DRAINAGE OF STIFF CLAYS.
Clay not impervious, or it could not be wet and dried.--Puddling,
what is.--Water will stand over Drains on Puddled Soil.--Cracking
of Clays by Drying.--Drained Clays improve by time.--Passage of
Water through Clay makes it permeable.--Experiment by Mr.
Pettibone, of Vermont.--Pressure of Water in saturated Soil.
It is a common impression that clay is impervious to water, and that,
therefore, a clay soil cannot be drained, especially by deep
under-drains. A moment's reflection will satisfy any one that such land
is not absolutely impervious. We find such land is wet in Spring, at any
depth; and, in the latter part of Summer, we find it comparatively dry.
How comes it wet, at any time, if water does not go into it? And how
comes it dry, at any time, if water does not come out of it?
In treating of the power of the soil to absorb moisture, we have shown
that a clay soil will absorb more than half its weight and bulk of
water, and that it holds more water than any other soil, with, perhaps,
the single exception of peat.
The facts, however, that clay may be wet, and may be dried, and that it
readily absorbs large quantities of water, though they prove
conclusively that it is not impervious to water, yet do not prove that
water will pass through it with sufficient rapidity to answer the
practical purposes of drainage for agriculture. This point can only be
satisfactorily determined by experiment. It is not necessary, however,
that each farmer should try the experiment for himself; because,
although we are very apt to think our own case an exception to all
general rules, it is not really probable that any new kind of clay will
be discovered hereafter, that is so different from all other clay that
is known, that established principles will not apply to it. So far as
our own observation extends, owners of clay farms always over-estimate
the difficulty of draining their land. There are certain notorious facts
with regard to clay, which mislead the judgment of men on this point.
One of these facts is, that clay is used for stopping water, by the
process called _puddling_. Puddled clay is used for the bottom of ponds,
and of canals, and of reservoirs, and, for such purposes, is regarded as
nearly, or quite impervious.
We see that, on our clay fields, water stands upon the surface,
especially in the ruts of wheels, and on headlands much trodden, late in
the season, and when, in other places, it has disappeared. This is due,
also, to puddling.
Puddling is merely the working of wet clay, or other soil, by beating,
or treading, or stirring, until its particles are so finely divided that
water has an exceedingly slow passage between them, with ordinary
pressure. We see the effect of this operation on common highways, where
water often stands for many days in puddles, because the surface has
been ground so fine, and rendered so compact, by wheels and horses, that
the water cannot find passage. This, however, is not the natural
condition of any clay; nor can any clay be kept in this condition,
except by being constantly wet. If once dried, or subjected to the
action of frost, the soil resumes its natural condition of porosity, as
will be presently explained. They who object to deep drainage, or to the
possibility of draining stiff clays, point to the fact that water may be
seen standing directly over the drains, on thorough-drained fields. We
have seen this on our own fields. In one instance, we had, after laying
tiles through a field, at 50 feet intervals, in the same Autumn, when
the land was wet, teamed across it a large quantity of soil for compost,
with a heavy ox-team. The next Spring, the water stood for many days in
that track which passed across tile-drains, after it had disappeared
elsewhere in the field. A fine crop of Indian corn grew on the field
that year, but the effect of the puddling was visible the whole season.
"One inch of wet and worked clay," says a scientific writer, "will
prevent water from passing through, so long as it is kept wet, as
effectually as a yard will do."
"If," says Gisborne, "you eat off turnips with sheep, if you plow
the land, or cart on it, or in any way puddle it, when it is wet,
of course the water will lie on the surface, and will not go to
your drains. A four-foot drain may go very near a pit, or a
water-course, without attracting water from either, because
water-courses almost invariably puddle their beds; and the same
effect is produced in pits by the treading of cattle, and even by
the motion of the water produced by wind. A very thin film of
puddle, always wet on one side, is impervious, _because it cannot
crack_."
In those four words, we find an allusion to the whole mystery of the
drainage of clays--a key which unlocks the secret by which the toughest
of these soils may be converted, as by a fairy charm, to fields of
waving grain.
CRACKING OF CLAYS BY DRYING.
