STORING WATER IN THE SOIL
The large amount of water required for the production of plant
substance is taken from the soil by the roots. Leaves and stems do
not absorb appreciable quantities of water. The scanty rainfall of
dry-farm districts or the more abundant precipitation of humid
regions must, therefore, be made to enter the soil in such a manner
as to be readily available as soil-moisture to the roots at the
right periods of plant growth.
In humid countries, the rain that falls during the growing season is
looked upon, and very properly, as the really effective factor in
the production of large crops. The root systems of plants grown
under such humid conditions are near the surface, ready to absorb
immediately the rains that fall, even if they do not soak deeply
into the soil. As has been shown in Chapter IV, it is only over a
small portion of the dry-farm territory that the bulk of the scanty
precipitation occurs during the growing season. Over a large portion
of the arid and semiarid region the summers are almost rainless and
the bulk of the precipitation comes in the winter, late fall, or
early spring when plants are not growing. If the rains that fall
during the growing season are indispensable in crop production, the
possible area to be reclaimed by dry-farming will be greatly
limited. Even when much of the total precipitation comes in summer,
the amount in dry-farm districts is seldom sufficient for the proper
maturing of crops. In fact, successful dry-farming depends chiefly
upon the success with which the rains that fall during any season of
the year may be stored and kept in the soil until needed by plants
in their growth. The fundamental operations of dry-farming include a
soil treatment which enables the largest possible proportion of the
annual precipitation to be stored in the soil. For this purpose, the
deep, somewhat porous soils, characteristic of arid regions, are
unusually well adapted.
Alway's demonstration
An important and unique demonstration of the possibility of bringing
crops to maturity on the moisture stored in the soil at the time of
planting has been made by Alway. Cylinders of galvanized iron, 6
feet long, were filled with soil as nearly as possible in its
natural position and condition Water was added until seepage began,
after which the excess was allowed to drain away. When the seepage
had closed, the cylinders were entirely closed except at the
surface. Sprouted grains of spring wheat were placed in the moist
surface soil, and 1 inch of dry soil added to the surface to prevent
evaporation. No more water was added; the air of the greenhouse was
kept as dry as possible. The wheat developed normally. The first ear
was ripe in 132 days after planting and the last in 143 days. The
three cylinders of soil from semiarid western Nebraska produced 37.8
grams of straw and 29 ears, containing 415 kernels weighing 11.188
grams. The three cylinders of soil from humid eastern Nebraska
produced only 11.2 grams of straw and 13 ears containing 114
kernels, weighing 3 grams. This experiment shows conclusively that
rains are not needed during the growing season, if the soil is well
filled with moisture at seedtime, to bring crops to maturity.
What becomes of the rainfall?
The water that falls on the land is disposed of in three ways:
First, under ordinary conditions, a large portion runs off without
entering the soil; secondly, a portion enters the soil, but remains
near the surface, and is rapidly evaporated back into the air; and,
thirdly, a portion enters the lower soil layers, from which it is
removed at later periods by several distinct processes. The run-off
is usually large and is a serious loss, especially in dry-farming
regions, where the absence of luxuriant vegetation, the somewhat
hard, sun-baked soils, and the numerous drainage channels, formed by
successive torrents, combine to furnish the rains with an easy
escape into the torrential rivers. Persons familiar with arid
conditions know how quickly the narrow box canyons, which often
drain thousands of square miles, are filled with roaring water after
a comparatively light rainfall.
The run-off
The proper cultivation of the soil diminishes very greatly the loss
due to run-off, but even on such soils the proportion may often be
very great. Farrel observed at one of the Utah stations that during
a torrential rain--2.6 inches in 4 hours--the surface of the summer
fallowed plats was packed so solid that only one fourth inch, or
less than one tenth of the whole amount, soaked into the soil, while
on a neighboring stubble field, which offered greater hindrance to
the run-off, 1-1/2 inches or about 60 per cent were absorbed.
