Henry F. French

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."

"If," says Gisborne, "you eat off turnips with sheep, if you plow the land, or cart on it, or in any way puddle it, when it is wet, of course the water will lie on the surface, and will not go to your drains. A four-foot drain may go very near a pit, or a water-course, without attracting water from either, because water-courses almost invariably puddle their beds; and the same effect is produced in pits by the treading of cattle, and even by the motion of the water produced by wind. A very thin film of puddle, always wet on one side, is impervious, because it cannot crack."

In those four words, we find an allusion to the whole mystery of the drainage of clays—a key which unlocks the secret by which the toughest of these soils may be converted, as by a fairy charm, to fields of waving grain.

CRACKING OF CLAYS BY DRYING.

"In drying under the influence of the sun," says Prof. Johnston, "soils shrink in, and thus diminish in bulk, in proportion to the quantity of clay, or of peaty matter, they contain. Sand scarcely diminishes at all in bulk by drying; but peat shrinks one-fifth in bulk, and strong agricultural clay nearly as much." By laying drains in land, we take from it that portion of the water that will run out at the bottom. The sun, by evaporation, then takes out a portion at the top. The soil is thus contracted, and, as the ends of the field cannot approach each other, both soil and subsoil are torn apart, and divided by a network of cracks and fissures. Every one who is familiar with clay land, or who has observed the bottom of a ditch or frog pond by the roadside, must have observed these cracks, thus caused by the contraction of the soil in drying. The same contraction occurs in drier land, by cold, in Winter; by which, in cold regions, deep rents are made in the earth, and reports, like those of cannon, are often heard. The cracking by drying, however, is more quiet in its effects, merely dividing the ground, noiselessly, into smaller and smaller masses, as the process proceeds. Were it not for this process, it may well be doubted whether clay lands could be effectually drained at all. Nature, however, seems to second our efforts here, for we have seen that the stiffer the clay, the greater the contraction, and the more the soil is split up and rendered permeable by this operation.

These cracks are found, by observation, to commence at the drains, and extend further and further, in almost straight lines, into the subsoil, forming so many minor drains, or feeders, all leading to the tiles. These main fissures have numerous smaller ones diverging from them, so that the whole mass is divided and subdivided into the most minute portions. The main fissures gradually enlarge, as the dryness increases, and, at the same time, lengthen out; so that, in a very dry season, they may be traced the whole way between the drains. The following cut will give some idea of these cracks, or fissures, as they exist in a dry time:

Fig.98.—Cracking of Clays by Drainage.

In the United States, the supply of rain is far less uniform than in England, and much severer droughts are experienced. Thus the contraction, and consequent cracking of the soil, must be greater here than in that country.

In laying drains more than four feet deep, in the stiffest clay which the author has seen, in a neighborhood furnishing abundance of brick and potter's clay, these cracks were seen to extend to the very bottoms of the drains, not in single fissures from top to bottom, but in innumerable seams running in all directions, so that the earth, moved with the pick-axe, came up in little cubes and flakes, and could be separated into pieces of an inch or less diameter. This was on a ridge which received no water except from the clouds, having no springs in or upon it, yet so nearly impervious to water, that it remained soft and muddy till late in June. In Midsummer, however, under our burning sun, it had, by evaporation, been so much dried as to produce the effect described.

In clay that has never been dried, as for instance, that found under wet meadows from which the water has but recently been drawn, we should not, of course, expect to find these cracks. Accordingly, we find sometimes in clay pits, excavated below the permanent water-line, and in wells, that the clay is in a compact mass, and tears apart without exhibiting anything like these divisions.

We should not expect that, on such a clay, the full effect of drainage would be at once apparent. The water falling on the surface would very slowly find its way downward, at first. But after the heat of Summer, aided by the drains underneath, had contracted and cracked the soil, passages for the water would soon be found, and, after a few years, the whole mass, to the depth of the drains, would become open and permeable. As an old English farmer said of his drains, "They do better year by year; the water gets a habit of coming to them." Although this be not philosophical language, yet the fact is correctly stated. Water tends towards the lowest openings. A deep well often diverts the underground stream from a shallower well, and lays it dry. A single railroad cut sometimes draws off the supply of water from a whole neighborhood. Passages thus formed are enlarged by the pressure of the water, and new ones are opened by the causes already suggested, till the drainage becomes perfect for all practical purposes. So much is this cracking process relied on to facilitate drainage, that skillful drainers frequently leave their ditches partly open, after laying the tiles, that the heat may produce the more effect during the first season.

