The organic agencies are animals and plants, both living and after death. In some respects these agencies tend to counteract the destructiveness of others, and the protective effects may be taken up first.
These have already been considered in part, in connection with the processes of weathering (see p. 100). The protective effects of organisms are almost entirely those of plants, since animals, on land at least, are not sufficiently abundant to be of any importance in this connection. A thick covering of vegetation, especially the elastic, matted roots of grassy turf, protects the soil against the mechanical wash of rain. How complete this protection often is, may be seen in the different effects produced by a heavy fall of rain upon a grass field and on the adjoining ploughed lands, or even on the roads. The roads may be so washed out as to be impassable, while the grass fields have not suffered at all. In certain of the western bad lands, the efficient protection given by grass is very well shown; where the grass has established itself thickly, the country is gently rolling, but where it is absent, the wild and broken bad lands are developed.
Forests also are very important conservers of the soil, especially on mountain sides and other steep slopes. The removal of forests only too often is followed by calamitous results.
Vegetation, especially grass, protects loose, light soils from the wind, and often this is the only means by which sand dunes can be held in place and prevented from overwhelming valuable lands. Even the banks of rivers and the seacoast may be efficiently protected by plants. Dense masses of seaweed growing on the rocks form an elastic buffer against the surf, and along low-lying tropical coasts the mangrove trees, with their interlacing aerial roots, so break the force of the waves that they cannot wash away even fine mud. The only protection afforded by animals that requires mention is in the case of coral reefs, which, thrown up along or parallel to the coast, shield it from the heaviest surf.
Fig. 81. - Erosion following removal of forest; Great Smoky Mts., Term.
(U.S. Bureau of Forestry).
The Destructive Effects of the organic agencies are decidedly subordinate to those of the other classes which have so far been considered, but they are not without importance. The products of vegetable decomposition in bogs and in beds of clay, muds on the sea-bottom, etc., are efficient means of chemical change, and observations show that the decay of animals in the deep sea is an agent of no mean importance in promoting the chemical changes which there take place. But even living animals and plants do an important work in disintegrating rocks, that should not be overlooked. Bacteria play a considerable, but not yet fully known, part in the surface decomposition of rocks and soils. Certain of these microscopic plants have the power of fixing the atmospheric nitrogen and converting it into nitric acid, while others are the indispensable agents of organic decomposition. Seeds germinating in the crevices of rocks, or the roots of trees which invade such crevices from above, wedge the rocks apart with the same irresistible power as is displayed by frost, and often large areas of rock are thus most, effectively broken up.
The roots of living plants also secrete an acid, which dissolves out some of the soluble constituents of rock, thus adding a chemical activity to the wedge-like mechanical effects of growth.
Fig. 82. - Soil destruction due to removal of forest; Mitchell Co., N.C.
(U.S. Bureau of Forestry).
Many marine animals bore into rocks, even the .hardest, and cause them to crumble, and on the land great numbers of animals continually bore and tunnel through the soil, allowing a freer access of air and water. In the tropics the soil is fairly alive with the multitude of burrowers. Earthworms are among the most important agents in work of this kind, and the last of Mr. Darwin's books was a most interesting one upon the geological work of worms. The worms swallow quantities of earth, for the sake of the organic matter which it contains, and grind it exceedingly fine in their muscular gizzards. This ground-up soil is always deposited on the surface, in the form of the coiled "worm-castings," so abundant in grassy places. Worms are thus continually undermining the soil, bringing up material from below and depositing it on the surface, while, by the collapse of the old burrows, the first surface gradually sinks. In England the material thus yearly brought to the surface varies from seven to eighteen tons per acre, which means an average annual addition of one-tenth to one-sixth of an inch. By this means the surface of the ground is constantly changed, and substances spread over the ground in the course of years make their way down into it, forming well-defined layers beneath the surface.
In the tropics ants and termites (so-called white ants) are even more active than worms in tunnelling the soil, and in many semi-arid plains burrowing mammals in incredible multitudes are continually working over the soil to great depths, as in the prairie-dog villages of our western plains. The occasional heavy rains thus penetrate to depths which could not otherwise be reached.
The surface of the land is everywhere attacked by the universally present atmosphere at a rate which differs much in different regions, depending upon climate, elevation above sea-level, and the resistant power of the rocks. The rain chemically decomposes the rocks, converting them into soil, and mechanically washing this soil to lower levels and into the streams. Frost shatters the rocks into smaller and smaller fragments. In arid regions the extreme changes of temperature break up the- rocks much as does the expansive force of freezing water, while the wind transports immense volumes of sand and dust, which cut and carve and wear away the exposed rocks. Underground waters, especially when heated, do an important work of solution and decomposition, and, under favourable circumstances, cause the dislodgment of great masses of earth and rock in land-slips and rock-slides. Rivers excavate valleys and serve as the great agents of transportation, bearing the waste of the land to the sea, and glaciers do similar work in a highly characteristic manner. The sea cuts into its coasts by the action of waves, deepening its bed in shallow places by tidal currents, and in the case of a slowly sinking land may plane down great areas to a flat, gently sloping surface.
Animals and plants add an important quota to the general work of destruction.
The annual waste of the land at the present time is estimated at 20 cubic kilometers (Penck), and, in past times, an incalculably great amount of material has been removed from the land. The Appalachian Mountain system has thus lost thicknesses of rock which vary in different regions from 8000 to 20,000 feet, and it is altogether probable that the average waste of all the continents amounts to several thousands of feet. The figures given for the basins of the Mississippi and Ganges show that such waste implies enormously long periods of time.