It is not possible under any condition to prevent the run-off
altogether, although it can usually be reduced exceedingly. It is a
common dry-farm custom to plow along the slopes of the farm instead
of plowing up and down them. When this is done, the water which runs
down the slopes is caught by the succession of furrows and in that
way the runoff is diminished. During the fallow season the disk and
smoothing harrows are run along the hillsides for the same purpose
and with results that are nearly always advantageous to the
dry-farmer. Of necessity, each man must study his own farm in order
to devise methods that will prevent the run-off.
The structure of soils
Before examining more closely the possibility of storing water in
soils a brief review of the structure of soils is desirable. As
previously explained, soil is essentially a mixture of disintegrated
rock and the decomposing remains of plants. The rock particles which
constitute the major portion of soils vary greatly in size. The
largest ones are often 500 times the sizes of the smallest. It would
take 50 of the coarsest sand particles, and 25,000 of the finest
silt particles, to form one lineal inch. The clay particles are
often smaller and of such a nature that they cannot be accurately
measured. The total number of soil particles in even a small
quantity of cultivated soil is far beyond the ordinary limits of
thought, ranging from 125,000 particles of coarse sand to
15,625,000,000,000 particles of the finest silt in one cubic inch.
In other words, if all the particles in one cubic inch of soil
consisting of fine silt were placed side by side, they would form a
continuous chain over a thousand miles long. The farmer, when he
tills the soil, deals with countless numbers of individual soil
grains, far surpassing the understanding of the human mind. It is
the immense number of constituent soil particles that gives to the
soil many of its most valuable properties.
It must be remembered that no natural soil is made up of particles
all of which are of the same size; all sizes, from the coarsest sand
to the finest clay, are usually present. These particles of all
sizes are not arranged in the soil in a regular, orderly way; they
are not placed side by side with geometrical regularity; they are
rather jumbled together in every possible way. The larger sand
grains touch and form comparatively large interstitial spaces into
which the finer silt and clay grains filter. Then, again, the clay
particles, which have cementing properties, bind, as it were, one
particle to another. A sand grain may have attached to it hundreds,
or it may be thousands, of the smaller silt grains; or a regiment of
smaller soil grains may themselves be clustered into one large grain
by cementing power of the clay. Further, in the presence of lime and
similar substances, these complex soil grains are grouped into yet
larger and more complex groups. The beneficial effect of lime is
usually due to this power of grouping untold numbers of soil
particles into larger groups. When by correct soil culture the
individual soil grains are thus grouped into large clusters, the
soil is said to be in good tilth. Anything that tends to destroy
these complex soil grains, as, for instance, plowing the soil when
it is too wet, weakens the crop-producing power of the soil. This
complexity of structure is one of the chief reasons for the
difficulty of understanding clearly the physical laws governing
soils.
Pore-space of soils
It follows from this description of soil structure that the soil
grains do not fill the whole of the soil space. The tendency is
rather to form clusters of soil grains which, though touching at
many points, leave comparatively large empty spaces. This pore space
in soils varies greatly, but with a maximum of about 55 per cent. In
soils formed under arid conditions the percentage of pore-space is
somewhere in the neighborhood of 50 per cent. There are some arid
soils, notably gypsum soils, the particles of which are so uniform
size that the pore-space is exceedingly small. Such soils are always
difficult to prepare for agricultural purposes.
It is the pore-space in soils that permits the storage of
soil-moisture; and it is always important for the farmer so to
maintain his soil that the pore-space is large enough to give him
the best results, not only for the storage of moisture, but for the
growth and development of roots, and for the entrance into the soil
of air, germ life, and other forces that aid in making the soil fit
for the habitation of plants. This can always be best accomplished,
as will be shown hereafter, by deep plowing, when the soil is not
too wet, the exposure of the plowed soil to the elements, the
frequent cultivation of the soil through the growing season, and the
admixture of organic matter. The natural soil structure at depths
not reached by the plow evidently cannot be vitally changed by the
farmer.
