerable amount of humus, which is chiefly decayed vegetation. A soil made of rock alone may contain all the mineral plant food that farm crops need, but it is apt to lack nitrogen and has not the right texture. The Evolution of a Soil. — Nothing in nature is more interesting than the gradual evolution of a fertile soil from a barren rock, and nothing is more significant of the illimitable wisdom of the Creator. The history of soil building reads something like this: In the beginning is a lofty cliff, mute witness of the eruptions and disturbances through which the earth passed in cooling. It is bare and desolate. No living thing finds nourishment upon it. For centuries the storms beat against it; ice, rain and sudden changes in temperature pry off great boulders, which crash into the valleys. In the course of time there come to be upon these boulders, and upon the rocks and stones split off from them, lichens and other humble plants that are able to send their minute root structures into the crevices and live upon the slight substances that are formed on the surface by weathering, together with what they can get from the air.

These lichens are very acid and are able to etch the rocks. They die and decay, leaving the beginning of a fertile soil in the crevices and upon the ledges. The growth of higher plants is thus made possible; perhaps the mosses gain a foothold. These in turn elaborate more of the rock for their own use and in turn die, enriching the soil with themselves. Now there is a pocket of soil upon the ledge which may be able to support such humble plants as ferns or saxifrage. Thus the process goes on from decade to decade and from century to century, the lower plants being succeeded by larger and more highly organised plants, as the rocks are made finer by weathering and are enriched by the decay of the plants that they nourish. Finally the soil can support mulleins, honeysuckles, or fir trees. Many years later it may be able to support a crop of corn, timothy, or apples. A fertile farm soil is the product of many agencies working through thousands of years. How Plants are Making Soil To-day. — Plants are helping to make fertile soil to-day as they have for centuries. Each year the forest floor receives a fresh carpet of leaves, and the older generations of trees fall to the ground and slowly pass into mold. Each year the grass in the meadows and the weeds by the roadside add their substance to the soil from which they have sprung, thereby enabling it to nurture other and lustier plants in succeeding years. Lichens spread their thin substance over rocks, and mosses take up the battle where the lichens leave off, just as of old.

Swamp lands and meadows are the most conspicuous examples of soils built mostly of plants. Lakes, ponds, streams and swamps are being filled in, not only with soil washed from surrounding higher land, but also with plants. The little pond that I skated upon as a boy is reduced to a mere mud hole now; the lilies, sedges, reeds, cattails and other aquatic and semi-aquatic plants have encroached upon it from the edges year by year, until now hay is cut where I used to catch bullheads. Most of the rich valleys and meadows of northern United States were once water-courses or glacial lakes. The weedy water's edge of today may be a sphagnum bog a century hence and a cabbage field in another hundred years. The mangrove swamp of this century, reaching trunk like roots into the sea, may be the tilled land of a future generation. Stems and Roots Split Rocks. — Plants also aid in soil building, to a considerable extent, by the pressure they exert upon rocks. The roots of trees often follow the crevices of rocks to a considerable depth, and by the force of growth help to widen them.

Even on top of the ground one may see many examples of rocks that have been rent by the growth of trees. Among greenhouse plants it is quite common to find pots that have been split apart by the growth of roots. But in many of the cases where rocks are split apart, and a tree is growing in the crevice, the rock was first split open by weathering and the tree then widened the crevice. The acids secreted by the roots of plants dissolve a small portion of plant food from the rocks that the roots embrace. Rocks that have been etched by root acids may be found in almost any tree-covered ledge. In these various ways plants are contributing to the up building of our agricultural soils. The peculiar . value of certain plants as soil binders must not be forgotten. One of the most efficient and certainly the most notorious of soil binders is "quack-grass," and its counterparts variously known as "Johnson-grass," "witch grass," "couch-grass," and other aliases. The evil reputation of this grass is due to the fact that it is extremely difficult to kill, because the long underground stems may root at any point. The smaller the pieces into which the roots are chopped by the irate husbandman, the more widely and thoroughly is the pest scattered. This is just the reason why "quack" is such an excellent soil binder; the tough, white root -stalks thread the soil in every direction, soon making a network of fibres, which prevent light soils from washing badly. Steep banks or slopes are sometimes held by establishing quack grass upon them ; the underground stems are chopped into small pieces and these are sown thickly. Several other grasses, notably Bermuda grass, are particularly serviceable in such cases.

In some sections, notably in Oregon, Eastern Massachusetts and Western Michigan, drifting sands are held by planting them with sedges or "beach grass." In Holland the dikes are planted with rushes to bind the soil. Willows and osiers planted on the banks of turbulent streams are effectual in preventing them from eating away their banks. Morning-glories and related plants are called bind-weeds, because the vines root at the joints and hold the soil tenaciously. A few horsetails planted in a wet place soon make a dense mat of roots which grasp the soil so firmly that it cannot wash away. These are only a few examples of plants that are particularly valuable for this purpose. All plants are soil binders to some extent, as well as soil makers ; they not only enrich it with their herbage, but also hold it with their roots and, if they lie close to the ground, with their herbage also. In hilly sections some plants may be used to great advantage in checking erosion.