THE WATER OF SOILS.—_Continued._
SURFACE, HYDROSTATIC AND GROUND WATER; PERCOLATION.
Since all the water of soils and plants is directly or indirectly
derived from the rainfall (including therein snow and hail), some
general points regarding this factor require first consideration. While
it is not the object of this work to discuss climatology in detail,
yet the times of the year and the manner in which precipitation comes,
acts upon and is disposed of in the soil under different climatic
conditions, must of necessity form an essential part of its subject
matter.
_Amount of rainfall._—The rain falling in the course of a year is
usually stated in the form of “inches” (or centimeters), implying
the height of the water column that would be shown at the end of the
year had it all been allowed to accumulate; or, the sum of all the
successive rains (including snow) observed during the year. Since
this amount ranges all the way from nothing, or a mere fraction of an
inch (as in portions of the Andes, and of the great African and Asian
deserts) to as much as 600 inches or fifty feet (Cherapundji in eastern
India), the adaptation of agricultural practice to the maintenance
of the proper moisture-supply to crops is largely a local question,
oftentimes of not inconsiderable difficulty. This is especially the
case where torrential rains, yielding several inches of rain in a few
hours, alternate with light, soaking rainfall, as is very commonly
the case in the interior of continents, and more especially in the
United States east of the Rocky Mountains. Westward of the same the
rainfall decreases so rapidly that at or about the one-hundredth
meridian (the longitude of Bismark and Pierre, Dakota, and Dodge City,
Kansas) we already reach the annual average of 20 inches, which is
commonly assumed to be the limit below which crops cannot safely be
grown without irrigation. The “cloudbursts” occasionally occurring
within these limits are usually confined to mountainous regions, and
the water they pour down on the dry soil is rarely of any direct
benefit to agriculture; hence they cannot be properly counted in the
general estimate of the effective rainfall. A region of high rainfall
(up to 100 inches and over), however, extends along the Pacific coast
from northern California through western Oregon and Washington across
British Columbia to Alaska, to seaward of the Sierra Nevada, Cascade,
and Alaskan coast ranges.
In the country east of the Mississippi river, the average annual
rainfall ranges from 30 inches in the region of the Great Lakes, and
45 to 50 inches on the north Atlantic coast, to 60 inches in Louisiana
and up to eighty in southern Florida. The average of the Mississippi
Valley and Atlantic coast States is usually stated at about 45 inches,
which is distributed more or less evenly throughout the year, excepting
usually from six to eight weeks of more scanty precipitation in the
latter part of August and in September—the “Indian summer” season; so
that the winter is the season of greatest total rainfall.
_Natural disposition of the Rain Water._—The rainfall is naturally
first disposed of in two ways, viz., a portion which is absorbed by
the soil, and another which is at once shed from the surface and
constitutes the “surface runoff.” The portion absorbed into the soil
is subsequently disposed of either by soakage downward into the
subdrainage and through springs and seepage[81] into the streams and
rivers; or by evaporation. The latter again occurs in two different
ways, viz., from the soil-surface itself, or through the roots and
leaves of plants. The importance of each of these modes is sufficiently
great to entitle each to detailed consideration.
_The Surface Runoff._—This portion of the disposal of rain may range
all the way from nothing to almost totality, according to the nature of
the soil and the condition of its surface.[82] Sandy soils, especially
when coarse, may absorb instantly even a very heavy rainfall. Heavy
clay soils when dry will at first also absorb quickly quite a heavy
precipitation; but as the beating of the raindrops compacts the
surface, the absorption quickly slows down, so that heavy downpours of
brief duration, while wetting thoroughly into a plastic mass the first
two or three inches of a clay soil, may leave all beneath dry, to be
very gradually moistened by the slow downward percolation against the
resistance of the air in the soil; while the greater part of the later
portion of the shower will drain off the surface in muddy runlets.
Certain soils classed as loams, having the property of crusting readily
by rain followed by sunshine (see chapter 7, p. 111), in heavy showers
behave hardly better than strong clay soils; shedding the water until
the soaked crust gives way, and is carried off in muddy streamlets.
Then begins the cutting-away of the soil that, in portions of the
Cotton States, as well as north of the Ohio river, has been the cause
of extensive devastation of once fruitful culture lands, the site of
which is now marked by “red washes” and gullies but too familiar to the
eye in many regions, especially of the southern United States.
