Part 27 (1/2)
[Ill.u.s.tration: Fig. 93.]
[Ill.u.s.tration: Fig. 94.]
”We pa.s.s on now to Fig. 93. Here we find a different state of matters. The ca.n.a.ls are open and freely supplied with air, while the pores are filled with water; and consequently you perceive that, while the seed _a_ has quite enough of air from the ca.n.a.ls, it can never be without moisture, as every particle of soil which touches it, is well supplied with this necessary ingredient. This, then, is the proper condition of soil for germination, and in fact for every period of the plant's development; and this condition occurs when soil is _moist_ but not _wet_--that is to say, when it has the color and appearance of being well watered, but when it is still capable of being crumbled to pieces by the hands, without any of its particles adhering together in the familiar form of mud.
”Turning our eyes to Fig. 94, we observe still another condition of soil. In this instance, as far as _water_ is concerned, the soil is in its healthy condition--it is moist, but not wet, the pores alone being filled with water. But where are the ca.n.a.ls? We see them in a few places, but in by far the greater part of the soil none are to be perceived; this is owing to the particles of soil having adhered together, and thus so far obliterated the interst.i.tial ca.n.a.ls, that they appear only like pores. This is the state of matters in every _clod of earth_, _b_; and you will at once perceive, on comparing it with _c_, which represents a stone, that these two differ only in possessing a few pores, which latter, while they may form a reservoir for moisture, can never act as vehicles for the _food_ of plants, as the roots are not capable of extending their fibres into the interior of a clod, but are at all times confined to the interst.i.tial ca.n.a.ls.
”With these four conditions before us, let us endeavor to apply them _practically_ to ascertain when they occur in our fields, and how those which are injurious may be obviated.
”The first of them, we perceive, is a state of too great dryness, _a very rare_ condition, in this climate at least; in fact, the only case in which it is likely to occur is in very coa.r.s.e sands, where the soil, being chiefly made up of pure sand and particles of flinty matter, contains comparatively much fewer pores; and, from the large size of the individual particles, a.s.sisted by their irregularity, the ca.n.a.ls are wider, the circulation of air freer, and, consequently, the whole is much more easily dried. When this state of matters exists, the best treatment is to leave all the stones which occur on the surface of the field, as they cast shades, and thereby prevent or r.e.t.a.r.d the evaporation of water.
”We will not, however, make any further observations on this very rare case, but will rather proceed to Fig. 92, a much more frequent, and, in every respect, more important condition of soil: I refer to an _excess of water_.
”When water is added to perfectly dry soil, it, of course, in the first instance, fills the interst.i.tial ca.n.a.ls, and from these enters the pores of each particle; and if the supply of water be not too great, the ca.n.a.ls speedily become empty, so that the whole of the fluid is taken up by the pores: this, we have already seen, is the _healthy_ condition of the soil. If, however, the supply of water be too great, as is the case when a spring gains admission into the soil, or when the sinking of the fluid through the ca.n.a.ls to a sufficient depth below the surface is prevented, it is clear that these also must get filled with water so soon as the pores have become saturated. This, then, is the condition of _undrained soil_.
”Not only are the pores filled, but the interst.i.tial ca.n.a.ls are likewise full; and the consequence is, that the whole process of the germination and growth of vegetables is materially interfered with. We shall here, therefore briefly state the injurious effects of an excess of water, for the purpose of impressing more strongly on your minds the necessity of thorough-draining, as the first and most essential step towards the improvement of your soil.
”The _first_ great effect of an excess of water is, that it produces a corresponding diminution of the amount of air beneath the surface, which air is of the greatest possible consequence in the nutrition of plants; in fact, if entirely excluded, germination could not take place, and the seed sown would, of course, either decay or lie dormant.
”_Secondly_, an excess of water is most hurtful, by reducing considerably the _temperature_ of the soil: this I find, by careful experiment, to be to the extent of six and a-half degrees Fahrenheit in Summer, which amount is equivalent to an elevation above the level of the sea of 1,950 feet.
”These are the two chief injuries of an excess of water in soil which affect the soil itself. There are very many others affecting the climate, &c.; but these not so connected with the subject in hand as to call for an explanation here.
”Of course, all these injurious effects are at once overcome by thorough-draining, the result of which is, to establish a direct communication between the interst.i.tial ca.n.a.ls and the drains, by which means it follows, that no water can remain any length of time in these ca.n.a.ls without, by its gravitation, finding its way into the drains.
”The 4th Fig. indicates badly-cultivated soil, or soil in which large unbroken clods exist; which clods, as we have already seen, are very little better than stones, on account of their impermeability to air and the roots of plants.
”Too much cannot be said in favor of pulverizing the soil; even thorough-draining itself will not supersede the necessity of performing this most necessary operation. The whole valuable effects of plowing, harrowing, grubbing, &c., may be reduced to this: and almost the whole superiority of _garden_ over _field_ produce is referable to the greater perfection to which this pulverizing of the soil can be carried.
”The whole success of the drill husbandry is owing, in a great measure, to its enabling you to stir up the soil well during the progress of your crop; which stirring up is of no value beyond its effects in more minutely pulverizing the soil, increasing, as far as possible, the size and number of the interst.i.tial ca.n.a.ls.