"In drying under the influence of the sun," says Prof. Johnston, "soils
shrink in, and thus diminish in bulk, in proportion to the quantity of
clay, or of peaty matter, they contain. Sand scarcely diminishes at all
in bulk by drying; but peat shrinks one-fifth in bulk, and strong
agricultural clay nearly as much." By laying drains in land, we take
from it that portion of the water that will run out at the bottom. The
sun, by evaporation, then takes out a portion at the top. The soil is
thus contracted, and, as the ends of the field cannot approach each
other, both soil and subsoil are torn apart, and divided by a network
of cracks and fissures. Every one who is familiar with clay land, or who
has observed the bottom of a ditch or frog pond by the roadside, must
have observed these cracks, thus caused by the contraction of the soil
in drying. The same contraction occurs in drier land, by cold, in
Winter; by which, in cold regions, deep rents are made in the earth, and
reports, like those of cannon, are often heard. The cracking by drying,
however, is more quiet in its effects, merely dividing the ground,
noiselessly, into smaller and smaller masses, as the process proceeds.
Were it not for this process, it may well be doubted whether clay lands
could be effectually drained at all. Nature, however, seems to second
our efforts here, for we have seen that the stiffer the clay, the
greater the contraction, and the more the soil is split up and rendered
permeable by this operation.
These cracks are found, by observation, to commence at the drains, and
extend further and further, in almost straight lines, into the subsoil,
forming so many minor drains, or feeders, all leading to the tiles.
These main fissures have numerous smaller ones diverging from them, so
that the whole mass is divided and subdivided into the most minute
portions. The main fissures gradually enlarge, as the dryness increases,
and, at the same time, lengthen out; so that, in a very dry season, they
may be traced the whole way between the drains. The following cut will
give some idea of these cracks, or fissures, as they exist in a dry
time:
[Illustration: Fig. 98.--Cracking of Clays by Drainage.]
Mr. Gisborne says: "Clay lands always shrink and crack with
drought; and the stiffer the clay, the greater the shrinking, as
brick-makers well know. In the great drought thirty-six 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 a
network of cracks. In the drained soil, the roots follow the
threads of vegetable mould which have been washed into the cracks,
and get an abiding tenure. Earth-worms follow either the roots or
the mould. Permanent schisms are established in the clay, and its
whole character is changed."
In the United States, the supply of rain is far less uniform than in
England, and much severer droughts are experienced. Thus the
contraction, and consequent cracking of the soil, must be greater here
than in that country.
In laying drains more than four feet deep, in the stiffest clay which
the author has seen, in a neighborhood furnishing abundance of brick and
potter's clay, these cracks were seen to extend to the very bottoms of
the drains, not in single fissures from top to bottom, but in
innumerable seams running in all directions, so that the earth, moved
with the pick-axe, came up in little cubes and flakes, and could be
separated into pieces of an inch or less diameter. This was on a ridge
which received no water except from the clouds, having no springs in or
upon it, yet so nearly impervious to water, that it remained soft and
muddy till late in June. In Midsummer, however, under our burning sun,
it had, by evaporation, been so much dried as to produce the effect
described.
In England, we learn, that these cracks extend to the depth of four
feet or more. Mr. Hewitt Davis stated in a public discussion, with
reference to draining strong soils, that, "he gave four feet as the
minimum depth of the drains in these soils, because he had always
found that the cracks and fissures formed by the drought and
changes of temperature, on the strongest clay, and which made these
soils permeable, extended below this depth, and the water from the
surface might be made to reach the drains at this distance."
In clay that has never been dried, as for instance, that found under wet
meadows from which the water has but recently been drawn, we should not,
of course, expect to find these cracks. Accordingly, we find sometimes
in clay pits, excavated below the permanent water-line, and in wells,
that the clay is in a compact mass, and tears apart without exhibiting
anything like these divisions.
We should not expect that, on such a clay, the full effect of drainage
would be at once apparent. The water falling on the surface would very
slowly find its way downward, at first. But after the heat of Summer,
aided by the drains underneath, had contracted and cracked the soil,
passages for the water would soon be found, and, after a few years, the
whole mass, to the depth of the drains, would become open and permeable.
As an old English farmer said of his drains, "They do better year by
year; the water gets a habit of coming to them." Although this be not
philosophical language, yet the fact is correctly stated. Water tends
towards the lowest openings. A deep well often diverts the underground
stream from a shallower well, and lays it dry. A single railroad cut
sometimes draws off the supply of water from a whole neighborhood.
Passages thus formed are enlarged by the pressure of the water, and new
ones are opened by the causes already suggested, till the drainage
becomes perfect for all practical purposes. So much is this cracking
process relied on to facilitate drainage, that skillful drainers
frequently leave their ditches partly open, after laying the tiles, that
the heat may produce the more effect during the first season.