[81] The quiet seepage from the banks and beds of streams plays a much
more important part in the increase of volume of flow than is commonly
supposed, because unperceived save by measurement of the tributaries
and comparison with the main streams. This is especially true of the
drainage in the arid region, where the deep and pervious soils favor
diffuse seepage as against definite spring flow.
[82] Toumey (Yearbook U. S. Dep’t Agr. 1903) states that in the San
Bernardino mountains in southern California, the first rainfall (in
December) was absorbed to the extent of 95% in forested areas, against
only 60% in the non-forested; but that later, after the soil had been
partially saturated, 60% only was absorbed in the forested land,
against 5% in the non-forested. While it is generally admitted that
forests diminish the runoff, Rafter (Relation of Rainfall to Runoff, U.
S. Geol. Survey Paper, No. 80, p. 53) contends that in New York State
the reverse is true.
[Illustration: FIG. 40.—Erosion in Mississippi Table Lands, causing
destruction of agricultural value both of Uplands and Valleys. (McGee,
12th Ann. Rept. U. S., 1890-91.)]
_Washing-away and Gullying in the Cotton
States._—Nowhere perhaps have these effects been so
severely felt as in portions of northwestern and central
Mississippi, and this case is so instructive as to deserve
a more detailed description. In the regions in question the
soil stratum consists of a yellow or brownish loam from
three to seven feet in original thickness, constituting a
very desirable class of gently rolling uplands, which at
one time claimed to be the best cotton-growing portion of
the State. It was originally covered with an open forest
of oaks, with an abundant growth of grasses that afforded
excellent pasture to deer and cattle; a natural park gay
with flowers during most of the season.
When these lands were taken into cultivation little or no
attention was paid to the direction of the furrows and rows
of corn and cotton; most commonly the plowing was done
“up-hill and down,” so that the “dead-furrow” afforded
a ready opportunity for the formation of washes cutting
into the subsoil, during the torrential rains sometimes
falling during the summers. Even when filled with soil by
plowing, these washes would frequently re-open during rains,
shedding the soil in a muddy flood upon the lower lands. The
washing-away of the surface soil, thus brought about, of
course diminished the production of the higher lands, which
were then commonly “turned out” and left without cultivation
or care of any kind. The crusted surface shed the rain water
into the old furrows, and the latter were quickly deepened
and widened into gullies—“red washes”—whose presence
rendered any resumption of cultivation difficult. In the
course of a few years the soil-stratum of brown loam was
penetrated into the loose or loosely cemented sand which
underlies it almost everywhere, and is very readily washed
away. Soon the water, gaining yearly in volume, undercut the
loam stratum so as to cause it to “cave” into gullies in
huge masses, which with the sand were carried into the
valleys adjacent, filling the beds of the streams so as to
cause their flow to disappear under the flood of sand. As
the evil progressed, large areas of uplands were denuded
completely of their loam or culture stratum, leaving nothing
but bare, arid sand, wholly useless for cultivation; while
the valleys were little better, the native vegetation having
been destroyed and only hardy weeds finding nourishment on
the sandy surface.
[Illustration: FIG. 40a.—Erosion in Mississippi Table Lands, causing
destruction of agricultural value both of Uplands and Valleys. (McGee,
12th Ann. Rept. U. S. G. S., 1890-91.)]
In this manner whole sections, and in some portions of the
State whole townships of the best class of uplands have
been transformed into sandy wastes, hardly reclaimable by
any ordinary means, and wholly changing the industrial
conditions of entire counties; whose county seats even in
some instances had to be changed, the old town and site
having, by the same destructive agencies, literally “gone
down hill.” This destruction of lands was greatly aggravated
by the civil war, during which, and for some time after,
large areas of lands once under cultivation were left to the
mercy of the elements.
_Injury in the arid regions._—In the arid regions, where the rainfall
frequently comes in heavy downpours or “cloudbursts,” immense damage
to pasture lands has been brought about by overstocking, in Arizona
and New Mexico; involving the destruction of the natural cover of
vegetation and the loosening of the surface especially by sheep; after
which a heavy rainfall will carry off the surface soil, the muddy water
being gathered largely in the trails made by cattle going to water.
