CHAPTER II. - HOW DRAINS ACT, AND HOW THEY AFFECT THE SOIL
For reasons which will appear, in the course of this work, the only sort of drain to which reference is here made is that which consists of a conduit of burned clay, (tile,) placed at a considerable depth in the subsoil, and enclosed in a compacted bed of the stiffest earth which can conveniently be found. Stone-drains, brush-drains, sod-drains, mole-plow tracks, and the various other devices for forming a conduit for the conveying away of the soakage-water of the land, are not without the support of such arguments as are based on the expediency of make-shifts, and are, perhaps, in rare cases, advisable to be used; but, for the purposes of permanent improvement, they are neither so good nor so economical as tile-drains. The arguments of this book have reference to the latter, (as the most perfect of all drains thus far invented,) though they will apply, in a modified degree, to all underground conduits, so long as they remain free from obstructions. Concerning stone-drains, attention may properly be called to the fact that, (contrary to the general opinion of farmers,) they are very much more expensive than tile-drains. So great is the cost of cutting the ditches to the much greater size required for stone than for tiles, of handling the stones, of placing them properly in the ditches, and of covering them, after they are laid, with a suitable barrier to the rattling down of loose earth among them, that, as a mere question of first cost, it is far cheaper to buy tiles than to use stones, although these may lie on the surface[pg 022] of the field, and only require to be placed in the trenches. In addition to this, the great liability of stone-drains to become obstructed in a few years, and the certainty that tile-drains will, practically, last forever, are conclusive arguments in favor of the use of the latter. If the land is stony, it must be cleared; this is a proposition by itself, but if the sole object is to make drains, the best material should be used, and this material is not stone.
A well laid tile-drain has the following essential characteristics:—1. It has a free outlet for the discharge of all water which may run through it. 2. It has openings, at its joints, sufficient for the admission of all the water which may rise to the level of its floor. 3. Its floor is laid on a well regulated line of descent, so that its current may maintain a flow of uniform, or, at least, never decreasing rapidity, throughout its entire length.
Land which requires draining, is that which, at some time during the year, (either from an accumulation of the rains which fall upon it, from the lateral flow, or soakage, from adjoining land, from springs which open within it, or from a combination of two or all of these sources,) becomes filled with water, that does not readily find a natural outlet, but remains until removed by evaporation. Every considerable addition to its water wells up, and soaks its very surface; and that which is added after it is already brim full, must flow off over the surface, or lie in puddles upon it. Evaporation is a slow process, and it becomes more and more slow as the level of the water recedes from the surface, and is sheltered, by the overlying earth, from the action of sun and wind. Therefore, at least during the periods of spring and fall preparation of the land, during the early growth of plants, and often even in midsummer, the water-table,—the top of the water of saturation,—is within a few inches of the surface, preventing the natural descent of roots, and, by reason of the small space to receive[pg 023] fresh rains, causing an interruption of work for some days after each storm.
If such land is properly furnished with tile-drains, (having a clear and sufficient outfall, offering sufficient means of entrance to the water which reaches them, and carrying it, by a uniform or increasing descent, to the outlet,) its water will be removed to nearly, or quite, the level of the floor of the drains, and its water-table will be at the distance of some feet from the surface, leaving the spaces between the particles of all of the soil above it filled with air instead of water. The water below the drains stands at a level, like any other water that is dammed up. Rain water falling on the soil will descend by its own weight to this level, and the water will rise into the drains, as it would flow over a dam, until the proper level is again attained. Spring water entering from below, and water oozing from the adjoining land, will be removed in like manner, and the usual condition of the soil, above the water-table, will be that represented in Fig. 3, the condition which is best adapted to the growth of useful plants.
In the heaviest storms, some water will flow over the surface of even the dryest beach-sand; but, in a well drained soil the water of ordinary rains will be at once absorbed, will slowly descend toward the water-table, and will be removed by the drains, so rapidly, even in heavy clays, as to leave the ground fit for cultivation, and in a condition for steady growth, within a short time after the rain ceases. It has been estimated that a drained soil has room between its particles for about one quarter of its bulk of water;—that is, four inches of drained soil contains free space enough to receive a rain-fall one inch in depth, and, by the same token, four feet of drained soil can receive twelve inches of rain,—-more than is known to have ever fallen in twenty-four hours, since the deluge, and more than one quarter of the annual rain-fall in the United States.
