LAND-DRAINAGE
“In the bottom of a flower-pot you will find a small round hole. Over
this hole it is customary to lay a bit of broken tile, and on this, if
the plant to occupy the pot is delicate, a few small pebbles. This
done, the pot is filled with vegetable mold. Why this hole, this bit of
tile, these pebbles? That is what we are now about to consider.
“Water is absolutely indispensable to plants, since it is the medium
that dissolves the various nutritive ingredients of the soil and thus
renders them capable of assimilation by the roots. Accordingly the soil
penetrated by these roots must be constantly supplied with sufficient
moisture either by rainfall or by artificial irrigation. But air is not
less indispensable. It disinfects the soil and by causing slow
combustion of the humus gives rise to a slight but uninterrupted
liberation of carbonic acid gas, one of the nutritive substances
required by vegetation. Should the roots be cut off from this
life-giving agency, they would languish and finally decay. Thus it is
that if vegetation is to thrive the soil in which it grows must have at
the same time both air and water. But if the bottom of the flower-pot
has no opening, or if its opening is stopped up, the water from the
watering-can will not flow through, nor will there be any air admitted
from below, and for lack of this the roots will decay. On the other
hand, if the water, after saturating the earth, runs out freely by the
hole in the bottom of the pot, the damp soil will become a sort of
sponge to which the air will have access from all sides, and the plant
will thrive.
“This reasoning applies to the most extensive agricultural operations
as well as to the care of a potted plant. After water has soaked into
the ground it should find some channel to carry it off; otherwise the
roots will decay for want of air. That is why clayey soils, which
retain water when they are once saturated, are unsuited to agriculture,
while light soils, having sand mixed with the clay and thus readily
allowing the water to drain off, are well adapted to it. For the same
reason, again, a sandy subsoil accelerates vegetation, and a clayey
subsoil retards it. A sandy subsoil offers the same advantage as a
flower-pot open at the bottom, whereas a clayey subsoil is like a
flower-pot closed at the bottom. In the first case the surplus of water
drains off and the air has free access; in the second the superabundant
moisture finds no outlet and the air cannot reach the roots.
“Now let us suppose we have a marshy soil to deal with. Because of the
stagnant water either on the surface or a little below it nothing can
grow on this piece of ground except rushes or other hardy plants
designed by nature for this kind of soil. Accordingly we proceed to dig
a number of small ditches, of a depth somewhat greater than that
attained by plant-roots, and we fill the bottom of these ditches with
small stones, on which we finally throw back the earth we have removed.
These underground ditches are suitably inclined, and all empty at the
lower end into a main canal. The water saturating the soil collects in
these ditches, filters through the layer of pebbles, and empties into
the main canal, which carries it off to some river or other stream. Our
marshy soil is now like the potful of earth with a hole at the bottom,
the bit of broken tile, and layer of little pebbles: the air has free
access and brings fertility with it. This operation of ours is called
drainage, a word formed from ‘drain,’ which is both a verb and a noun.
In the latter sense we apply it to the narrow ditch dug for carrying
off superfluous water.
“A drainage system like that just described is the simplest possible,
but there is one serious objection to it: the layer of small stones
soon becomes clogged with soil washed down by the water, and the latter
can no longer run off. Hence it is customary to use fagots instead of
stones, since they offer less obstruction. But still better results are
obtained with earthenware conduits laid in the ditches. Sometimes these
conduits take the form of drain-tiles such as are used on roofs, and
they rest on sills or ground-pieces of the same material; or, again,
they may be tubular in form, the successive sections loosely fitted
together so that the water to be carried off may enter where the
sections join.
“The effect of drainage is not merely to carry off the superfluous
water and thus promote the aëration of the soil to the depth reached by
the roots; it also keeps the soil cool and moist by the constant
presence of water in the drainage ditches or pipes. When a heap of sand
is watered at its base, the moisture is seen to mount higher and higher
until it reaches the top. In like manner the water collected in our
drainage ditches soaks into the upper soil in a dry time and thus
reaches the roots of plants growing there, so that water which is
superfluous or even harmful at certain periods is held in reserve and
gradually distributed at the right moment.
“Another advantage of a drainage system is that it prevents that
cooling of the soil which would result from prolonged evaporation. In
taking the form of vapor water chills the objects that help to promote
the evaporation. For this reason we feel a decided chill on emerging
from a bath; the film of moisture that covered us is passing off in
vaporous form. Similarly a constant evaporation at the surface of a
water-soaked tract of land chills the ground and we have a cold soil.
But if the water is carried off by proper drainage, evaporation ceases
and there is no further chilling of the surface soil. Now, a high
temperature is always favorable to vegetation.
“Draining is so beneficial that it is not confined to marshy ground,
which without it would be quite unproductive, but is applied also to
ordinary arable land. Wherever the soil is too clayey, or even where
the surface soil is good but the subsoil clayey, rain-water cannot
drain off readily and the ground remains soggy and cold. Eventually,
however, it dries up, but there being no way for the air to permeate
the soil, the latter is left hard and unyielding, so that the roots are
by turns drowned in liquid mud and held fast in a tenacious paste that
has been baked by the sun. Drainage overcomes these difficulties, and
consequently all rich soils that hold rain-water for some time before
infiltration are much improved by being properly drained.”