Thus gradually gullies are formed, which enlarging more and more become
ravines and cut up the pasture slopes into “bad lands,” useless equally
for pasture and for agriculture.[83] California, eastern Oregon and
Washington, and Montana, offer striking and lamentable examples of the
same destructive agencies.
[83] Open Range and Irrigation Farming. R. H Forbes, in Forester, Nos.
7, 9, 1902.
_Deforestation._—The deforestation of hill and mountain lands has, the
world over, led to similar results; causing not only the destruction
of pasture and agricultural lands, but also the conversion of streams,
flowing from springs and seepage all the year, into periodic torrents,
flooding the lowlands during rains by the rapid running-off of the
water from the bare and hard-baked mountain slopes, and then running
dry within a short time, so as not even to afford drinking water to
pasturing cattle in summer. Thus for half a century the unsolved
problem of the “correction of the waters of the Jura mountains” was
before the Swiss and French governments; and the great and costly
public work involving re-forestation, deflection of torrents and
filling-in of deep ravines and gullies, is not even yet nearly
completed. In Spain, which in the time of the Roman occupation was
largely a forest country with abundant rainfall, the same results are
seen, notably in the South, in the wide, and mostly dry, sandy beds
of streams once running deep and clear; and in the scarred hill-and
mountain-sides, and scant vegetation of low shrubs (“chaparral”) that
replace the once abundant tree growth, _e. g._, in Old and New Castile.
Unfortunately the lessons taught by the bitter experience of the old
world seem to require actual repetition in the new, before means of
prevention are even thought of.
_Prevention of Injury to Cultivated Lands from excessive Runoff._—The
fundamental remedy for the injurious effects of excessive runoff from
the land surface is, of course, to facilitate its absorption into the
soil to the utmost extent possible, by deep tillage; or in cases where
this is undesirable (as when in rainy climates excessive leaching of
the land is feared), to so direct and control the surface drainage
that its flow shall nowhere be so rapid as to carry with it any large
amounts of earth, or to wash out the furrows. To this end its fall must
be diminished by “circling,” _i. e._, plowing nearly at right angles
to the slope instead of up-and-down, and on steep slopes especially
also by maintaining open furrows or ditches having a gentle fall only,
into which the water can shed and flow off quietly in case the furrows,
left in plowing, prove insufficient to retain and shed gradually the
water they cannot hold permanently. The early adoption of this simple
expedient would have wholly prevented the enormous waste of fine
agricultural lands referred to above.
The underdraining of lands liable to washing is a costly but highly
effective means of preventing denudation; and the laying of underdrains
in gullies already formed, to prevent farther deepening, is among the
most obvious means of arresting farther damage. The beneficial effects
of underdrainage in conserving moisture will be discussed farther on.
ABSORPTION AND MOVEMENTS OF WATER IN SOILS.
The phenomena and laws of capillary ascent of water in soils, as
discussed in the preceding chapter, serve best to demonstrate the
general behavior of liquid water within different soils and their
several grain-sizes; because measurably independent of the physical
changes that almost unavoidably accompany the percolation of water
from above downward; whether such water comes in the form of rain,
or irrigation, or even when applied with the utmost precautions in
the laboratory. The “beating” of rains quickly compacts the surface
to a certain extent, varying with the nature of the soil, its
condition of more or less perfect tilth, and the degree of violence
with which the rain strikes the surface. When the latter has been
compacted by a previous rain and then dried, “baking” or incrusting
the surface, the latter may almost wholly shed a rain of brief
duration, which, had the surface been loose, would have been wholly
absorbed, materially benefiting the crop. Such surface-crusting is,
therefore, injurious in preventing the absorption of water from above;
and in addition, it serves to waste, by evaporation, the moisture
contained in the underlying soil and subsoil. For the crust being of
a finer (single-grain) texture than the tilled portion beneath, it
will forcibly abstract from the latter, by absorption, its capillary
moisture, and evaporating it at the upper surface, continue to deplete
the land, to the great injury of crop growth, until destroyed by
cultivation.[84]
[84] This effect is well illustrated by the behavior of a dry brick
laid upon a wet sponge. It will quickly absorb all the liquid moisture
contained in the latter, while the sponge will be wholly unable to take
any moisture from a fully-soaked brick.