Hygroscopic soil-water
Under normal conditions, a certain amount of water is always found
in all things occurring naturally, soils included. Clinging to every
tree, stone, or animal tissue is a small quantity of moisture
varying with the temperature, the amount of water in the air, and
with other well-known factors. It is impossible to rid any natural
substance wholly of water without heating it to a high temperature.
This water which, apparently, belongs to all natural objects is
commonly called hygroscopic water. Hilgard states that the soils of
the arid regions contain, under a temperature of 15 deg C. and an
atmosphere saturated with water, approximately 5-1/2 per cent of
hygroscopic water. In fact, however, the air over the arid region is
far from being saturated with water and the temperature is even
higher than 15 deg C., and the hygroscopic moisture actually found
in the soils of the dry-farm territory is considerably smaller than
the average above given. Under the conditions prevailing in the
Great Basin the hygroscopic water of soils varies from .75 per cent
to 3-1/2 per cent; the average amount is not far from 12 per cent.
Whether or not the hygroscopic water of soils is of value in plant
growth is a disputed question. Hilgard believes that the hygroscopic
moisture can be of considerable help in carrying plants through
rainless summers, and further, that its presence prevents the
heating of the soil particles to a point dangerous to plant roots.
Other authorities maintain earnestly that the hygroscopic soil-water
is practically useless to plants. Considering the fact that wilting
occurs long before the hygroscopic water contained in the soil is
reached, it is very unlikely that water so held is of any real
benefit to plant growth.
Gravitational water
It often happens that a portion of the water in the soil is under
the immediate influence of gravitation. For instance, a stone which,
normally, is covered with hygroscopic water is dipped into water The
hydroscopic water is not thereby affected, but as the stone is drawn
out of the water a good part of the water runs off. This is
gravitational water That is, the gravitational water of soils is
that portion of the soil-water which filling the soil pores, flows
downward through the soil under the influence of gravity. When the
soil pores are completely filled, the maximum amount of
gravitational water is found there. In ordinary dry-farm soils this
total water capacity is between 35 and 40 per cent of the dry weight
of soil.
The gravitational soil-water cannot long remain in that condition;
for, necessarily, the pull of gravity moves it downward through the
soil pores and if conditions are favorable, it finally reaches the
standing water-table, whence it is carried to the great rivers, and
finally to the ocean. In humid soils, under a large precipitation,
gravitational water moves down to the standing water-table after
every rain. In dry-farm soils the gravitational water seldom reaches
the standing water-table; for, as it moves downward, it wets the
soil grains and remains in the capillary condition as a thin film
around the soil grains.
To the dry-farmer, the full water capacity is of importance only as
it pertains to the upper foot of soil. If, by proper plowing and
cultivation, the upper soil be loose and porous, the precipitation
is allowed to soak quickly into the soil, away from the action of
the wind and sun. From this temporary reservoir, the water, in
obedience to the pull of gravity, will move slowly downward to the
greater soil depths, where it will be stored permanently until
needed by plants. It is for this reason that dry-farmers find it
profitable to plow in the fall, as soon as possible after
harvesting. In fact, Campbell advocates that the harvester be
followed immediately by the disk, later to be followed by the plow
The essential thing is to keep the topsoil open and receptive to a
rain.
Capillary soil-water
The so-called capillary soil-water is of greatest importance to the
dry-farmer. This is the water that clings as a film around a marble
that has been dipped into water. There is a natural attraction
between water and nearly all known substances, as is witnessed by
the fact that nearly all things may be moistened. The water is held
around the marble because the attraction between the marble and the
water is greater than the pull of gravity upon the water. The
greater the attraction, the thicker the film; the smaller the
attraction, the thinner the film will be. The water that rises in a
capillary glass tube when placed in water does so by virtue of the
attraction between water and glass. Frequently, the force that makes
capillary water possible is called surface tension.
