FLOOD AND SOIL
By G. V. JACKS THE catastrophic floods in the Mississippi and Ohio valleys were an act not of God, but of man. They were the direct result of soil erosion, caused by ruthless exploitation of soil fertility and now threatening the whole existence of the United States. Millions of tons of fertile soil are annually deposited in these rivers, raising their beds so that at the present time for long stretches they actually flow above the adjoining land and are only held in place by dams and levees which must continually be raised higher and higher as the river beds rise. And the higher the levees, the greater is the calamity when the Mississippi and Ohio break bounds. Furthermore, by removing the porous top-soil, erosion has enormously increased the run-off into rivers of rain-water which should normally percolate through the soil into underground reservoirs. The volume of water carried by the great American rivers is increasing every year. The calamity of flood is ceasing to be a hazard ; it is becoming a certainty.
Flood damage is the most spectacular consequence of soil erosion, but it is only one, and by no means the largest, of the many items in America's annual erosion bill. The United States National Resources Board calculates that 40o million dollars worth of fertile soil—representing irre- placeable capital wealth—is lost every year. One hundred million acres of some of the world's most productive land have already been turned into unreclaimable desert in the United States, and if erosion continues unchecked the country will be incapable of supporting its present population before the end of the century. The soil cannot be recovered ; even that part which is deposited on the land by floods is not the original soil, but merely an infertile mud which covers the plains and renders them useless when the floods subside. Another large part finds its way into reservoirs, silting them up in a few years and causing destruction to public water and irrigation supplies and hydro-electric power plants. All the soluble plant-food elements in the soil are irretrievably washed into the sea. A soil takes thousands of years to form, but it can be washed away in a day. In the worst afflicted areas whole counties which were once prosperous farmland now consist of cavernous gullies which have swallowed up not only the land but the very houses built upon it.
Widespread erosion is taking place not only throughout most of the United States, but also in South and East Africa, in India, in Australia and almost everywhere where man has attempted to subdue Nature in the last too years. In short, soil erosion is Nature's revolt against civilisation, and it is now unmistakably apparent that either Nature will win outright by making the lands uninhabitable or else our whole treatment of her will have to be drastically modified. It has been stated that no greater natural disaster has threat- ened mankind since the Flood—which, incidentally, may well have been caused by the soil erosion that undoubtedly destroyed ancient civilisations in Asia and America.
The main cause of soil erosion is destruction of the natural vegetation that everywhere affords the soil adequate protection against the continuous eroding action of water and wind. A study of the distribution of plants over the earth's surface has shown that each region has a natural vegetation just sufficient to prevent erosion under the pre- vailing conditions of soil and climate. Substitution of any other vegetation for the natural type results either in soil erosion—as when forest or grassland is cultivated—or soil deterioration—as when forest replaces grass, particularly on the prairies and steppes, which have the finest arable soils in the world. Worst of all is when the wild vegetation is completely destroyed or replaced by cultivated and care- fully weeded crops which necessitate most of the soil surface being left bare. Man must cultivate the soil in order to live, but in his blind attempts to transplant a mature civilisation into unprepared ground he has forgotten the immutable law that the land must be recompensed for everything taken from it.
The onset of soil erosion is insidious and often not noticed until the land is already .so ruined that reclamation is impos- sible. When, for instance, prairie grassland is ploughed up, sheet erosion sets in on even the gentlest slope, removing perhaps an inch of soil in a season, but giving no indication that anything untoward has happened until after some years the plough turns up only the barren subsoil. The more noticeable gully erosion which alters water-courses and may even divert great rivers, does not usually occur until sheet erosion is well advanced. Sheet and gully erosion are both caused by the action of rain on unprotected soil ; even a light rain will cause some erosion, particularly if it falls on dry, powdery soil, but heavy rains do far more damage in proportion to the amount of water that falls. In the tropics it is not uncommon for one torrential rain to remove all the soil from a newly-cleared slope before it has produced a single crop.
Another main cause of erosion is overstocking of pasture- land, resulting in rapid destruction of the vegetation cover. Overstocking is a severe, indeed a desperate, problem in Africa particularly, where it is a direct consequence of the white man's invasion. Before his advent, inter-tribal wars, disease and the native practice of shifting cultivation pre- served an equilibrium between the cattle population and the land's carrying capacity. Now, the efficiency of medical, veterinary and missionary services, wholesale land alienation, and more intensive agriculture on the European system have entirely dislocated the delicate equilibrium, and erosion is proceeding at an appalling rate.
Wind also plays havoc with unprotected soil, particularly in extensive level regions whose flatness renders them less liable to water erosion. Wind erosion is greatly increased by the pulverisation of the soil produced by continuous cultivation, and is a growing menace in North America, South Africa and Australia, where droughts are frequent. The great dust storms in America in May, 1934, when thousands of farms were ruined and the Atlantic cities were darkened by clouds of soil blown a thousand miles from the prairies, may have marked a turning-point in American history. For it was then that the politician in Washington and the industrialist in New York, choked by the falling cloud of wasted wealth, first really woke to the danger that was imperilling the country's immediate prosperity and future existence.
The battle with erosion must at no very distant date cause a complete revolution in thought and behaviour among the peoples of the afflicted countries, whose economy is at present based on the wholesale export of irreplaceable soil- fertility in exchange for the amenities of civilisation. The desolation produced by erosion has to be seen to be believed. That young nations, in the full vigour of youth, will ulti- mately overcome the imminent danger confronting them cannot be doubted, and their victory will leave a deeper impress than all the wars in history on the evolution of human society, for we are witnessing the opening rounds of a gigantic conflict with Nature comparable with, but far greater than, those which ended in the conquest of drought and the emergence of Egyptian, Mesopotamian and Peruvian civilisations, and in the conquest of the forest and the ocean, on which our Atlantic civilisation was founded. Compre- hensive scientific planning with land salvation as its first objective is now the only choice before the countries most affected. Erosion control, or " conservation "—to use a more general term—will dominate their entire future development, and the results will be very different from anything the world has yet seen or imagined. Men have dreamed of a posterity dominated by helicopters, hygienic clothing and beautiful cities of skyscrapers, but present indications are rather that the next, and perhaps the first really scientific, civilisation will be based on more prosaic things such as contour terraces, afforestation, dams and, above all, the maintenance and improvement of grass. Throughout the world tardy steps arc at last being taken to meet the erosion menace. As the danger increases, the conservation movement will everywhere gather force and will ultimately re-mould the destiny of man. If today we are watching the twilight of a worn-out civilisation, sated with power it cannot use, we can already trace the dawn of a new one, to which the old has given birth.
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