The flow of irrigation water produces the same compacting effect, but
to a less extent; the more as, unlike rain water, irrigation water
usually contains a certain amount of alkaline and earth salts, which
tend to prevent the diffusion of clay and of fine sediments, and
therefore the disintegration of the soil-floccules into single grains.
Nevertheless, it is in some soils as necessary to cultivate after
surface-irrigation as after rains, in order to prevent great waste of
moisture by evaporation.
_Determination of rate of percolation._—When water is allowed to
soak into an air-dry soil column without sensible shock or motion,
from a constant level, we obtain the nearest approach to a definite
determination of the relative permeability of soils to water under
the conditions usual in the arid region. A number of determinations
thus made is tabulated in the diagram given below, which embodies the
observations made by Mr. A. V. Stubenrauch[85] in connection with a
more extended investigation.
[85] Rep’t Calif. Exp’t Station for 1898 to 1901, p. 165.
As these experiments were made with soils not in their
field condition, but gently broken up with a rubber pestle,
a standard of compactness was established by weighing the
quantity which could conveniently be settled into a tube
space of 100 centimeters capacity by tapping the sides and
bottom of the tube, without touching the soil itself. In
this way the following standards were established: For the
University Adobe soil, 140 grams; for the Yuba loam soil,
110 grams; for the Stanislaus sandy soil, 170 grams. Tubes
1½ inches wide were used, and the soils were introduced
in bulk, inside of a cylinder of stiff paper upon which
previously to rolling it up the soils had been thoroughly
mixed. After introducing the soil-filled paper roll it
was gently withdrawn, leaving the soil column in the tube
as uniform as before; a condition almost impossible of
fulfilment when the soil is introduced piece-meal. The tubes
were, of course, left open at the lower end, using a wire
netting to keep the soil column in, so that the air could
escape freely before the descending water column.
The results thus obtained do not, of course, apply directly
to the same soils undisturbed in place in the field; where,
moreover, the air is confined by the wetting of the surface
and thus directly opposes penetration of the water. Still,
they doubtless give a correct idea of their _relative_
permeability for water when in the tilled condition. The
water level was automatically maintained at the depth
of half an inch above the surface of the soil columns.
Pore-spaces given are calculated from volume-weight and
specific gravity.
This diagram shows plainly that there is no direct relation between
the total pore-space in a soil and the facility of water-penetration.
The highest pore-space, in the fine-grained alluvial loam, allows more
rapid percolation than the heavy clay or adobe soil, but is greatly
exceeded by the coarser sandy soil. In all it is very apparent that the
downward movement slows down as the water descends, doubtless because
the great friction in a longer column gradually diminishes the effect
of hydrostatic pressure. It may be presumed that at a certain distance
from the surface the downward movement becomes practically uniform, and
independent of the pressure from above.
[Illustration: FIG. 41.—Diagram showing differences in rates of
percolation through different soils.]
_Summary._—Two salient points are revealed by even a cursory inspection
of the preceding diagram, viz.:
1. The downward percolation is most rapid in the same soils in which
the capillary ascent is quickest, that is, in the coarse, sandy soil.
2. The rapidity of percolation decreases materially as the wetted soil
column increases in length.
The first point is readily foreseen and needs no comment.
As regards the second, it results from the fact that as
the wetted column lengthens the frictional resistance
increasingly counteracts the effects of the hydrostatic
pressure from above, until the water’s descent becomes but
little more rapid than would be its lateral diffusion,
or its ascent at the end of a similar column supplied by
capillary rise from below. In both cases the frictional
resistance has so far counteracted the effect of gravity
that the capillary coefficients of the soil-material become
the controlling factors of the water movement.
_Influence of Variety of Grain-sizes._—King (Physics of Agriculture,
pp. 159, 160), compared the rapidity of the percolation of water
through definitely graded pure sands on the one hand, and a sandy loam
and a clay soil on the other. The materials were arranged in 8-foot
columns fully saturated with water at the outset, and then allowed
to drain freely. The following abridged table shows the tenor of his
results:
TABLE SHOWING RELATIVE RAPIDITY OF PERCOLATION IN PURE
SANDS AND SOILS, IN INCHES OF WATER DRAINED OFF.
===================+==================+==============+============
Diameter of Uniform| First | Second | Total
Sand Grains. | 30 minutes. | 30 minutes. |in one hour.