As was stated in the previous chapter, the water which reaches the soil may be considered under two heads:
1st—That which reaches its surface, whether directly by rain, or by the surface flow of adjoining land.
2d—That which reaches it below the surface, by springs and by soakage from the lower portions of adjoining land.
The first of these is beneficial, because it contains fresh air, carbonic acid, ammonia, nitric acid, and heat, obtained from the atmosphere; and the flowage water contains, in addition, some of the finer or more soluble parts of the land over which it has passed. The second, is only so much dead water, which has already given up, to other soil, all that ours could absorb from it, and its effect is chilling and hurtful. This being the case, the only interest we can have in it, is to keep it down from the surface, and remove it as rapidly as possible.
The water of the first sort, on the other hand, should be arrested by every device within our reach. If the land is steep, the furrows in plowing should be run horizontally along the hill, to prevent the escape of the water over the surface, and to allow it to descend readily into the ground. Steep grass lands may have frequent, small, horizontal ditches for the same purpose. If the soil is at all heavy, it should not, when wet, be trampled by animals, lest it be puddled, and thus made less absorptive. If in cultivation, the surface should be kept loose and open, ready to receive all of the rain and irrigation water that reaches it.
In descending through the soil, this water, in summer, gives up heat which it received from the air and from the heated surface of the ground, and thus raises the temperature of the lower soil. The fertilizing matters which it has obtained from the air,—carbonic acid, ammonia and nitric acid,—are extracted from it, and held for the use of growing plants. Its fresh air, and the air which follows the descent of the water-table, carries oxygen to the organic and[pg 025] mineral parts of the soil, and hastens the rust and decay by which these are prepared for the uses of vegetation. The water itself supplies, by means of their power of absorption, the moisture which is needed by the particles of the soil; and, having performed its work, it goes down to the level of the water below, and, swelling the tide above the brink of the dam, sets the drains running, until it is all removed. In its descent through the ground, this water clears the passages through which it flows, leaving a better channel for the water of future rains, so that, in time, the heaviest clays, which will drain but imperfectly during the first one or two years, will pass water, to a depth of four or five feet, almost as readily as the lighter loams.
Now, imagine the drains to be closed up, leaving no outlet for the water, save at the surface. This amounts to a raising of the dam to that height, and additions to the water will bring the water-table even with the top of the soil. No provision being made for the removal of spring and soakage water, this causes serious inconvenience, and even the rain-fall, finding no room in the soil for its reception, can only lie upon, or flow over, the surface,—not yielding to the soil the fertilizing matters which it contains, but, on the contrary, washing away some of its finer and looser parts. The particles of the soil, instead of being furnished, by absorption, with a healthful amount of moisture, are made unduly wet; and the spaces between them, being filled with water, no air can enter, whereby the chemical processes by which the inert minerals, and the roots and manure, in the soil are prepared for the use of vegetation, are greatly retarded.
Instead of carrying the heat of the air, and of the surface of the ground, to the subsoil, the rain only adds so much to the amount of water to be evaporated, and increases, by so much, the chilling effect of evaporation.
Instead of opening the spaces of the soil for the more free passage of water and air, as is done by descending water, that which ascends by evaporation at the surface brings up soluble matters, which it leaves at the point where it becomes a vapor, forming a crust that prevents the free entrance of air at those times when the soil is dry enough to afford it space for circulation.
Instead of crumbling to the fine condition of a loam, as it does, when well drained, by the descent of water through it, heavy clay soil, being rapidly dried by evaporation, shrinks into hard masses, separated by wide cracks.
In short, in wet seasons, on such land, the crops will be greatly lessened, or entirely destroyed, and in dry seasons, cultivation will always be much more laborious, more hurried, and less complete, than if it were well drained.
The foregoing general statements, concerning the action of water in drained, and in undrained land, and of the effects of its removal, by gravitation, and by evaporation, are based on facts which have been developed by long practice, and on a rational application of well know principles of science. These facts and principles are worthy of examination, and they are set forth below, somewhat at length, especially with reference to Absorption and Filtration; Evaporation; Temperature; Drought; Porosity or Mellowness; and Chemical Action.
