Farm drainage Part 29

CHAPTER XVI.

POWER OF SOILS TO ABSORB AND RETAIN MOISTURE.

Why does not Drainage make the Land too Dry?--Adhesive Attraction.--The Finest Soils exert most Attraction.--How much Water different Soils hold by Attraction.--Capillary Attraction, Ill.u.s.trated.--Power to Imbibe Moisture from the Air.--Weight Absorbed by 1,000 lbs. in 12 Hours.--Dew, Cause of.--Dew Point.--Cause of Frost.--Why Covering Plants Protects from Frost.--Dew Imparts Warmth.--Idea that the Moon Promotes Putrefaction.--Quant.i.ty of Dew.

The first and most natural objection made, by those not practically familiar with drainage operations, to the whole system is, that the drains will draw out so much of the water from the soil, as to leave it too dry for the crops.

If a cask be filled with round stones, or with musket b.a.l.l.s, or with large shot, and with water to the surface, and then an opening be made at the bottom of the cask, all the water, except a thin film adhering to the surface of the vessel and its contents, will immediately run out.

If now, the same cask be filled with the dried soil of any cultivated field, and this soil be saturated with water, a part only of the water can be drawn out at the bottom. The soil in the cask will remain moist, retaining more or less of the water, according to the character of the soil.

Why does not the water all run out of the soil, and leave it dry? An answer may be found in the books, which is, in reality, but a re-statement of the fact, by reference to a principle of nature, by no means intelligible to finite minds, called attraction. If two substances are placed in close contact with each other, they cannot be separated without a certain amount of force.

"If we wet the surfaces of two pieces of gla.s.s, and place them in contact, we shall find that they adhere to each other, and that, independently of the effect of the pressure of the air, they oppose considerable resistance to any attempt to separate them. Again, if we bring any substance, as the blade of a knife, in contact with water, the water adheres to the blade in a thin film, and remains, by what is termed _adhesive attraction_. This property resides in the surface of bodies, and is in proportion to the extent of its surface.

"Soils possess this property, in common with all other bodies, and possess it, in a greater or less degree, according to the aggregate surface which the particles of a given bulk present. Thus, clay may, by means of kneading, be made to contain so large a quant.i.ty of water, as that, at last, it may almost be supposed to be divided into infinitesimally thin layers, having each a film of water adhering to it on either side. Such soils, again, as sand or chalk, the particles of which are coa.r.s.er exert a less degree of adhesive attraction for water."--_Cyc. of Ag._, 695.

Professor Schubler, of Tubingen, gives the results of experiments upon this point. By dropping water upon dried soils of different kinds, until it began to drop from the bottom, he found that 100 lbs. of soil held by attraction, as follows:

Sand 25 lbs. of water.

Loamy Soil 40 "

Clay Loam 50 "

Pure Clay 70 "

Mr. Shedd, of Boston, gives the result of a recent experiment of his own on this point. He writes thus:

"I have made an experiment with a soil of ordinary tenacity, to ascertain how much water it would hold in suspension, with the following result: One cubic foot of earth held 0.4826434 cubic feet of water; three feet of dry soil of that character will receive 1.44793 ft. vertical depth of water before any drains off, or seventeen and three-quarter inches, equal to nearly six month's rain-fall. One cubic foot of earth held 3.53713 gallons of water, or if drains are three feet deep, one square foot of surface would receive 10.61 gallons of water, before saturation. Other soils would sustain a greater or less quant.i.ty, according to their character."

Besides this power of retaining water, when brought into contact with it, the soil has, in common with other porous bodies, the power of drawing up moisture, or of absorbing it, independent of gravitation, or of the weight of the water which aids to carry it down into the soil.