Whenever there is a sufficient amount of water available, a thin
film of water is found around every soil grain; and where the soil
grains touch, or where they are very near together, water is held
pretty much as in capillary tubes. Not only are the soil particles
enveloped by such a film, but the plant roots foraging in the soil
are likewise covered; that is, the whole system of soil grains and
roots is covered, under favorable conditions, with a thin film of
capillary water. It is the water in this form upon which plants draw
during their periods of growth. The hygroscopic water and the
gravitational water are of comparatively little value in plant
growth.
Field capacity of soils for capillary water
The tremendously large number of soil grains found in even a small
amount of soil makes it possible for the soil to hold very large
quantities of capillary water. To illustrate: In one cubic inch of
sand soil the total surface exposed by the soil grains varies from
42 square inches to 27 square feet; in one cubic inch of silt soil,
from 27 square feet to 72 square feet, and in one cubic inch of an
ordinary soil the total surface exposed by the soil grains is about
25 square feet. This means that the total surface of the soil grains
contained in a column of soil 1 square foot at the top and 10 feet
deep is approximately 10 acres. When even a thin film of water is
spread over such a large area, it is clear that the total amount of
water involved must be large It is to be noticed, therefore, that
the fineness of the soil particles previously discussed has a direct
bearing upon the amount of water that soils may retain for the use
of plant growth. As the fineness of the soil grains increases, the
total surface increases' and the water-holding capacity also
increases.
Naturally, the thickness of a water film held around the soil grains
is very minute. King has calculated that a film 275 millionths of an
inch thick, clinging around the soil particles, is equivalent to
14.24 per cent of water in a heavy clay; 7.2 per cent in a loam;
5.21 per cent in a sandy loam, and 1.41 per cent in a sandy soil.
It is important to know the largest amount of water that soils can
hold in a capillary condition, for upon it depend, in a measure, the
possibilities of crop production under dry-farming conditions. King
states that the largest amount of capillary water that can be held
in sandy loams varies from 17.65 per cent to 10.67 per cent; in clay
loams from 22.67 per cent to 18.16 per cent, and in humus soils
(which are practically unknown in dry-farm sections) from 44.72 per
cent to 21.29 per cent. These results were not obtained under
dry-farm conditions and must be confirmed by investigations of arid
soils.
The water that falls upon dry-farms is very seldom sufficient in
quantity to reach the standing water-table, and it is necessary,
therefore, to determine the largest percentage of water that a soil
can hold under the influence of gravity down to a depth of 8 or 10
feet--the depth to which the roots penetrate and in which root
action is distinctly felt. This is somewhat difficult to determine
because the many conflicting factors acting upon the soil-water are
seldom in equilibrium. Moreover, a considerable time must usually
elapse before the rain-water is thoroughly distributed throughout
the soil. For instance, in sandy soils, the downward descent of
water is very rapid; in clay soils, where the preponderance of fine
particles makes minute soil pores, there is considerable hindrance
to the descent of water, and it may take weeks or months for
equilibrium to be established. It is believed that in a dry-farm
district, where the major part of the precipitation comes during
winter, the early springtime, before the spring rains come, is the
best time for determining the maximum water capacity of a soil. At
that season the water-dissipating influences, such as sunshine and
high temperature, are at a minimum, and a sufficient time has
elapsed to permit the rains of fall and winter to distribute
themselves uniformly throughout the soil. In districts of high
summer precipitation, the late fall after a fallow season will
probably be the best time for the determination of the field-water
capacity.
Experiments on this subject have been conducted at the Utah Station.
As a result of several thousand trials it was found that, in the
spring, a uniform, sandy loam soil of true arid properties
contained, from year to year, an average of nearly 16-1/2 per cent
of water to a depth of 8 feet. This appeared to be practically the
maximum water capacity of that soil under field conditions, and it
may be called the field capacity of that soil for capillary water.
Other experiments on dry-farms showed the field capacity of a clay
soil to a depth of 8 feet to be 19 per cent; of a clay loam, to be
18 per cent; of a loam, 17 per cent; of another loam somewhat more
sandy, 16 per cent; of a sandy loam, 14-1/2 per cent; and of a very
sandy loam, 14 per cent. Leather found that in the calcareous arid
soil of India the upper 5 feet contained 18 per cent of water at the
close of the wet season.