Absorption and Filtration.—The process of under-draining is a process of absorption and filtration, as distinguished from surface-flow and evaporation. The completeness with which the latter are prevented, and the former promoted, is the measure of the completeness of the improvement. If water lie on the surface of the ground until evaporated, or if it flow off over the surface, it will do harm; if it soak away through the soil, it will do good. The rapidity and ease with which it is absorbed, and, therefore, the extent to which under-draining is successful, depend[pg 027] on the physical condition of the soil, and on the manner in which its texture is affected by the drying action of sun and wind, and by the downward passage of water through it.
In drying, all soils, except pure sands, shrink, and occupy less space than when they are saturated with water. They shrink more or less, according to their composition, as will be seen by the following table of results obtained in the experiments of Schuebler:
| 1,000 Parts of | Will Contract Parts. | 1,000 Parts of | Will Contract Parts. |
| Strong Limey Soil | 50. | Pure Clay | 183. |
| Heavy Loam | 60. | Peat | 200. |
| Brick Maker's Clay | 85. |
Professor Johnson estimates that peat and heavy clay shrink one-fifth of their bulk.
If soil be dried suddenly, from a condition of extreme wetness, it will be divided into large masses, or clods, separated by wide cracks. A subsequent wetting of the clods, which is not sufficient to expand it to its former condition, will not entirely obliterate the cracks, and the next drying will be followed by new fissures within the clods themselves; and a frequent repetition of this process will make the network of fissures finer and finer, until the whole mass of the soil is divided to a pulverulent condition. This is the process which follows the complete draining of such lands as contain large proportions of clay or of peat. It is retarded, in proportion to the amount of the free water in the soil which is evaporated from the surface, and in proportion to the trampling of the ground, when very wet. It is greatly facilitated by frost, and especially by deep frost.
The fissures which are formed by this process are, in time, occupied by the roots of plants, which remain and decay, when the crop has been removed, and which prevent the soil from ever again closing on itself so completely as before their penetration; and each season's crop adds new roots[pg 028] to make the separation more complete and more universal; but it is only after the water of saturation, which occupies the lower soil for so large a part of the year, has been removed by draining, that roots can penetrate to any considerable depth, and, in fact, the cracking of undrained soils, in drying, never extends beyond the separation into large masses, because each heavy rain, by saturating the soil and expanding it to its full capacity, entirely obliterates the cracks and forms a solid mass, in which the operation has to be commenced anew with the next drying.
Mr. Gisborne, in his capital essay on "Agricultural Drainage," which appeared in the Quarterly Review, No. CLXXI, says: "We really thought that no one was so ignorant as not to be aware that clay lands always shrink and crack with drought, and the stiffer the clay the greater the shrinking, as brickmakers well know. In the great drought, 36 years ago, we saw in a very retentive soil in the Vale of Belvoir, cracks which it was not very pleasant to ride among. This very summer, on land which, with reference to this very subject, the owner stated to be impervious, we put a walking stick three feet into a sun-crack, without finding a bottom, and the whole surface was what Mr. Parkes, not inappropriately, calls a network of cracks. When heavy rain comes upon a soil in this state, of course the cracks fill, the clay imbibes the water, expands, and the cracks are abolished. But if there are four or five feet parallel drains in the land, the water passes at once into them and is carried off. In fact, when heavy rain falls upon clay lands in this cracked state, it passes off too quickly, without adequate filtration. Into the fissures of the undrained soil the roots only penetrate to be perished by the cold and wet of the succeeding winter; but in the drained soil the roots follow the threads of vegetable mold which have been washed into the cracks, and get an abiding tenure. Earth[pg 029] worms follow either the roots or the mold. Permanent schisms are established in the clay, and its whole character 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 friable 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 produced by the fibers of the growing crop intersecting every particle of the soil, which they never could do before 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[pg 030] 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 impossible 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 frequently 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, distributed through a certain depth, and ordinarily, in all[pg 031] 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 [pg 032] 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 one-half 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[pg 033] 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[pg 034] from below, was 6-1/2° Far. warmer than a similar soil undrained, from which the water had to be removed by evaporation. This difference of 6-1/2° is equal to a difference of elevation of 1,950 feet.
