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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 evapora tion, 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; Evapora tion; Temperature; Drought; Porosity or Mellowness; and Chemical Action.

ABSORPTION AND FILTRATION.-The process of underdraining is a process of absorption and filtration, as distinguished from surface-flow and evaporation. The com pleteness 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, there fore the extent to which under-draining is successful, de

pend 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:

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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 divived into large masses, or clods, sep arated 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 propor. tion 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

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.

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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 ig"norant 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 66 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 fol "low the threads of vegetable mold which have been "washed into the cracks, and get an abiding tenure. Earth

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Permanent

worms follow either the roots or the mold. "schisms are established in the clay, and its whole charao"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."

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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."

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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

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