It may be concluded, therefore, that the field-water capacities of
ordinary dry-farm soils are not very high, ranging from 15 to 20 per
cent, with an average for ordinary dry-farm soils in the
neighborhood of 16 or 17 per cent. Expressed in another way this
means that a layer of water from 2 to 3 inches deep can be stored in
the soil to a depth of 12 inches. Sandy soils will hold less water
than clayey ones. It must not be forgotten that in the dry-farm
region are numerous types of soils, among them some consisting
chiefly of very fine soil grains and which would; consequently,
possess field-water capacities above the average here stated. The
first endeavor of the dry-farmer should be to have the soil filled
to its full field-water capacity before a crop is planted.
Downward movement of soil-moisture
One of the chief considerations in a discussion of the storing of
water in soils is the depth to which water may move under ordinary
dry-farm conditions. In humid regions, where the water table is near
the surface and where the rainfall is very abundant, no question has
been raised concerning the possibility of the descent of water
through the soil to the standing water. Considerable objection,
however, has been offered to the doctrine that the rainfall of arid
districts penetrates the soil to any great extent. Numerous writers
on the subject intimate that the rainfall under dry-farm conditions
reaches at the best the upper 3 or 4 feet of soil. This cannot be
true, for the deep rich soils of the arid region, which never have
been disturbed by the husbandman, are moist to very great depths. In
the deserts of the Great Basin, where vegetation is very scanty,
soil borings made almost anywhere will reveal the fact that moisture
exists in considerable quantities to the full depth of the ordinary
soil auger, usually 10 feet. The same is true for practically every
district of the arid region.
Such water has not come from below, for in the majority of cases the
standing water is 50 to 500 feet below the surface. Whitney made
this observation many years ago and reported it as a striking
feature of agriculture in arid regions, worthy of serious
consideration. Investigations made at the Utah Station have shown
that undisturbed soils within the Great Basin frequently contain, to
a depth of 10 feet, an amount of water equivalent to 2 or 3 years of
the rainfall which normally occurs in that locality. These
quantities of water could not be found in such soils, unless, under
arid conditions, water has the power to move downward to
considerably greater depths than is usually believed by dry-farmers.
In a series of irrigation experiments conducted at the Utah Station
it was demonstrated that on a loam soil, within a few hours after an
irrigation, some of the water applied had reached the eighth foot,
or at least had increased the percentage of water in the eighth
foot. In soil that was already well filled with water, the addition
of water was felt distinctly to the full depth of 8 feet. Moreover,
it was observed in these experiments that even very small rains
caused moisture changes to considerable depths a few hours after the
rain was over. For instance, 0.14 of an inch of rainfall was felt to
a depth of 2 feet within 3 hours; 0.93 of an inch was felt to a
depth of 3 feet within the same period.
To determine whether or not the natural winter precipitation, upon
which the crops of a large portion of the dry-farm territory depend,
penetrates the soil to any great depth a series of tests were
undertaken. At the close of the harvest in August or September the
soil was carefully sampled to a depth of 8 feet, and in the
following spring similar samples were taken on the same soils to the
same depth. In every case, it was found that the winter
precipitation had caused moisture changes to the full depth reached
by the soil auger. Moreover, these changes were so great as to lead
the investigators to believe that moisture changes had occurred to
greater depths.
In districts where the major part of the precipitation occurs during
the summer the same law is undoubtedly in operation; but, since
evaporation is most active in the summer, it is probable that a
smaller proportion reaches the greater soil depths. In the Great
Plains district, therefore, greater care will have to be exercised
during the summer in securing proper water storage than in the Great
Basin, for instance. The principle is, nevertheless, the same. Burr,
working under Great Plains conditions in Nebraska, has shown that
the spring and summer rains penetrate the soil to the depth of 6
feet, the average depth of the borings, and that it undoubtedly
affects the soil-moisture to the depth of 10 feet. In general, the
dry-farmer may safely accept the doctrine that the water that falls
upon his land penetrates the soil far beyond the immediate reach of
the sun, though not so far away that plant roots cannot make use of
it.