It has been found, by experiments made in England, that the average evaporation of water from wet soils is equal to a depth of two inches per month, from May to August, inclusive; in America it must be very much greater than this in the summer months, but this is surely enough for the purposes of illustration, as two inches of water, over an acre of land, would weigh about two hundred tons. The amount of heat required to evaporate this is immense, and a very large part of it is taken from the soil, which, thereby, becomes cooler, and less favorable for a rapid growth. It is usual to speak of heavy, wet lands as being "cold," and it is now seen why they are so.
If none of the water which falls on a field is removed by drainage, (natural or artificial,) and if none runs off from the surface, the whole rain-fall of a year must be removed by evaporation, and the cooling of the soil will be proportionately great. The more completely we withdraw this water from the surface, and carry it off in underground drains, the more do we reduce the amount to be removed by evaporation. In land which is well drained, the amount evaporated, even in summer, will not be sufficient to so lower the temperature of the soil as to retard the growth of plants; the small amount dried out of the particles of the soil, (water of absorption,) will only keep it from being raised to too great a heat by the mid-summer sun.
An idea of the amount of heat lost to the soil, in the evaporation of water, may be formed from the fact that to evaporate, by artificial heat, the amount of water contained in a rain-fall of two inches on an acre, (200 tons,) would require over 20 tons of coal. Of course a considerable—probably by far the larger,—part of the heat taken up in[pg 035] the process of evaporation is furnished by the air; but the amount abstracted from the soil is great, and is in direct proportion to the amount of water removed by this process; hence, the more we remove by draining, the more heat we retain in the ground.
The season of growth is lengthened by draining, because, by avoiding the cooling effects of evaporation, germination is more rapid, and the young plant grows steadily from the start, instead of struggling against the retarding influence of a cold soil.
Temperature.—The temperature of the soil has great effect on the germination of seeds, the growth of plants, and the ripening of the crops.
Gisborne says: "The evaporation of 1 lb. of water lowers the temperature of 100 lbs. of soil 10°,—that is to say, that, if to 100 lbs. of soil, holding all the water it can by attraction, but containing no water of drainage, is added 1 lb. of water which it has no means of discharging, except by evaporation, it will, by the time that it has so discharged it, be 60° colder than it would have been, if it had the power of discharging this 1 lb. by filtration; or, more practically, that, if rain, entering in the proportion of 1 lb. to 100 lbs. into a retentive soil, which is saturated with water of attraction, is discharged by evaporation, it lowers the temperature of that soil 10°. If the soil has the means of discharging that 1 lb. of water by filtration, no effect is produced beyond what is due to the relative temperatures of the rain and of the soil."
It has been established by experiment that four times as much heat is required to evaporate a certain quantity of water, as to raise the same quantity from the freezing to the boiling point.
It is, probably, in consequence of this cooling effect of evaporation, that wet lands are warmest when shaded,[pg 036] because, under this condition, evaporation is less active. Such lands, in cloudy weather, form an unnatural growth, such as results in the "lodging" of grain crops, from the deficient strength of the straw which this growth produces.
In hot weather, the temperature of the lower soil is, of course, much lower than that of the air, and lower than that of the water of warm rains. If the soil is saturated with water, the water will, of course, be of an even temperature with the soil in which it lies, but if this be drained off, warm air will enter from above, and give its heat to the soil, while each rain, as it falls, will also carry its heat with it. Furthermore, the surface of the ground is sometimes excessively heated by the summer sun, and the heat thus contained is carried down to the lower soil by the descending water of rains, which thus cool the surface and warm the subsoil, both beneficial.
Mr. Josiah Parkes, one of the leading draining engineers of England, has made some experiments to test the extent to which draining affects the temperature of the soil. The results of his observations are thus stated by Gisborne: "Mr. Parkes gives the temperature on a Lancashire flat moss, but they only commence 7 inches below the surface, and do not extend to mid-summer. At that period of the year the temperature, at 7 inches, never exceeded 66°, and was generally from 10° to 15° below the temperature of the air in the shade, at 4 feet above the earth. Mr. Parkes' experiments were made simultaneously, on a drained, and on an undrained portion of the moss; and the result was, that, on a mean of 35 observations, the drained soil at 7 inches in depth was 10° warmer than the undrained, at the same depth. The undrained soil never exceeded 47°, whereas, after a thunder storm, the drained reached 66° at 7 inches, and 48° at 31 inches. Such were the effects, at an early period of the year, on a black bog. They suggest some[pg 037] idea of what they were, when, in July or August, thunder rain at 60° or 70° falls on a surface heated to 130°, and carries down with it, into the greedy fissures of the earth, its augmented temperature. These advantages, porous soils possess by nature, and retentive ones only acquire them by drainage."