As to the depth of drains in stiff clays, enough has already been said,
under the appropriate title. In England, the weight of authority is in
favor of four-foot drains. In this country, a less depth has thus far,
in general, been adopted in practice, but it is believed that this has
been because a greater depth has not been tried. It is understood, that
the most successful drainers in the State of New York, have been
satisfied with three-foot drains, not, as it is believed, because there
is any instance on record, in this country, of the failure of four-foot
drains, but because the effect of more shallow drains has been so
satisfactory, that it has been thought a useless expense to go deeper.
To Mr. Johnston and to Mr. Delafield, of Seneca County, the country is
greatly indebted for their enterprise and leadership in the matter of
drainage. Mr. Johnston gives it as his opinion, that "three feet is deep
enough, if the bottom is hard enough to lay tiles on; if not, go
deeper."
Without intimating that any different mode of drainage than that
adopted, would have been better on Mr. Johnston's farm, we should be
unwilling to surrender, even to the opinion of Mr. Johnston and his
friends, our conviction that, in general, three-foot drains are too
shallow. Mr. Johnston expressly disclaims any experience in draining a
proper clay soil. In the _Country Gentleman_, of June 10th, 1848, he
says:
"In a subsoil that is impervious to water, either by being a red
clay, blue clay, or hard-pan, within a foot of the surface, I would
recommend farmers to feel their way very cautiously in draining. If
tiles and labor were as low here as in Great Britain, we could
afford to make drains sixteen feet apart in such land, and then, by
loosening the soil, say twenty inches deep, by the subsoil plow, I
think such land might be made perfectly dry; but I don't think the
time is yet come, considering the cost of tiles and labor, to
undertake such an outlay; but still it might pay _in the end_. I
have found only a little of red clay subsoil in draining my farm. I
never had any blue clay on my farm, or hard-pan, to trouble me; but
I can readily perceive that it must be equally bad to drain as the
tenacious red clay. If I were going to purchase another farm, I
would look a great deal more to the subsoil than the surface soil.
If the subsoil is right, the surface soil, I think, cannot be
wrong."
In the same paper, under date of July 8th, Mr. Johnston says, "The only
experience I have had in digging into soils, to judge of draining out of
this county (Seneca), was in Niagara." He states the result of his
observations thus:
"A few inches below the surface I found a stiff blue clay for about
ten inches deep, and as impervious to water as so much iron.
Underneath that blue clay, I found a red clay, apparently
impervious to water; but, as water could not get through the blue,
I could only guess at that; and, after spending the greater part of
the day, with five men digging holes from four to five feet deep, I
found I knew no more how such land could be drained, than a man who
had never seen a drain dug. I advised the gentleman to try a few
experiments, by digging a few ditches, as I laid them out, and
plowing as deep as possible with a subsoil plow, but to get no tile
until he saw if he could get a run of water. He paid my traveling
expenses, treated me very kindly and I have heard nothing from him
since.
"Now, if your correspondent's soil and subsoil is similar to that
soil I would advise him to feel his way cautiously in draining.
Certainly, no man would be fool enough to dig ditches and lay tile,
if there is no water to carry off."
In the _Country Gentleman_ of Nov. 18th, 1858, we find an interesting
statement, by John S. Pettibone, of Manchester, Vermont, partly in reply
to the statement of Mr. Johnston.
The experiment by Mr. Pettibone, showing the increased permeability of
clay, merely by the passage of water through it, is very interesting. He
says, in his letter to the editor:
"When so experienced a drainer as Mr. Johnston expresses an opinion
that some soils cannot be drained, it is important we should know
what the soil is which cannot be drained. He uses the word _stiff
blue_ clay, as descriptive of the soil which cannot be drained. * * *
"I had taken a specimen of what I thought to be _stiff blue clay_.
That clay, when wet, as taken out, would hold water about as well
as iron: yet, from experiments I have made, I am confident that
such clay soil can be drained, and at much less expense than a
hard-pan soil. Water will pass through such clay, and the clay
become dry; and after it becomes once dry, water will, I am
convinced, readily pass down through such stiff blue clay. The
specimen was taken about three feet below the surface, and on a
level with a brook which runs through this clay soil. I filled a
one hundred-pound nail-keg with clay taken from the same place. It
was so wet, that by shaking, it came to a level, and water rose to
the top of the clay. I had made holes in the bottom of the keg, and
set it up on blocks. After twenty-four hours I came almost to the
conclusion Mr. Johnston did, that water would not pass through this
clay. This trial was during the hot, dry weather last Summer. After
some ten or twelve days the clay appeared to be dry. I then made a
basin-like excavation in the top of the clay, and put water in, and
the water disappeared rather slowly. I filled the basin with water
frequently, and the oftener I filled it, the more readily it passed
off. I left it for more than a week, when we had a heavy shower.
