Imágenes de páginas
PDF
EPUB

( * worms follow either the roots or the mold. Permanent “schisms are established in the clay, and its whole charac“ter is changed. An old farmer in a midland county began “with 20-inch drains across the hill, and, without ever “reading a word, or, we believe, conversing with any one “on the subject, poked his way, step by step, to four or “five feet drains, in the line of steepest descent. Showing “us his drains this spring, he said: “They do better year “by year; the water gets a habit of coming to them ’—a very “correct statement of fact, though not a very philosophical “explanation.”

Alderman Mechi, of Tiptree Hall, says: “Filtration “may be too sudden, as is well enough shown by our hot “sands and gravels; but I apprehend no one will ever “fear rendering strong clays too porous and manageable. “The object of draining is to impart to such soils the “mellowness and dark color of self drained, rich and fria“ble soil. That perfect drainage and cultivation will do “this,is a well known fact. I know it in the case of my “own garden. How it does so I am not chemist enough “to explain in detail; but it is evident the effect is pro“duced by the fibers of the growing crop intersecting “every particle of the soil, which they never could do be“fore draining; these, with their excretions, decompose on “removal of the crop, and are acted on by the alternating “air and water, which also decompose and change, in a “degree, the inorganic substances of the soil. Thereby “drained land, which was, before, impervious to air and “water, and consequently unavailable to air and roots, “to worms, or to vegetable or animal life, becomes, by “drainage, populated by both, and is a great chemical “laboratory, as our own atmosphere is subject to all the “changes produced by animated nature.”

Experience proves that the descent of water through the soil renders it more porous, so that it is easier for the water falling afterward to pass down to the drains, but no very satisfactory reason for this has been presented, beyond that which is connected with the cracking of the soil. The fact is well stated in the following extract from a letter to the Country Gentleman : “A simple experiment will convince any farmer that the “best means of permanently deepening and mellowing the “soil is by thorough drainage, to afford a ready exit for all “surplus moisture. Let him take in spring, while wet, a “quantity of his hardest soil, such as it is almost impossi“ble to plow in summer, such as presents a baked and “brick-like character under the influence of drought, and “place it in a box or barrel, open at the bottom, and fre* quently during the season let him saturate it with water. “He will find it gradually becoming more and more porous “and friable, holding water less and less perfectly as the “experiment proceeds, and in the end it will attain a state “best suited to the growth of plants from its deep and “mellow character.” It is equally a fact that the ascent of water in the soil, together with its evaporation at the surface, has the effect of making the soil impervious to rains, and of covering the land with a crust of hard, dry earth, which forms a barrier to the free entrance of air. So far as the formation of crust is concerned, it is doubtless due to the fact that the water in the soil holds in solution certain mineral matters, which it deposits at the point of evaporation, the collection of these finely divided matters serving to completely fill the spaces between the particles of soil at the surface, pasting them together, as it were. How far below the surface this direct action extends, cannot be definitely determined; but the process being carried on for successive years, accumulating a quantity of these fine particles, each season, they are, by cultivation, and by the action of heavy showers falling at a time when the soil is more or less dry, dis. tributed through a certain depth, and ordinarily, in all

probability, are most largely deposited at the top of the subsoil. It is found in practice that the first foot in depth of retentive soils is more retentive than that which lies below. If this opinion as to the cause of this greater imperviousness is correct, it will be readily seen how water, descending to the drains, by carrying these soluble and finer parts downward and distributing them more equally through the whole, should render the soil more porous.

Another cause of the retention of water by the surface soil, often a very serious one, is the puddling which clayey lands undergo by working them, or feeding cattle upon them, when they are wet. This is always injurious. By draining, land is made fit for working much earlier in the spring, and is sooner ready for pasturing after a rain, but, no matter how thoroughly the draining has been done, if there is much clay in the soil, the effect of the improvement will be destroyed by plowing or trampling, while very wet; this impervious condition will be removed in time, of course, but, while it lasts, it places us as completely at the mercy of the weather as we were before a ditch was dug. In connection with the use of the word impervious, it should be understood that it is not used in its strict sense, for no substance which can be wetted by water is really impervious, and the most retentive soil will become wet. Gisborne states the case clearly when he says: “Is your “subsoil moister after the rains of mid-winter, than it is “after the drought of mid-summer ? If it is, it will drain.” The proportion of the rain-fall which will filtrate through the soil to the level of the drains, varies with the composition of the soil, and with the effect that the draining has had upon them. In a very loose, gravelly, or sandy soil, which has a perfect outlet for water below, all but the heaviest falls of rain will sink at once, while on a heavy clay, no matter | how well it is drained, the process of filtration will be much more slow, and if the land be steeply inclined, some of the water of ordinarily heavy rains must flow off over the surface, unless, by horizontal plowing, or catch drains on the surface, its flow be retarded until it has time to enter the soil.

The power of drained soils to hold water, by absorption, is very great. A cubic foot of very dry soil, of favorable character, has been estimated to absorb within its particles, —holding no free water, or water of drainage, about onehalf its bulk of water; if this is true, the amount required to moisten a dry soil, four feet deep, giving no excess to be drained away, would amount to a rain fall of from 20 to 30 inches in depth. If we consider, in addition to this, the amount of water drained away, we shall see that the soil has sufficient capacity for the reception of all the rain water that falls upon it.

In connection with the question of absorption and filtration, it is interesting to investigate the movements of water in the ground. The natural tendency of water, in the soil as well as out of it, is to descend perpendicularly toward the center of the earth If it meet a flat layer of gravel lying upon clay, and having a free outlet, it will follow the course of the gravel,-laterally,–and find the outlet; if it meet water which is dammed up in the soil, and which has an outlet at a certain elevation, as at the floor of a drain, it will raise the general level of the water; and force it out through the drain; if it meet water which has no outlet, it will raise its level until the soil is filled, or until it accumulates sufficient pressure, (head,) to force its way through the adjoining lands, or until it finds an outlet at the surface.

The first two cases named represent the condition which it is desirable to obtain, by either natural or artificial drainage; the third case is the only one which makes drainage necessary. It is a fixed rule that water, descending in the soil, will find the lowest outlet to which there exists a channel through which it can flow, and that if, after heavy rains, it rise too near the surface of the ground, the proper remedy is to tap it at a lower level, and thus remove the water table to the proper distance from the surface. This subject will be more fully treated in a future chapter, in considering the question of the depth, and the intervals, at which drains should be placed.

Evaporation,-By evaporation is meant the process by which a liquid assumes the form of a gas or vapor, or “dries up.” Water, exposed to the air, is constantly undergoing this change. It is changed from the liquid form, and becomes a vapor in the air. Water in the form of vapor occupies nearly 2000 times the space that it filled as a liquid. As the vapor at the time of its formation is of the same temperature with the water, and, from its highly expanded condition, requires a great amount of heat to maintain it as vapor, it follows that a given quantity of water contains, in the vapory form, many times as much heat as in the liquid form. This heat is taken from surrounding substances, from the ground and from the air, which are thereby made much cooler. For instance, if a shower moisten the ground, on a hot summer day, the drying up of the water will cool both the ground and the air. If we place a wet cloth on the head, and hasten the evaporation of the water by fanning, we cool the head; if we wrap a wet napkin around a pitcher of water, and place it in a current of air, the water in the pitcher is made cooler, by giving up its heat to the evaporating water of the napkin; when we sprinkle water on the floor of a room, its evaporation cools the air of the room.

So great is the effect of evaporation, on the temperature

of the soil, that Dr. Madden found that the soil of a

drained field, in which most of the water was removed

« AnteriorContinuar »