Drained land, being more open to atmospheric circulation, and having lost the water which prevented the temperature of its lower portions from being so readily affected by the temperature of the air as it is when dry, will freeze to a greater depth in winter and thaw out earlier in the spring. The deep freezing has the effect to greatly pulverize the lower soil, thus better fitting it for the support of vegetation; and the earlier thawing makes it earlier ready for spring work.
Drought.—At first thought, it is not unnatural to suppose that draining will increase the ill effect of too dry seasons, by removing water which might keep the soil moist. Experience has proven, however, that the result is exactly the opposite of this. Lands which suffer most from drought are most benefited by draining,—more in their greater ability to withstand drought than in any other particular.
The reasons for this action of draining become obvious, when its effects on the character of the soil are examined. There is always the same amount of water in, and about, the surface of the earth. In winter there is more in the soil than in summer, while in summer, that which has been dried out of the soil exists in the atmosphere in the form of a vapor. It is held in the vapory form by heat, which may be regarded as braces to keep it distended. When vapor comes in contact with substances sufficiently colder than itself, it gives up its heat,—thus losing its braces,—contracts, becomes liquid water, and is deposited as dew.
Many instances of this operation are familiar to all.
For instance, a cold pitcher in the summer robs the vapor in the air of its heat, and causes it to be deposited on its own surface,—of course the water comes from the atmosphere, not through the wall of the pitcher; if we breathe on a knife blade, it condenses, in the same manner, the moisture of the breath, and becomes covered with a film of-water; stone-houses are damp in summer, because the inner surface of their walls, being cooler than the atmosphere, causes its moisture to be deposited in the manner described;[2] nearly every night, in summer, the cold earth receives moisture from the atmosphere in the form of dew; a single large head of cabbage, which at night is very cold, often condenses water to the amount of a gill or more.
The same operation takes place in the soil. When the air is allowed to circulate among its lower and cooler, (because more shaded,) particles, they receive moisture by the same process of condensation. Therefore, when, by the aid of under-drains, the lower soil becomes sufficiently loose and open, to allow a circulation of air, the deposit of atmospheric moisture will keep it supplied with water, at a point easily accessible to the roots of plants.
If we wish to satisfy ourselves that this is practically correct, we have only to prepare two boxes of finely pulverized soil,—one three or four inches deep,—and the other fifteen or twenty inches deep, and place them in the sun, at midday, in summer. The thinner soil will soon be completely dried, while the deeper one, though it may have been previously dried in an oven, will soon accumulate a[pg 039] large amount of water on those particles which, being lower and better sheltered from the sun's heat than the particles of the thin soil, are made cooler.
We have seen that even the most retentive soil,—the stiffest clay,—is made porous by the repeated passage of water from the surface to the level of the drains, and that the ability to admit air, which plowing gives it, is maintained for a much longer time than if it were usually saturated with water which has no other means of escape than by evaporation at the surface. The power of dry soils to absorb moisture from the air may be seen by an examination of the following table of results obtained by Schuebler, who exposed 1,000 grains of dried soil of the various kinds named to the action of the air:
| Kind of Soil. | Amount of Water Absorbed in 24 Hours. |
| Common Soil | 22 grains. |
| Loamy Clay | 26 grains. |
| Garden Soil | 45 grains. |
| Brickmakers' Clay | 30 grains. |
The effect of draining in overcoming drought, by admitting atmospheric vapor will, of course, be very much increased if the land be thoroughly loosened by cultivation, and especially if the surface be kept in an open and mellow condition.
In addition to the moisture received from the air, as above described, water is, in a porous soil, drawn up from the wetter subsoil below, by the same attractive force which acts to wet the whole of a sponge of which only the lower part touches the water;—as a hard, dry, compact sponge will absorb water much less readily than one which is loose and open, so the hard clods, into which undrained clay is dried, drink up water much less freely than they will do after draining shall have made them more friable.