-------------------+------------------+--------------+------------
.475 mm. | 10.25 | 4.68 | 14.93
.155 “ | 5.67 | 4.52 | 10.19
.083 “ | 1.21 | .85 | 2.06
===================+===========+======+===+==========+============
|First 21-23| First 10 |Second 10 | Total in
Soils. | hours. | days | days | about
| |following.|following.| 505 hours.
-------------------+-----------+----------+----------+------------
Sandy loam | 2.64 | 5.07 | .91 | 8.62
Clay loam | 1.96 | 2.11 | .49 | 4.56
-------------------+-----------+----------+----------+------------
This table is very instructive in showing the great difference in the
rapidity of percolation in materials of uniform, even-sized grains,
as compared with such as contain particles of many different sizes,
in which the interspaces of the larger ones are filled more or less
closely by the smaller sizes of particles (see chapter 7, p. 109).
While it is true that we have no definite physical analysis of the
soils here used, the differences are so great as to be sufficiently
striking. Compare the percolation through the sand of .155 mm. uniform
grain-size (a fine sand), during the first half hour, with that through
the sandy loam during the first 21 hours. Twice as much water has
passed from the sand as from the soil in one forty-second part of the
time. Comparing similarly the finest sand, .083 mm. in diameter, with
the clay loam, we find the difference to be as one to seventy-three.
It is thus evident that but for the variously assorted sizes of the
soil-particles, water would not be held long enough to supply plant
growth.
_Percolation in Natural Soils._—In artificial percolation experiments,
as well as during a fall of rain, the gradual settling of the fully
wetted soil-column produces a compacting of that portion of the mass,
that increasingly impedes the downward penetration. The effect of this
under natural conditions is readily seen in the fact that after the
first, rapid absorption of falling rain by the soil when in good tilth,
there is a gradual slackening of the process even when the rain is fine
and slow, causing a perceptible increase of the runoff until, should
the rain continue for some time, the absorption becomes so slow as to
cause all, or nearly all the water to drain off the surface. The soil
is then called “saturated,” having really arrived at that point right
at the surface, and to a depth varying according to the duration and
amount of rain, and the natural perviousness of the land.
When the rain ceases, the visible saturation of the surface usually
soon disappears in cultivated soils, and the zone of saturation begins
to descend. The progress of this descent may be very strikingly
observed in a series of holes (post-holes) dug or bored across a
ridge; as indicated in the subjoined schematic diagram, in which the
successive dotted lines represent the levels of the descending “bottom
water” at successive intervals, as derived from the observation of the
water levels in the several holes.[86]
[86] The exact record of these observations was unfortunately destroyed
by fire; the soil was a heavy clay, and it took ten days before the
water disappeared from the lowest hole.
[Illustration: FIG. 42.—Percolation in clay land after heavy rain.]
It will be seen that while at first the upper surface of
the zone of saturation coincides with the surface of the
ground, in falling it descends most rapidly on the highest
ground, while at the lower levels the holes may remain full
or overflowing; the drainage taking place sideways as well
as vertically. The curved surface connecting the levels in
the several holes gradually flattens, rapidly at first, then
progressively more slowly; the water disappearing entirely,
first from the holes lying highest, then successively from
those at lower levels; those located in valleys or drainage
channels remaining full until surface-water ceases to run
in such channels. But even after liquid water has ceased to
be visible in the holes, the descent of the water continues
within that portion of the soil, tending (unless more rain
should come before that time), to establish the condition
of equilibrium as existing in the soil columns shown in
the diagram on p. 205, chapt. 11; such as results from the
capillary ascent of water from below, but having above it a
column of soil of minimum water-content, of greater or less
height according to the length of time allowed for the water
to descend. This is a very common state of things during the
long summer droughts in the arid region, when neither rain
nor irrigation has added to the water supply in the soil for
many months, and yet ordinary deciduous fruit trees mature
their normal crops. Frequently, however, before this state
of equilibrium is reached, evaporation from the surface so
draws upon the water supply within the first few feet, as to
reduce the soil to undersaturation at the lowest point of
the descending column, so stopping farther descent and soon
reversing the direction of the movement. The latter is the
usual condition of scantily irrigated ground.
