Upon our own land, we have had two or three instances of obstruction by sand, very soon after the tiles were laid, and always at the junction of drains imperfectly secured with bricks, before we had procured proper branch-pipes for the purpose.
A little experience will enable the proprietor at once to detect any failure of his drains, and to apply the proper remedy. Obstructions from silt and sand are much more likely to occur during the first season after the drains are laid, than afterwards, because the earth is loose about the pipes, and more liable to be washed into the joints, than after it has become compact.
On the whole, we believe the danger to tile-drains, of obstruction, is very little, provided good tiles are used, and proper care is exercised in laying them.
CHAPTER XIX
DRAINAGE OF STIFF CLAYS.
Clay not impervious, or it could not be wet and dried.—Puddling, what is.—Water will stand over Drains on Puddled Soil.—Cracking of Clays by Drying.—Drained Clays improve by time.—Passage of Water through Clay makes it permeable.—Experiment by Mr. Pettibone, of Vermont.—Pressure of Water in saturated Soil.
It is a common impression that clay is impervious to water, and that, therefore, a clay soil cannot be drained, especially by deep under-drains. A moment's reflection will satisfy any one that such land is not absolutely impervious. We find such land is wet in Spring, at any depth; and, in the latter part of Summer, we find it comparatively dry. How comes it wet, at any time, if water does not go into it? And how comes it dry, at any time, if water does not come out of it?
In treating of the power of the soil to absorb moisture, we have shown that a clay soil will absorb more than half its weight and bulk of water, and that it holds more water than any other soil, with, perhaps, the single exception of peat.
The facts, however, that clay may be wet, and may be dried, and that it readily absorbs large quantities of water, though they prove conclusively that it is not impervious to water, yet do not prove that water will pass through it with sufficient rapidity to answer the practical purposes of drainage for agriculture. This point can only be satisfactorily determined by experiment. It is not necessary, however, that each farmer should try the experiment for himself; because, although we are very apt to think our own case an exception to all general rules, it is not really probable that any new kind of clay will be discovered hereafter, that is so different from all other clay that is known, that established principles will not apply to it. So far as our own observation extends, owners of clay farms always over-estimate the difficulty of draining their land. There are certain notorious facts with regard to clay, which mislead the judgment of men on this point. One of these facts is, that clay is used for stopping water, by the process called puddling. Puddled clay is used for the bottom of ponds, and of canals, and of reservoirs, and, for such purposes, is regarded as nearly, or quite impervious.
We see that, on our clay fields, water stands upon the surface, especially in the ruts of wheels, and on headlands much trodden, late in the season, and when, in other places, it has disappeared. This is due, also, to puddling.
Puddling is merely the working of wet clay, or other soil, by beating, or treading, or stirring, until its particles are so finely divided that water has an exceedingly slow passage between them, with ordinary pressure. We see the effect of this operation on common highways, where water often stands for many days in puddles, because the surface has been ground so fine, and rendered so compact, by wheels and horses, that the water cannot find passage. This, however, is not the natural condition of any clay; nor can any clay be kept in this condition, except by being constantly wet. If once dried, or subjected to the action of frost, the soil resumes its natural condition of porosity, as will be presently explained. They who object to deep drainage, or to the possibility of draining stiff clays, point to the fact that water may be seen standing directly over the drains, on thorough-drained fields. We have seen this on our own fields. In one instance, we had, after laying tiles through a field, at 50 feet intervals, in the same Autumn, when the land was wet, teamed across it a large quantity of soil for compost, with a heavy ox-team. The next Spring, the water stood for many days in that track which passed across tile-drains, after it had disappeared elsewhere in the field. A fine crop of Indian corn grew on the field that year, but the effect of the puddling was visible the whole season. "One inch of wet and worked clay," says a scientific writer, "will prevent water from passing through, so long as it is kept wet, as effectually as a yard will do."