To the west of Mesopotamia is the valley containing the promised land of Palestine—it, also, has fallen from its former splendor, and is a desert compared with the days of its greatest prosperity. Still further west and south lies the land of Egypt, in the valley of the Nile. It was the fostermother of science, and the shaker of empires. It has fallen likewise; and a blight has come upon the soil, until it bears the appearance of a sandy waste. Over the sites of other famous nations of antiquity, in Europe and Asia, hovers, today, the spirit of desolation.
The same story is told on the American continent. Peru, the land of the Incas, once populous, powerful, wealthy, is today largely a wilderness. Mexico, the Aztec home, is now a vast desert, in spite of the evidence, through the discovered ruins of mighty cities and gigantic temples, that it was once the home of a strong people. Central America tells a similar story. It seems to be a general fact that wherever a large people lived formerly, there, today, a desert often occurs.
However, these countries are deserts only because human effort is no longer applied to them; by proper treatment the lands would again be raised to the flourishing condition that prevailed in their prosperous days. Intrinsically the soils are extremely fertile, but are dry and require the application of water to make the fertility suitable for the use of crops. The soils of Babylon, Assyria, Egypt, Peru and Mexico, raise crops of wonderful yields when properly irrigated; and there is abundant proof that in former days irrigation was practiced in these countries on a scale far larger than in Utah or in any other country of the present day.
Many of the old irrigation canals of Babylon still exist, and prove the magnitude of the practice, there, of the art of irrigation. The old historians, also, agree in explaining the ingenious devices by which whole rivers were turned from their courses to flow over the soil. In Egypt, likewise, irrigation was more commonly practiced in the past than it is today; though even now a large portion of the soil of that country is made to yield crops by the artificial application of water. In Peru, Central America, and Mexico, the irrigation canals that remain from prehistoric days are even more wonderful as feats of engineering and as evidences of a populous and enlightened condition of the country than the massive temples and extensive cities that are also found. In the construction of these canals every precaution, apparently, was taken to have the water applied to the lands in the right manner, and to reduce the loss to a minimum. In some places immense canals remain, that are tiled for miles, on sides and bottom, in order to render them watertight, and thus prevent any loss by seepage.
Instead of saying, then, that the countries where most great nations have lived are now deserts, we may as well say that most great nations have lived in countries where irrigation was necessary; in fact, that history indicates that a dense population, and high culture, usually go hand in hand with a soil that thirsts for water. What can science, the great explainer, say on this subject?
II.
"Science moves, but slowly, slowly, moving on from point to point."—Locksley Hall.
A plant feeds in two ways—by its leaves, and by its roots. The leaves feed from the air; the roots from the soil. In the air is found a colorless, heavy gas, known as carbon dioxide, which is made up partly of the element of carbon, or charcoal. When an animal or a plant is burned at a low heat, it first chars, showing the presence of charcoal; then if the burning be continued, it disappears, with the exception of the ash, as the gas, carbon dioxide. Since animal and vegetable matters are constantly being burned upon the earth's surface, naturally the air contains a perceptible quantity of carbon dioxide. The leaves of a living plant, waving back and forth, draw into themselves the carbon dioxide with which they come into contact, and there break it up and take the carbon away from it. The carbon thus obtained by the leaves is built into the many ingredients of a plant, and carried to the parts that are in greatest need. The plant is able to do this by virtue of the peculiar properties of the green coloring matter in all its leaves, leaf green; which acts, however, only in the presence of bright sunlight. Since one-half or more of the dry matter of a plant is carbon, the importance of the leaf-air-feeding of a plant may be understood.
The water which a plant contains and the incombustible portions, the mineral matters or ash, are taken directly from the soil by means of the roots. The old idea that vegetable mould and other corbonaceous matters are also taken from the soil by the roots has been shown to be erroneous. The mineral portions of a plant are of the highest value to the life of the plant—without them, in fact, it languishes and dies. If a soil on which a plant is growing contains, for instance, no iron, the leaves become pale, soon white, and finally they lose the power of appropriating carbon from the air. If potash is absent from the soil, the plants growing upon it will develop in an imperfect manner and finally die. It has been found by careful experiment that seven mineral substances must be found in every soil, if it shall support the life of plants, namely: (1) Potash; (2) lime; (3) magnesia; (4) oxide of iron or iron rust; (5) sulphuric acid or oil of vitriol; (6, phosophoric acid, and (7) nitric acid or aqua fortis. The fertility of any soil or soil district is determined by the quantity of these indispensable ash ingredients contained by it.
All soils are produced by the breaking down of the mountains under the influence of weathering. The broken down rock is washed into the hollows and lowlands by the rains and floods of melted snow, and there forms soil. Soil may, therefore, be defined, in a general way, as pulverized rock. Nearly all rocks contain the elements above enumerated as being essential to a plant's life; and nearly every soil will, consequently, be in possession of them. Rocks, however, in being subjected to the action of weathering, undergo other changes than mere pulverization. The potash, lime and other plant foods held by a rock are in an insoluble condition, and can not be taken up with any ease by the plant roots. As the rock is pulverized in the process of weathering, it is also made more soluble, and the juices of the plant roots can then absorb the needed foods with greater facility. This process of making the soil more soluble, continues while time lasts, and every year will find the soil more soluble than the year before, if there are no opposing actions. Therefore, the fertility of a soil is determined not only by the quantity of plant food it contains, but also by the condition of solubility the soil constituents are in.