After the shower I examined the keg, and not a drop of water was to
be seen. I then took a chisel and cut a hole six inches down. I
took out a piece like the one I dried in the house, and laid that
up till it was perfectly dry. There was a plain difference between
the appearance of the two pieces. The texture, I should say, was
quite different. That through which the water had passed, after it
had been dried, was more open and porous. It did not possess so
much of the blue cast. In less than one hour after the rain fell,
the clay taken six inches from the top of the keg would crumble by
rubbing in the hand."
When we observe the effect of heat in opening clays to water by
cracking, and the effect of the water itself, aided, as it doubtless is,
by the action of the air, in rendering the soil permeable, we hardly
need feel discouraged if the question rested entirely on this evidence;
but when we consider that thousands upon thousands of acres of the
stiffest clays have been, in England and Scotland, rescued from utter
barrenness by drainage, and made to yield the largest crops, we should
regard the question of practicability as settled. The only question left
for decision is whether, under all the circumstances of each
particular case, the operation of draining our clay lands will be
expedient--whether their increased value will pay the expense. It is
often objected to deep drains in clays, that it is so far down to the
drains that the water cannot readily pass through so large a mass. If
we think merely of a drop of rain falling on the surface, and obliged to
find its devious way through the mazes of cracks and particles till it
gains an outlet at the bottom of four feet of clay, it does seem a
discouraging journey for the poor little solitary thing; but there is a
more correct view of the matter, which somewhat relieves the difficulty.
All the water that will run out of the soil has departed; but the soil
holds a vast amount still, by attraction. The rain begins to fall; and
when the soil is saturated, a portion passes into the drain; but it is,
by no means, the water which last fell upon the surface, but that which
was next the drain before the rain fell. If you pour water into a tube
that is nearly full, the water which will first run from the other end
is manifestly not that which you pour in. So the ground is full of
little tubes, open at both ends, in which the water is held by
attraction. A drop upon the surface drives out a drop at the lower end,
into to the drain, and so the process goes on--the drains beginning to
run as soon as the rain commences, and ceasing to flow only when the
principle of attraction balances the power of gravitation.
PRESSURE OF WATER IN THE SOIL.
In connection with the passage of water through clay soil, it may be
appropriate to advert to the question sometimes mooted, whether in a
soil filled with water, at four feet depth, there is the same pressure
as there would be, at the same depth, in a river or pond. The pressure
of fluids on a given area, is, ordinarily, in proportion to their
vertical height; and the pressure of a column of water, four feet high,
would be sufficient to drive the lower particles into an opening like a
drain, with considerable force, and the upper part of such a column
would essentially aid the lower part in its downward passage. Does this
pressure exist? Mr. Gisborne speaks undoubtingly on this point, thus:
"We will assume the drain to be four feet deep, and the water-table
to be at one foot below the surface of the earth. Every particle of
water which lies at three feet below the water-table, has on it the
pressure of a column of water three feet high. This pressure will
drive the particle in any direction in which it finds no
resistance, with a rapidity varying inversely to the friction of
the medium through which the column acts. The bottom of our drains
will offer no resistance, and into it particles of water will be
pushed, in conformity with the rule we have stated; rapidly, if the
medium opposes little friction; slowly, if it opposes much. The
water so pushed in runs off by the drain, the column of pressure
being diminished in proportion to the water which runs off."
Mr. Thomas Arkell, in a paper read before the Society of Arts, in 1855,
says, on this point:
"The pressure due to a head of water of four or five feet, may be
imagined from the force with which water will come through the
crevices of a hatch, with that depth of water above it. Now, there
is the same pressure of water to enter the vacuum in the
pipe-drain, as there is against the hatches, supposing the land to
be full to the surface."
We do not find any intimation that there is any error in the view
advanced by the learned gentleman quoted; and if there is none, we have
an explanation of the faculty which water seems to have, of finding its
way into drainpipes. Yet, we feel bound to confess, that, aside from
authority, we should have supposed that the pressure due to a column of
pure water, would be essentially lessened, by the interposition of solid
matter between its particles.