The source of this underground moisture is the "water table,"—the level of the soil below the influence of the[pg 040] drains,—and this should be so placed that, while its water will easily rise to a point occupied by the feeding roots of the crop, it should yield as little as possible for evaporation at the surface.
Another source of moisture, in summer, is the deposit of dew on the surface of the ground. The amount of this is very difficult to determine, and accurate American experiments on the subject are wanting. Of course the amount of dew is greater here than in England, where Dr. Dalton, a skillful examiner of atmospheric phenomena, estimates the annual deposit of dew to equal a depth of five inches, or about one-fifth of the rain-fall. Water thus deposited on the soil is absorbed more or less completely, in proportion to the porosity of the ground.
The extent to which plants will be affected by drought depends, other things being equal, on the depth to which they send their roots. If these lie near the surface, they will be parched by the heat of the sun. If they strike deeply into the damper subsoil, the sun will have less effect on the source from which they obtain their moisture. Nothing tends so much to deep rooting, as the thorough draining of the soil. If the free water be withdrawn to a considerable distance from the surface, plants,—even without the valuable aid of deep and subsoil plowing,—will send their roots to great depths. Writers on this subject cite many instances in which the roots of ordinary crops "not mere hairs, but strong fibres, as large as pack-thread," sink to the depth of 4, 6, and in some instances 12 or 14 feet. Certain it is that, in a healthy, well aerated soil, any of the plants ordinarily cultivated in the garden or field will send their roots far below the parched surface soil; but if the subsoil is wet, cold, and soggy, at the time when the young crop is laying out its plan of future action, it will perforce accommodate its roots to the limited space which the comparatively dry surface soil affords.
It is well known among those who attend the meetings of the Farmers' Club of the American Institute, in New York, that the farm of Professor Mapes, near Newark, N.J., which maintains its wonderful fertility, year after year, without reference to wet or dry weather, has been rendered almost absolutely indifferent to the severest drought, by a course of cultivation which has been rendered possible only by under-draining. The lawns of the Central Park, which are a marvel of freshness, when the lands about the Park are burned brown, owe their vigor mainly to the complete drainage of the soil. What is true of these thoroughly cultivated lands, it is practicable to attain on all soils, which, from their compact condition, are now almost denuded of vegetation in dry seasons.
Porosity or Mellowness.—An open and mellow condition of the soil is always favorable for the growth of plants. They require heat, fresh air and moisture, to enable them to take up the materials on which they live, and by which they grow. We have seen that the heat of retentive soils is almost directly proportionate to the completeness with which their free water is removed by underground draining, and that, by reason of the increased facility with which air and water circulate within them, their heat is more evenly distributed among all those parts of the soil which are occupied by roots. The word moisture, in this connection, is used in contradistinction to wetness, and implies a condition of freshness and dampness,—not at all of saturation. In a saturated, a soaking-wet soil, every space between the particles is filled with water to the entire exclusion of the atmosphere, and in such a soil only aquatic plants will grow. In a dry soil, on the other hand, when the earth is contracted into clods and baked, almost as in an oven,—one of the most important conditions for growth being wanting,—nothing can thrive, save those plants which ask of the earth only an anchoring place, and seek their nourishment from the air. Both air[pg 042] plants and water plants have their wisely assigned places in the economy of nature, and nature provides them with ample space for growth. Agriculture, however, is directed to the production of a class of plants very different from either of these,—to those which can only grow to their greatest perfection in a soil combining, not one or two only, but all three of the conditions named above. While they require heat, they cannot dispense with the moisture which too great heat removes; while they require moisture, they cannot abide the entire exclusion of air, nor the dissipation of heat which too much water causes. The interior part of the pellets of a well pulverized soil should contain all the water that they can hold by their own absorptive power, just as the finer walls of a damp sponge hold it; while the spaces between these pellets, like the pores of the sponge, should be filled with air.
In such a soil, roots can extend in any direction, and to considerable depth, without being parched with thirst, or drowned in stagnant water, and, other things being equal, plants will grow to their greatest possible size, and all their tissues will be of the best possible texture. On rich land, which is maintained in this condition of porosity and mellowness, agriculture will produce its best results, and will encounter the fewest possible chances of failure. Of course, there are not many such soils to be found, and such absolute balance between warmth and moisture in the soil cannot be maintained at all times, and under all circumstances, but the more nearly it is maintained, the more nearly perfect will be the results of cultivation.