_Ground or Bottom Water, Water Table._—During and after long-continued
and abundant rains, the zone of supersaturation continues to descend
until it finally reaches a more or less permanent level, varying
somewhat from season to season, but on the whole usually definable for
each region and locality; being the depth to which wells must be sunk
in order to secure a fairly permanent water supply. This is called
the water table, ground water, bottom water, or “first water.”[87]
The proportion of the rainfall that reaches the permanent water level
varies enormously, of course, in different soils and at different
times. With brief and moderate rains, in soils of high water-holding
power and slow percolation, it may never reach the bottom-water level;
this is very commonly the case in the arid regions. Where, as in the
humid regions, rains are frequent or much prolonged, one half and even
more may finally reach the permanent level; runoff and evaporation
disposing of the balance.
[87] In contradistinction to other levels or “streams” of water which
may usually be found lower down, separated from the first water by some
impervious stratum of clay, hardpan or rock, and very commonly under
sufficient pressure to rise somewhat higher than the point at which it
was struck, owing to connection with higher-lying sources of supply.
When such pressure is sufficient to cause an overflow at the surface of
the ground, we have “Artesian” water as commonly understood.
_Lysimeters._—For the determination of the amount of
water percolating to given depths, water-tight receptacles
called lysimeters are usually employed. The best way to
establish such receptacles is to isolate a unit-area
(usually a square meter) by digging all around it to the
depth desired, then surrounding it with a metal sheet
soldered tightly at the cut edges, and finally driving in a
sharp-edged, stiff metal sheet so as to form the bottom when
soldered to the upright walls; leaving on one side an outlet
for the percolating water, which is then received into a
measuring receptacle somewhat like a rain gauge.
Hall (The Soil, p. 75) states that at Rothamstead, where an average
rainfall of 31.3 inches is distributed rather uniformly through the
season, and where the soil is a moderately clayey loam, a little less
than half percolates through 20 inches of soil, and about 45% through
60 inches.
_Surface of Ground Water; Variations._—The surface of the water table,
however, is rarely level except in level and very uniform ground, or
after long periods of drought. The undulations of its surface conform,
in general, to that of the ground surface, but are less abrupt; so that
the water lies nearer to the surface in low than in high ground, as is
indicated in the diagram above.
King[88] has shown, moreover, that the level of the ground water shows
sensible variations due to increased or diminished barometric pressure,
as well as to variations of temperature in the soil, which cause the
air in the pores to expand or contract to a degree sufficient to bring
about variations in the flow of springs and underdrains to the extent
of 8 and 15% respectively, in conformity with the daily changes of
temperature and pressure.
[88] Physics of Agriculture, p. 270.
_The Depth of the Ground Water most Favorable to Crops_ cannot be
stated in a general manner, as it depends materially upon the nature of
the crop, its root habit, and the nature of the soil. As has already
been said, the amount of soil-moisture most favorable to plant growth
is about half of the maximum it can hold; and this condition, as is
shown in the table in chapter 11, p. 208, is reached about the middle
of the maximum height to which the water can rise by capillarity from
the water level. Below this point the access of air to the roots
becomes too limited, and in case of continuous rains the root-ends
would soon begin to suffer from want of aeration. On “sub-irrigated”
land, therefore, which is generally considered desirable, crops must
be carefully selected with respect to their root habits. Thus while
alfalfa needs considerable moisture to do its best, its deep-rooting
habit renders it undesirable when the ground water is at less than five
feet depth; but red clover may be grown even with the water level at
three feet.
In clayey soils root-penetration is always less than in sandy lands;
and although in the former the capillary ascent of water goes higher
than in the latter, yet its movement in clays is so much slower than in
sandy materials that unless water is within comparatively easy reach,
the plants may suffer from drought. Experience has long ago fixed the
proper depth at which to lay underdrains limiting the rise of bottom
water, at from three to four and a half or even five feet in clay
soils; greater depths are only exceptionally used, partly because the
laying of drains then becomes too expensive.
A mass of four feet of clay-loam soil is commonly, then, considered
as sufficient to supply the needs of a crop; it being understood that
in the humid region at least, such soils are usually the richest in
plant food, so that a deeper range of the root system is not called
for. It is quite otherwise in the sandy soils of the same region, which
being usually poor in plant food, must afford a deeper penetration in
order that an adequate amount of the same shall be within reach of the
roots. Sandy lands, then, should be deep in order to repay cultivation;
and fortunately this is usually the case. But when this is otherwise;
when for instance a sandy soil four feet in depth is underlaid by
impervious clay, underdrains may be quite as necessary as in the clay
lands; since the depth of actually available soil mass would otherwise
be reduced to two or two and a half feet only, by the water stagnating
on the clay surface and rising from 16 to 24 inches in the sand. Soils
thus shallowed can with difficulty be maintained in good productive
condition even by the most energetic fertilization.