Importance of a moist subsoil
In the consideration of the downward movement of soil-water it is to
be noted that it is only when the soil is tolerably moist that the
natural precipitation moves rapidly and freely to the deeper soil
layers. When the soil is dry, the downward movement of the water is
much slower and the bulk of the water is then stored near the
surface where the loss of moisture goes on most rapidly. It has been
observed repeatedly in the investigations at the Utah Station that
when desert land is broken for dry-farm purposes and then properly
cultivated, the precipitation penetrates farther and farther into
the soil with every year of cultivation. For example, on a dry-farm,
the soil of which is clay loam, and which was plowed in the fall of
1904 and farmed annually thereafter, the eighth foot contained in
the spring of 1905, 6.59 per cent of moisture; in the spring of
1906, 13.11 per cent, and in the spring of 1907, 14.75 per cent of
moisture. On another farm, with a very sandy soil and also plowed in
the fall of 1904, there was found in the eighth foot in the spring
of 1905, 5.63 per cent of moisture, in the spring of 1906, 11.41 per
cent of moisture, and in the spring of 1907, 15.49 per cent of
moisture. In both of these typical cases it is evident that as the
topsoil was loosened, the full field water capacity of the soil was
more nearly approached to a greater depth. It would seem that, as
the lower soil layers are moistened, the water is enabled, so to
speak, to slide down more easily into the depths of the soil.
This is a very important principle for the dry farmer to understand.
It is always dangerous to permit the soil of a dry-farm to become
very dry, especially below the first foot. Dry-farms should be so
manipulated that even at the harvesting season a comparatively large
quantity of water remains in the soil to a depth of 8 feet or more.
The larger the quantity of water in the soil in the fall, the more
readily and quickly will the water that falls on the land during the
resting period of fall, winter, and early spring sink into the soil
and move away from the topsoil. The top or first foot will always
contain the largest percentage of water because it is the chief
receptacle of the water that falls as rain or snow but when the
subsoil is properly moist, the water will more completely leave the
topsoil. Further, crops planted on a soil saturated with water to a
depth of 8 feet are almost certain to mature and yield well.
If the field-water capacity has not been filled, there is always the
danger that an unusually dry season or a series of hot winds or
other like circumstances may either seriously injure the crop or
cause a complete failure. The dry-farmer should keep a surplus of
moisture in the soil to be carried over from year to year, just as
the wise business man maintains a sufficient working capital for the
needs of his business. In fact, it is often safe to advise the
prospective dry-farmer to plow his newly cleared or broken land
carefully and then to grow no crop on it the first year, so that,
when crop production begins, the soil will have stored in it an
amount of water sufficient to carry a crop over periods of drouth.
Especially in districts of very low rainfall is this practice to be
recommended. In the Great Plains area, where the summer rains tempt
the farmer to give less attention to the soil-moisture problem than
in the dry districts with winter precipitation farther West, it is
important that a fallow season be occasionally given the land to
prevent the store of soil moisture from becoming dangerously low.
To what extent is the rainfall stored in soils?
What proportion of the actual amount of water falling upon the soil
can be stored in the soil and carried over from season to season?
This question naturally arises in view of the conclusion that water
penetrates the soil to considerable depths. There is comparatively
little available information with which to answer this question,
because the great majority of students of soil moisture have
concerned themselves wholly with the upper two, three, or four feet
of soil. The results of such investigations are practically useless
in answering this question. In humid regions it may be very
satisfactory to confine soil-moisture investigations to the upper
few feet; but in arid regions, where dry-farming is a living
question, such a method leads to erroneous or incomplete
conclusions.
Since the average field capacity of soils for water is about 2.5
inches per foot, it follows that it is possible to store 25 inches
of water in 10 feet of soil. This is from two to one and a half
times one year's rainfall over the better dry-farming sections.