This power is called _capillary attraction_, from the hair-like tubes used in early experiments. If very minute tubes, open at both ends, are placed upright, partly immersed in a vessel of water, the water rises in the tubes perceptibly higher than its general surface in the vessel. A sponge, from which water has been pressed out, held over a basin of water, so that its lower part touches the surface, draws up the water till it is saturated. A common flower-pot, with a perforated bottom, and filled with dry earth, placed in a saucer of water, best ill.u.s.trates this point. The water rises at once to a common level in the pot and outside. This represents the water-table in the soil of our fields. But, from this level, water will continue to rise in the earth in the pot, till it is moistened to the surface, and this, too, is by capillary attraction.

The tendency of water to ascend, however, is not the same in all soils.

In coa.r.s.e gravelly soils, the principle may not operate perfectly, because the interstices are too large, the weight of the water overcoming the power of attraction, as in the cask of stones or shot. In very fine clay, on the other hand, although it be absorptive and retentive of water, yet the particles are so fine, and the s.p.a.ces between them so small, that this attraction, though sure, would be slow in operation. A loamy, light, well pulverized soil, again, would perhaps furnish the best medium for the diffusion of water in this way.

It is impossible to set limits to so uncertain a power as this of capillary attraction. We see that in minute gla.s.s tubes, it has power to raise water a small fraction of an inch only. We see that, in the sponge or flower-pot, it has power to raise water many inches; and we know that, in the soil, moisture is thus attracted upwards several feet.

By observing a saturated sponge in a saucer, we shall see that, although moist at the top, it holds more and more water to the bottom. So, in the saturated earth in a flower-pot, the earth, merely moist at the surface, is wet mud just above the water-table. So, in drained land, the capillary force which retained the water in the soil to the height of a few inches, is no longer able to sustain it, when the height is increased to feet, and a portion descends into the drain, leaving the surface comparatively dry.

Thus, it would seem, that draining may modify the force of capillary attraction, while it cannot affect that of adhesive attraction. It may drain off surplus water, but, unaided, can never render any arable land too dry. If, however, the surplus water be speedily taken off by drainage, and the capillary attraction be greatly impaired, so that little water is drawn upwards by its force, will not the soil soon become parched by the heat of the sun, or, in other words, by evaporation?

Without stopping in this place, to speak of evaporation, we may answer, that, in our burning Summer heat, the earth would be burnt up too dry for any vegetation, were it not for a beneficent arrangement of Providence, which counteracts the effect of the sun's rays, and of which we will now make mention.

_Power to imbibe moisture from the air._--We have spoken, in another place, of the absorption, by drained land, of fertilizing substances from the atmosphere. Dry soil has, too, a wonderful power of deriving moisture from the same source.

"When a portion of soil," says Johnston, "is dried carefully over boiling water, or in an oven, and is then spread out upon a sheet of paper in the open air, it will gradually drink in watery vapor from the atmosphere, and will thus increase in weight.

"In hot climates and in dry seasons, this property is of great importance, restoring as it does, to the thirsty soil, and bringing within the reach of plants, a portion of the moisture, which, during the day, they had so copiously exhaled."

Different soils possess this power in unequal degrees. During a night of 12 hours, and when the air is moist, according to Schubler, 1000 lbs. of perfectly dry

Quartz sand will gain 0 lbs.

Calcareous sand 2 "

Loamy soil 21 "

Clay loam 25 "

Pure agricultural clay 27 "

Sir Humphrey Davy found, that the power of attraction for water, generally proved an index to the agricultural value of soils. It is, however, but one means of judging of their value. Peaty soils and strong clays are very absorbent of water, although not always the best for cultivation.

Sir H. Davy gives the following results of his experiments. When made perfectly dry, 1000 lbs. of a

Very fertile soil from East Lothian, gained in an hour 18 lbs.

Very fertile soil from Somersetshire 16 "

Soil, worth 45s., (rent) from Ess.e.x 13 "

Sandy soil, worth 28s., from Ess.e.x 11 "

Coa.r.s.e sand, worth 15s. 8 "

Soil of Bagshot Heath 3 "

"This sort of attraction, however," suggests a writer in the Cyclopedia of Agriculture, "it may be believed, depends upon other causes besides the attraction of adhesion. The power of attraction, which certain substances exhibit for the _vapor_ of water, is more akin to the force which enables certain porous bodies to absorb and retain many times their volume of the different gases; as charcoal, of ammonia, of which it is said to absorb ninety times its own bulk."