_Moisture supplied by tap roots._—In most cases, sandy lands do not
require underdraining; and in them, root-penetration may reach to
extraordinary depths in the case of certain plants, especially when
taprooted. Thus the roots of alfalfa (lucern) are very commonly found
to reach depths of twenty to twenty-five feet, and even sixty feet has
been credibly reported for the same plant in the arid region. It is
obvious that for such plants, a high level of bottom water is wholly
undesirable, since they are enabled to obtain their moisture supply
from great depths, and can thus utilize for their nutrition much larger
soil-masses than can shallow-rooted plants.
_Reserve of Capillary Water._—It must be remembered that it is not
only, nor usually, the bottom water that supplies moisture to plant
growth; for all soils of proper texture for cultivation retain within
them a certain amount of capillary moisture after the ground water has
reached its permanent level (see this chap. p. 226), and when the tap
or main roots are plentifully supplied with water, the upper and chief
feeding roots draw but lightly upon the moisture within their immediate
reach for the purpose of leaf evaporation. This fact can be plainly
observed in the arid region, when on the advent of the summer drought,
young plantlets whose tap roots have reached a certain depth continue
to flourish and develop, while others practically of the same age, but
slightly behind, quickly succumb, though the _feeding_ roots of both
may draw upon the same soil layer. It is especially in sandy soils that
moisture is naturally thus conserved in the upper layers, because of
the failure of the water to rise by capillary ascent so as to evaporate
from the surface layer. It is often surprising to find a good amount
of moisture in the sandy soils of desert regions at the depth of eight
or ten inches, when the surface is so hot as to scorch the fingers;
and this moisture continues very uniformly to great depths, probably
to bottom water lying twenty or more feet below the surface, which in
such materials may readily be reached by taprooted plants such as the
“sagebrush” (Artemisia tridentata), the saltbushes (Atriplex) and
others.
_Injurious Rise of Bottom Water resulting from Irrigation._—In the
deep, pervious sandy lands of the arid region, especially where the
rainfall is very low and can wet the soil annually only to two or three
feet depth, the substrata are sometimes found to be barely moist to
depths of thirty and forty feet, and the short-lived spring vegetation
carries off during its growth all the moisture supplied by the winter
rains. When such lands are subjected to irrigation and the ditches
carrying the water are simply dug into the natural sandy land, the
thirsty soil absorbs the water greedily, so that even a considerable
volume of water makes but slow progress toward the farther end of
the canals. Gradually, as the rapidity of absorption decreases, the
diminution of flow becomes less sensible, but still the loss thus
experienced may be a very considerable percentage of the whole supply.
Thus in the Great Valley of California, as well as in portions of
Wyoming (Bull. 61, p. 32), the permanent loss from seepage is in the
case of some extensive irrigation systems estimated at fully 50 per
cent. When such lands have a considerable slope, the injury commonly
ends with the loss of the water, which in many cases is again gathered
and utilized at a lower level. But when the lands have but a slight
slope, the drainage may become so slow as to permit of the gradual
rise of the seepage water in the substrata, until finally it may come
to within a few feet of, or actually to the surface.
_Consequences of the Swamping of Irrigated Lands._—The injurious
consequences of this swamping of the irrigated lands may readily
be imagined. The first effect is usually noted in the sickening or
dying-out of orchards and vineyards, consequent upon the submergence
of the deeper roots, which in such lands frequently reach to from
fifteen to twenty feet below the surface. But even where pre-existing
plantations are not in question, the shallowing of the soil- and
subsoil-strata from which the plants may draw their nourishment,
constitutes a most serious injury to the cultural value of the land. It
has become unsuited to deep-rooted crops; and where the natural soil,
alone, would have perpetuated fertility for many years, fertilization
becomes necessary within a short time. The injury becomes doubly great
when, as is frequently the case, the rising bottom water brings up
with it to the surface soil the alkali salts which previously were
distributed throughout many feet of substrata, frequently rendering
profitable cultivation impossible where formerly the most luxuriant
crops were grown.