Theoretically, therefore, there is no reason why the rainfall of one
season or more could not be stored in the soil. Careful
investigations have borne out this theory. Atkinson found, for
example, at the Montana Station, that soil, which to a depth of 9
feet contained 7.7 per cent of moisture in the fall contained 11.5
per cent in the spring and, after carrying it through the summer by
proper methods of cultivation, 11 per cent.
It may certainly be concluded from this experiment that it is
possible to carry over the soil moisture from season to season. The
elaborate investigations at the Utah Station have demonstrated that
the winter precipitation, that is, the precipitation that comes
during the wettest period of the year, may be retained in a large
measure in the soil. Naturally, the amount of the natural
precipitation accounted for in the upper eight feet will depend upon
the dryness of the soil at the time the investigation commenced. If
at the beginning of the wet season the upper eight feet of soil are
fairly well stored with moisture, the precipitation will move down
to even greater depths, beyond the reach of the soil auger. If, on
the other hand, the soil is comparatively dry at the beginning of
the season, the natural precipitation will distribute itself through
the upper few feet, and thus be readily measured by the soil auger.
In the Utah investigations it was found that of the water which fell
as rain and snow during the winter, as high as 95-1/2 per cent was
found stored in the first eight feet of soil at the beginning of the
growing season. Naturally, much smaller percentages were also found,
but on an average, in soils somewhat dry at the beginning of the dry
season, more than three fourths of the natural precipitation was
found stored in the soil in the spring. The results were all
obtained in a locality where the bulk of the precipitation comes in
the winter, yet similar results would undoubtedly be obtained where
the precipitation occurs mainly in the summer. The storage of water
in the soil cannot be a whit less important on the Great Plains than
in the Great Basin. In fact, Burr has clearly demonstrated for
western Nebraska that over 50 per cent of the rainfall of the spring
and summer may be stored in the soil to the depth of six feet.
Without question, some is stored also at greater depths.
All the evidence at hand shows that a large portion of the
precipitation falling upon properly prepared soil, whether it be
summer or winter, is stored in the soil until evaporation is allowed
to withdraw it Whether or not water so stored may be made to remain
in the soil throughout the season or the year will be discussed in
the next chapter. It must be said, however, that the possibility of
storing water in the soil, that is, making the water descend to
relatively great soil depths away from the immediate and direct
action of the sunshine and winds, is the most fundamental principle
in successful dry-farming.
The fallow
It may be safely concluded that a large portion of the water that
falls as rain or snow may be stored in the soil to considerable
depths (eight feet or more). However, the question remains, Is it
possible to store the rainfall of successive years in the soil for
the use of one crop? In short, Does the practice of clean fallowing
or resting the ground with proper cultivation for one season enable
the farmer to store in the soil the larger portion of the rainfall
of two years, to be used for one crop? It is unquestionably true, as
will be shown later, that clean fallowing or "summer tillage" is one
of the oldest and safest practices of dry-farming as practiced in
the West, but it is not generally understood why fallowing is
desirable.
Considerable doubt has recently been cast upon the doctrine that one
of the beneficial effects of fallowing in dry-farming is to store
the rainfall of successive seasons in the soil for the use of one
crop. Since it has been shown that a large proportion of the winter
precipitation can be stored in the soil during the wet season, it
merely becomes a question of the possibility of preventing the
evaporation of this water during the drier season. As will be shown
in the next chapter, this can well be effected by proper
cultivation.
There is no good reason, therefore, for believing that the
precipitation of successive seasons may not be added to water
already stored in the soil. King has shown that fallowing the soil
one year carried over per square foot, in the upper four feet, 9.38
pounds of water more than was found in a cropped soil in a parallel
experiment; and, moreover, the beneficial effect of this. water
advantage was felt for a whole succeeding season. King concludes,
therefore, that one of the advantages of fallowing is to increase
the moisture content of the soil. The Utah experiments show that the
tendency of fallowing is always to increase the soil-moisture
content. In dry-farming, water is the critical factor, and any
practice that helps to conserve water should be adopted. For that
reason, fallowing, which gathers soil-moisture, should be strongly
advocated. In Chapter IX another important value of the fallow will
be discussed.
