Loss Of Water By Seepage

The free water of all soils is continually passing downward in obedience to the law of gravitation. Near the surface it seeps down slowly, but as it goes deeper it gathers volume and power. If the soil is shallow, it soon strikes hardpan and overflows as surface drainage. If the soil is deep, it may sink down many hundreds of feet until it comes to some kind of a check or channel; perhaps a stratum of rock, perhaps a layer of coarse gravel. Down this it passes, joining forces with other underground currents, as the rill joins the brook and the brook joins the creek. This channel may lead it many miles away to where the stratum of rock or gravel comes to the surface. Then it gushes forth as a spring near the base of some hill, or on the bottom of some lake. Or it may not come to the surface but seek a lower level and there seep upward through the soil because of the pressure of other water behind it. Just as surface water flows down hillsides and collects in valleys, so underground water may sink through the soil of the mountain, hill, knoll or ridge, until it reaches the levels, where it may be pushed up towards the surface again by the pressure of water behind it.

Many thousands of acres of farm lands are thus sub-irrigated, or watered from below, by water that has seeped down from higher land, perhaps many miles away. When drained, these soils become very productive, not only because of the equable supply of water that they receive from below, but also because this water, having perhaps travelled a long distance in seeking its level, has dissolved much plant food from the soil through which it has passed. Loss of Plant Food in Seepage Water. — This latter phase of the seepage of soil water has a very important bearing upon the fertility of the land. The water in the soil, both free and film, is not pure but has in it various salts and elements that it has dissolved from the soil. Some of these are plant foods. The nitrates, containing that most expensive of plant foods, nitrogen, are most likely to be carried off in this way; also the phosphates and the potash salts to some extent. Most any kind of farm plant will grow very well in the water caught from a drain tile, and nothing else, showing that this water contains as much plant food as that which the plants in the soil draw up through their roots. The coarser a soil is, and the less humus it contains, the less able is it to retain the rain that falls upon it. It takes longer for water to seep down through clay soils, in which the spaces between the particles are very small, than through a sandy soil, in which the spaces between the grains are much larger. This is why sandy soils are leachy. The loss of water from clayey soils, through seepage, is much facilitated by the burrows of earthworms and the decay of roots, both of which open channels; also, to a considerable extent, by the numerous cracks that appear in all clay soils as they dry. These cracks are often very large on the surface; smaller though less numerous cracks are found for several feet below. With the exception of very sandy soils, the loss of water by seepage is not likely to occur during the growing season. In most parts of the country the upper soil becomes so dry during the summer that the summer rainfall is mostly taken up or evaporated before it is lost by seepage.

It is during the season when vegetation is dormant or inactive, which is usually when the precipitation is largest, that the loss of water by seepage, and the loss of the plant food dissolved in this water, is likely to be largest. The loss of soil water by seepage can be prevented, in part, by judicious farm practice. If a leachy soil is filled with humus, either from manure or from decaying vegetation, the large spaces between the grains are clogged and water sinks through the soil less rapidly. An open, porous soil may also be compacted by rolling, which reduces the size of the spaces by crushing the grains together. Liming a sandy soil may have a slight effect in the same direction. If the soil is not left bare during the winter when a crop is not growing upon it, but is kept covered with a catch crop, as rye, the roots and herbage of this crop hold much of the water that otherwise would be lost. These operations are discussed at length in succeeding chapters.