Theoretically of course it is perfectly easy to avoid or remedy these
troubles. It is only necessary to render the ditches water-tight by
puddling with clay, cement, or otherwise. But the heavy cost of this
improvement forms a serious obstacle to its adoption by the ditch
companies who are not themselves owners of land. Thus, extensive areas
of lands which when first irrigated were among the most productive,
have in the course of eight or ten years become almost valueless to
their owners, to whom legislation thus far affords but distant promise
of relief; although the case seems in equity to fall clearly within the
limits of the laws governing trespass.
_Permanent Injury to Certain Lands._—In cases like those alluded to the
remedy usually available for higher ground-water does not always afford
relief, even when otherwise available. Long-continued submergence
produces in many soils effects which cannot easily, if at all, be
overcome by subsequent aeration. This is most emphatically true of
soils containing a large proportion of ferric hydrate in the finely
divided form in which it is usually present in “red” soils.
The first effect of the stagnation of water in such lands (as
already explained in a former chapter 3, p. 45) is to set up a
reductive (bacterial) fermentation of the organic matter of the soil,
transforming the ferric into ferrous hydrate, which in the presence of
the carbonic acid simultaneously formed, becomes ferrous carbonate,
readily soluble in carbonated water. That this compound is poisonous
to plant growth, has been stated (chap. 3, p. 46). The carbonates of
lime and magnesia are simultaneously dissolved by the same, as is
also calcic phosphate, the usual form in which phosphoric acid is
present in the soil. Under the influence of partial aeration from
the surface, the ferrous carbonate is slowly re-transformed into
ferric hydrate, aggregated in the form of spots or concretions of
“bog ore” (see chapter 5, p. 66). In this process the greater part of
the phosphoric acid of the soil is also abstracted from its general
mass and concentrated in the bog ore (chap. 5, p. 65), in which it is
wholly unavailable to vegetation, and cannot be made available while
in the ground, by any known process. The soil is therefore permanently
impoverished in phosphoric acid; it is also deprived of its content of
ferric hydrate, and is transferred from the class of “red” to that of
“white” soils, well known everywhere to be unthrifty and to require
early fertilization. Not only is this true, because of their almost
invariable poverty in phosphoric acid, but also usually in lime, which
like the iron, if not leached out, is aggregated into concretions
in the subsoil, leaving the surface soil depleted of this important
ingredient. The humus, also, is either destroyed or at least “soured”
at the same time.
_Reduction of Sulfates._—Should such a soil contain
any considerable amount of sulfates, especially in the form
of gypsum or calcic (or magnesic) sulfate, the reductive
process results in the formation of iron pyrite (ferric
sulfid, chap. 5, p. 75); while at the same time the soil is
often sufficiently impregnated with sulfuretted hydrogen as
to be readily perceived by the odor, or by the blackening of
a silver coin. This is very commonly the case in sea-coast
marshes, where a hole made with a stick thrust into the mud
will be found to give forth both carburetted and sulfuretted
hydrogen, while a careful washing of the soil will reveal
the presence of minute crystals of iron pyrite. Hence the
need of prolonged aeration of marsh soils, effecting the
peroxidation of the ferrous compounds, and the conversion of
the pyrite first into ferrous sulfate, and subsequently into
innocuous, yellow, insoluble ferric oxy-sulfate.
_Ferruginous Lands._—The injurious effect of the swamping of
ferruginous lands has been especially conspicuous in some of the
irrigated rolling lands of the Sierra Foothills of California, where
orchards planted in relatively low ground and in full bearing have
succumbed to the poisonous effects of the ferrous carbonate formed
in the subsoil, long before the water had risen so high that, had
the trees been grown afterwards, they would have adapted their
root system to the existing conditions and fared moderately well
at least. Underdrainage of the lower lands is, of course, the only
possible remedy for this state of things, although even then the
root-penetration is much more restricted, and therefore natural
fertility of much shorter duration, than would have been the case
without the rise of the irrigation water.
It is thus clear that in the practice of irrigation, the liability
of injury to the lower ground by “swamping” through the rise of the
ground water should always be kept in view; that, in fact, irrigation
and provision for drainage should always go hand in hand. The legal
provisions facilitating the rights-of-way for irrigation ditches should
be made equally cogent with respect to drainage.