”Lest any one should suppose that the contents of these interst.i.tial ca.n.a.ls must be so minute that their whole amount can be of but little consequence, I may here notice the fact, that, in moderately well pulverized soil, they amount to no less than one-fourth of the whole bulk of the soil itself; for example, 100 cubic inches of _moist_ soil (that is, of soil in which the pores are filled with water while the ca.n.a.ls are filled with air), contain no less than 25 cubic inches of air. According to this calculation, in a field pulverized to the depth of eight inches, a depth perfectly attainable on most soils by careful tillage, every imperial acre will retain beneath its surface no less than 12,545,280 cubic inches of air. And, to take one more element into the calculation, supposing the soil were not properly drained, the sufficient pulverizing of an additional inch in depth would increase the escape of water from the surface by upwards of one hundred gallons a day.”
_Drainage improves the quality of crops._ In a dry season, we frequently hear the farmer boast of the quality of his products. His hay-crop, he says, is light, but will ”spend” much better than the crop of a wet season; his potatoes are not large, but they are sound and mealy.
Indeed, this topic need not be enlarged upon. Every farmer knows that his wheat and corn are heavier and more sound when grown upon land sufficiently drained.
_Drainage prevents drought._ This proposition is somewhat startling at first view. How can draining land make it more moist? One would as soon think of watering land to make it dry. A drought is the enemy we all dread. Professor Espy has a plan for producing rain, by lighting extensive artificial fires. A great objection to his theory is, that he cannot limit his showers to his own land, and all the public would never be ready for a shower on the same day. If we can really protect our land from drought, by under-draining it, everybody may at once engage in the work without offence to his neighbor.
If we take up a handfull of rich soil of almost any kind, after a heavy rain, we can squeeze it hard enough with the hand to press out drops of water. If we should take of the same soil a large quant.i.ty, after it was so dry that not a drop of water could be pressed out by hand, and subject it to the pressure of machinery, we should force from it more water. Any boy, who has watched the process of making cider with the old-fas.h.i.+oned press, has seen the pomace, after it had been once pressed apparently dry and cut down, and the screw applied anew to the ”cheese,”
give out quant.i.ties of juice. These facts ill.u.s.trate, first, how much water may be held in the soil by attraction. They show, again, that more water is held by a pulverized and open soil, than by a compact and close one. Water is held in the soil between the minute particles of earth. If these particles be pressed together compactly, there is no s.p.a.ce left between them for water. The same is true of soil naturally compact. This compactness exists more or less in most subsoils, certainly in all through which water does not readily pa.s.s. Hence, all these subsoils are rendered more permeable to water by being broken up and divided; and more retentive by having the particles of which they are composed separated, one from another--in a word, by pulverization. This increased capacity to contain moisture by attraction, is the greatest security against drought. The plants, in a dry time send their rootlets throughout the soil, and flourish in the moisture thus stored up for their time of need. The pulverization of drained land may be produced, partly by deep, or subsoil plowing, which is always necessary to perfect the object of thorough-draining; but it is much aided, in stiff clays, also, by the shrinkage of the soil by drying.
Drainage resists drought, again, by the very deepening of the soil of which we have already spoken. The roots of plants, we have seen, will not extend into stagnant water. If, then, as is frequently the case, even on sandy plains, the water-line be, in early Spring, very near the surface, the seed may be planted, may vegetate, and throw up a goodly show of leaves and stalks, which may flourish as long as the early rains continue; but, suddenly, the rains cease; the sun comes out in his June brightness; the water-line lowers at once in the soil; the roots have no depth to draw moisture from below, and the whole field of clover, or of corn, in a single week, is past recovery. Now, if this light, sandy soil be drained, so that, at the first start of the crop, there is a deep seed-bed free from water, the roots strike downward, at once, and thus prepare for a drought. The writer has seen upon deep-trenched land in his own garden, parsnips, which, before midsummer, had extended downward three feet, before they were as large as a common whiplash; and yet, through the Summer drought, continued to thrive till they attained in Autumn a length, including tops, of about seven feet, and an extraordinary size. A moment's reflection will satisfy any one that, the dryer the soil in Spring, the deeper will the roots strike, and the better able will be the plant to endure the Summer's drought.
Again, drainage and consequent pulverization and deepening of the soils increase their capacity to absorb moisture from the atmosphere, and thus afford protection against drought. Watery vapor is constantly, in all dry weather, rising from the surface of the earth; and plants, in the day-time, are also, from their leaves and bark, giving off moisture which they draw from the soil. But Nature has provided a wonderful law of compensation for this waste, which would, without such provision, parch the earth to barrenness in a single rainless month.
The capacity of the atmosphere to take up and convey water, furnishes one of the grandest ill.u.s.trations of the perfect work of the Author of the Universe. ”All the rivers run into the sea, yet the sea is not full;” and the sea is not full, because the numerous great rivers and their millions of tributaries, ever flowing from age to age, convey to the ocean only as much water as the atmosphere carries back in vapor, and discharges upon the hills. The warmer the atmosphere, the greater its capacity to hold moisture. The heated, thirsty air of the tropics drinks up the water of the ocean, and bears it away to the colder regions, where, through condensation by cold, it becomes visible as a cloud; and as a huge sponge pressed by an invisible hand, the cloud, condensed still further by cold, sends down its water to the earth in rain.