Page:Popular Science Monthly Volume 73.djvu/12

8 formation of the four feet of soil next to the rock over our average domain, where such depth obtains, it will probably be none too conservative. To preserve a good working soil-depth, with such an estimate, surface wastage should not exceed some such rate as one inch in a thousand years. If one chooses to indulge in a more liberal estimate of the soil-forming rate, it will still appear, under any intelligent estimate, that surface wastage is a serious menace to the retention of our soils under present modes of management. Historical evidence enforces this danger. In the Orient there are large tracts almost absolutely bare of soil now, which formerly bore flourishing populations. Long-tilled lands generally bear testimony of like import. Much more than mere loss of fertility is here menaced; it is the loss of, the soil-body itself, a loss almost beyond repair. When our soils are gone, we too must go, unless we shall find some way to feed on raw rock or its equivalent. The immense tonnage of soil-material carried out to sea annually by our rivers, even when allowance is made for laudable wash, and for material derived from the river channels, is an impressive warning of the danger of excessive soil-waste. Nor is this all; the wash from one acre often buries the fertile portion of another acre, or of several. Sometimes one's loss is another's gain, but all too frequently one's loss is another's disaster.

If the atmospheric waters may not run off the surface freely without serious menace, where may they go and what may they do consistent with our welfare? The answer lies in a return to the study of the origin and internal work of soils. For necessary brevity, let us neglect all secondary soils, or overplacements, and consider simply the origin and activities of primary soils derived from primary rocks. The action of air and water in producing soil from such rock is partly chemical and partly physical. Certain rock substances are made soluble and become plant food or plant poisons, while others remain relatively insoluble, but are reduced to a finely divided state and form the earthy element of the soil.

Some of the soluble substances thus formed at the base of soils are necessary plant food, while some are harmful; but what is more to the point, all are harmful if too concentrated. There is need, therefore, that enough water pass through the forming soil, and on down to the ground-water and out through the under-drainage, to carry away the excess of these products. An essential part of the best adjustment is thus seen to lie in a proper apportionment of the amount of water which goes through the soils. If this be not enough, the plants will suffer from saline excess; if it be too much, the plants may suffer from saline deficiency.

When evaporation from the surface is active and prolonged, waters which had previously gone down to the zone of soil-formation and