As to the depth of drains in stiff clays, enough has already been said, under the appropriate title. In England, the weight of authority is in favor of four-foot drains. In this country, a less depth has thus far, in general, been adopted in practice, but it is believed that this has been because a greater depth has not been tried. It is understood, that the most successful drainers in the State of New York, have been satisfied with three-foot drains, not, as it is believed, because there is any instance on record, in this country, of the failure of four-foot drains, but because the effect of more shallow drains has been so satisfactory, that it has been thought a useless expense to go deeper. To Mr. Johnston and to Mr. Delafield, of Seneca County, the country is greatly indebted for their enterprise and leadership in the matter of drainage. Mr. Johnston gives it as his opinion, that "three feet is deep enough, if the bottom is hard enough to lay tiles on; if not, go deeper."

Without intimating that any different mode of drainage than that adopted, would have been better on Mr. Johnston's farm, we should be unwilling to surrender, even to the opinion of Mr. Johnston and his friends, our conviction that, in general, three-foot drains are too shallow. Mr. Johnston expressly disclaims any experience in draining a proper clay soil. In the Country Gentleman, of June 10th, 1848, he says:

In the same paper, under date of July 8th, Mr. Johnston says, "The only experience I have had in digging into soils, to judge of draining out of this county (Seneca), was in Niagara." He states the result of his observations thus:

In the Country Gentleman of Nov. 18th, 1858, we find an interesting statement, by John S. Pettibone, of Manchester, Vermont, partly in reply to the statement of Mr. Johnston.

The experiment by Mr. Pettibone, showing the increased permeability of clay, merely by the passage of water through it, is very interesting. He says, in his letter to the editor:

When we observe the effect of heat in opening clays to water by cracking, and the effect of the water itself, aided, as it doubtless is, by the action of the air, in rendering the soil permeable, we hardly need feel discouraged if the question rested entirely on this evidence; but when we consider that thousands upon thousands of acres of the stiffest clays have been, in England and Scotland, rescued from utter barrenness by drainage, and made to yield the largest crops, we should regard the question of practicability as settled. The only question left for decision is whether, under all the circumstances of each particular case, the operation of draining our clay lands will be expedient—whether their increased value will pay the expense. It is often objected to deep drains in clays, that it is so far down to the drains that the water cannot readily pass through so large a mass. If we think merely of a drop of rain falling on the surface, and obliged to find its devious way through the mazes of cracks and particles till it gains an outlet at the bottom of four feet of clay, it does seem a discouraging journey for the poor little solitary thing; but there is a more correct view of the matter, which somewhat relieves the difficulty.

All the water that will run out of the soil has departed; but the soil holds a vast amount still, by attraction. The rain begins to fall; and when the soil is saturated, a portion passes into the drain; but it is, by no means, the water which last fell upon the surface, but that which was next the drain before the rain fell. If you pour water into a tube that is nearly full, the water which will first run from the other end is manifestly not that which you pour in. So the ground is full of little tubes, open at both ends, in which the water is held by attraction. A drop upon the surface drives out a drop at the lower end, into to the drain, and so the process goes on—the drains beginning to run as soon as the rain commences, and ceasing to flow only when the principle of attraction balances the power of gravitation.

PRESSURE OF WATER IN THE SOIL.

In connection with the passage of water through clay soil, it may be appropriate to advert to the question sometimes mooted, whether in a soil filled with water, at four feet depth, there is the same pressure as there would be, at the same depth, in a river or pond. The pressure of fluids on a given area, is, ordinarily, in proportion to their vertical height; and the pressure of a column of water, four feet high, would be sufficient to drive the lower particles into an opening like a drain, with considerable force, and the upper part of such a column would essentially aid the lower part in its downward passage. Does this pressure exist? Mr. Gisborne speaks undoubtingly on this point, thus:

Mr. Thomas Arkell, in a paper read before the Society of Arts, in 1855, says, on this point:

We do not find any intimation that there is any error in the view advanced by the learned gentleman quoted; and if there is none, we have an explanation of the faculty which water seems to have, of finding its way into drainpipes. Yet, we feel bound to confess, that, aside from authority, we should have supposed that the pressure due to a column of pure water, would be essentially lessened, by the interposition of solid matter between its particles.