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,
Illustrated.--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.--Quantity 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 balls, 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 glass, 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 quantity
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 coarser exert a less degree of adhesive
attraction for water."--_Cyc. of Ag._, 695.
Professor Schuebler, 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 quantity, 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 illustrates
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 coarse 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 spaces
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 glass 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 Schuebler, 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 Essex 13 "
Sandy soil, worth 28s., from Essex 11 "
Coarse 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 constitution; 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 quantity 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.