Here again, we find in the soil, an inexplicable but beneficent power, by which it supplies itself with moisture when it most needs it.

Warm air is capable of holding more vapor than cooler air, and the very heat of Summer supplies it with moisture by evaporation from land and water. As the air is cooled, at nightfall, it must somewhere deposit the water, which the hand of the Unseen presses out of it by condensation.

The sun-dried surface of fertile, well drained soil, is in precisely the condition best adapted to receive the refreshing draught, and convey it to the thirsting plants.

We may form some estimate of the vast amount absorbed by an acre of land in a dry season, by considering that the clay loam, in the above statement, absorbed in 12 hours a fortieth part of its own weight.

OF DEW.

Dew is one of the most ordinary forms in which moisture is deposited in and upon the soil, in its natural conditions. The absorbent power of artificially-dried soils, as has been seen, seems to depend much upon their chemical const.i.tution; and that topic has been considered, without special reference to the comparative temperature of the soil and atmosphere. The soil, as we have seen, absorbs moisture from the air, when both are of the same temperature, the amount absorbed depending also upon the physical condition of the soil, and upon the comparative moisture of the soil and atmosphere.

The deposition of dew results from a different law. All bodies throw off, at all times, heat, by radiation, as it is termed. In the day-time, the sun's rays warm the earth, and the air is heated by it, and that nearest the surface is heated most. Evaporation is constantly going on from the earth and water, and loads the air with vapor, and the warmer the air, the more vapor it will hold.

When the sun goes down, the earth still continues to throw off heat by radiation, and soon becomes cooler than the air, unless the same amount of heat be returned, by radiation from other surfaces. Becoming cooler than the air, the soil or plants cool the air which comes in contact with them; and thus cooled to a certain point, the air cannot hold all the vapor which it absorbed while warmer, and part of it is deposited upon the soil, plant, or other cool surface. This is dew; and the temperature at which the air is saturated with vapor, is called the dew-point. If saturated at a given temperature with vapor, the air, when cooled below this point, must part with a portion of the vapor, in some way; in the form of rain or mist, if in the air; in the form of dew, if on the surface of the earth.

If, however, other surfaces, at night, radiate as much heat back to the earth as it throws off, the surface of the earth is not thus cooled, and there is no dew. Clouds radiate heat to the earth, and, therefore, there is less dew in cloudy than in clear nights. If the temperature of the earth sinks below the freezing-point, the aqueous vapor is frozen, and is then called _frost_.

To radiate back a portion of the heat thus thrown off by the soil and plants, gardeners cover their tender plants and vines with mats or boards, or even with thin cloth, and thus protect them from frost. If the covering touch the plants, they are often frozen, the heat being conducted off, by contact, to the covering, and thence radiated. Dew then is an effect, but not a cause, of cold. It imparts warmth, because it can be deposited only on objects cooler than itself.

It has been supposed by many that the light of the moon promotes putrefaction. Pliny and Plutarch both affirm this to be true. Dew, by supplying moisture in the warm season, aids this process of decay. We have seen that dew is most abundant in clear nights; and although all clear nights are not moonlight nights, yet all moonlight nights are clear nights; and this, perhaps, furnishes sufficient grounds for this belief, as to the influence of the moon.

The quant.i.ty of dew deposited is not easily measured. It has, however, been estimated by Dr. Dalton, to amount, in England, to five inches of water in a year, or 500 tons to the acre, equal to about one quarter of our rain-fall during the six summer months!

Deep and well-pulverized soils attract much more moisture, in every form, from the atmosphere, than shallow and compact soils. They, in fact, expose a much larger surface to the air. This is the reason why stirring the ground, even in the Summer drought, refreshes our fields of Indian corn.