Chemical Action in the Soil.—Plants receive certain of their constituents from the soil, through their roots. The raw materials from which these constituents are obtained are the minerals of the soil, the manures which are artificially applied, water, and certain substances which are taken from the air by the absorptive action of the soil,[pg 043] or are brought to it by rains, or by water flowing over the surface from other land.
The mineral matters, which constitute the ashes of plants, when burned, are not mere accidental impurities which happen to be carried into their roots in solution in the water which supplies the sap, although they vary in character and proportion with each change in the mineral composition of the soil. It is proven by chemical analysis, that the composition of the ashes, not only of different species of plants, but of different parts of the same plant, have distinctive characters,—some being rich in phosphates, and others in silex; some in potash, and others in lime,—and that these characters are in a measure the same, in the same plants or parts of plants, without especial reference to the soil on which they grow. The minerals which form the ashes of plants, constitute but a very small part of the soil, and they are very sparsely distributed throughout the mass; existing in the interior of its particles, as well as upon their surfaces. As roots cannot penetrate to the interior of pebbles and compact particles of earth, in search of the food which they require, but can only take that which is exposed on their surfaces, and, as the oxydizing effect of atmospheric air is useful in preparing the crude minerals for assimilation, as well as in decomposing the particles in which they are bound up,—a process which is allied to the rusting of metals,—the more freely atmospheric air is allowed, or induced, to circulate among the inner portions of the soil, the more readily are its fertilizing parts made available for the use of roots. By no other process, is air made to enter so deeply, nor to circulate so readily in the soil, as by under-draining, and the deep cultivation which under-draining facilitates.
Of the manures which are applied to the land, those of a mineral character are affected by draining, in the same manner as the minerals which are native to the soil;[pg 044] while organic, or animal and vegetable, manures, (especially when applied, as is usual, in an incompletely fermented condition,) absolutely require fresh supplies of atmospheric air, to continue the decomposition which alone can prepare them for their proper effect on vegetation.
If kept saturated with water, so that the air is excluded, animal manures lie nearly inert, and vegetable matters decompose but incompletely,—yielding acids which are injurious to vegetation, and which would not be formed in the presence of a sufficient supply of air. An instance is cited by H. Wauer where sheep dung was preserved, for five years, by excessive moisture, which kept it from the air. If the soil be saturated with water in the spring, and, in summer, (by the compacting of its surface, which is caused by evaporation,) be closed against the entrance of air, manures will be but slowly decomposed, and will act but imperfectly on the crop,—if, on the other hand, a complete system of drainage be adopted, manures, (and the roots which have been left in the ground by the previous crop,) will be readily decomposed, and will exercise their full influence on the soil, and on the plants growing in it.
Again, manures are more or less effective, in proportion as they are more or less thoroughly mixed with the soil. In an undrained, retentive soil, it is not often possible to attain that perfect tilth, which is best suited for a proper admixture, and which is easily given after thorough draining.
The soil must be regarded as the laboratory in which nature, during the season of growth, is carrying on those hidden, but indispensable chemical separations, combinations, and re-combinations, by which the earth is made to bear its fruits, and to sustain its myriad life. The chief demand of this laboratory is for free ventilation. The[pg 045] raw material for the work is at hand,—as well in the wet soil as in the dry; but the door is sealed, the damper is closed, and only a stray whiff of air can, now and then, gain entrance,—only enough to commence an analysis, or a combination, which is choked off when half complete, leaving food for sorrel, but making none for grass. We must throw open door and window, draw away the water in which all is immersed, let in the air, with its all destroying, and, therefore, all re-creating oxygen, and leave the forces of nature's beneficent chemistry free play, deep down in the ground. Then may we hope for the full benefit of the fertilizing matters which our good soil contains, and for the full effect of the manures which we add.
With our land thoroughly improved, as has been described, we may carry on the operations of farming with as much certainty of success, and with as great immunity from the ill effects of unfavorable weather, as can be expected in any business, whose results depend on such a variety of circumstances. We shall have substituted certainty for chance, as far as it is in our power to do so, and shall have made farming an art, rather than a venture.