In view of the discussion in this chapter it is easily understood
why students of soil-moisture have not found a material increase in
soil-moisture due to fallowing. Usually such investigations have
been made to shallow depths which already were fairly well filled
with moisture. Water falling upon such soils would sink beyond the
depth reached by the soil augers, and it became impossible to judge
accurately of the moisture-storing advantage of the fallow. A
critical analysis of the literature on this subject will reveal the
weakness of most experiments in this respect.
It may be mentioned here that the only fallow that should be
practiced by the dry-farmer is the clean fallow. Water storage is
manifestly impossible when crops are growing upon a soil. A healthy
crop of sagebrush, sunflowers, or other weeds consumes as much water
as a first-class stand of corn, wheat, or potatoes. Weeds should be
abhorred by the farmer. A weedy fallow is a sure forerunner of a
crop failure. How to maintain a good fallow is discussed in Chapter
VIII, under the head of Cultivation. Moreover, the practice of
fallowing should be varied with the climatic conditions. In
districts of low rainfall, 10-15 inches, the land should be clean
summer-fallowed every other year; under very low rainfall perhaps
even two out of three years; in districts of more abundant rainfall,
15-20 inches, perhaps one year out of every three or four is
sufficient. Where the precipitation comes during the growing season,
as in the Great Plains area, fallowing for the storage of water is
less important than where the major part of the rainfall comes
during the fall and winter. However, any system of dry-farming that
omits fallowing wholly from its practices is in danger of failure in
dry years.
Deep plowing for water storage
It has been attempted in this chapter to demonstrate that water
falling upon a soil may descend to great depths, and may be stored
in the soil from year to year, subject to the needs of the crop that
may be planted. By what cultural treatment may this downward descent
of the water be accelerated by the farmer? First and foremost, by
plowing at the right time and to the right depth. Plowing should be
done deeply and thoroughly so that the falling water may immediately
be drawn down to the full depth of the loose, spongy, plowed soil,
away from the action of the sunshine or winds. The moisture thus
caught will slowly work its way down into the lower layers of the
soil. Deep plowing is always to be recommended for successful
dry-farming.
In humid districts where there is a great difference between the
soil and the subsoil, it is often dangerous to turn up the lifeless
subsoil, but in arid districts where there is no real
differentiation between the soil and the subsoil, deep plowing may
safely be recommended. True, occasionally, soils are found in the
dry-farm territory which are underlaid near the surface by an inert
clay or infertile layer of lime or gypsum which forbids the farmer
putting the plow too deeply into the soil. Such soils, however' are
seldom worth while trying for dry-farm purposes. Deep plowing must
be practiced for the best dry-farming results.
It naturally follows that subsoiling should be a beneficial practice
on dry-farms. Whether or not the great cost of subsoiling is offset
by the resulting increased yields is an open question; it is, in
fact, quite doubtful. Deep plowing done at the right time and
frequently enough is possibly sufficient. By deep plowing is meant
stirring or turning the soil to a depth of six to ten inches below
the surface of the land.
Fall plowing far water storage
It is not alone sufficient to plow and to plow deeply; it is also
necessary that the plowing be done at the right time. In the very
great majority of cases over the whole dry-farm territory, plowing
should be done in the fall. There are three reasons for this: First,
after the crop is harvested, the soil should be stirred immediately,
so that it can be exposed to the full action of the weathering
agencies, whether the winters be open or closed. If for any reason
plowing cannot be done early it is often advantageous to follow the
harvester with a disk and to plow later when convenient. The
chemical effect on the soil resulting from the weathering, made
possible by fall plowing, as will be shown in Chapter IX, is of
itself so great as to warrant the teaching of the general practice
of fall plowing. Secondly, the early stirring of the soil prevents
evaporation of the moisture in the soil during late summer and the
fall. Thirdly, in the parts of the dry-farm territory where much
precipitation occurs in the fall, winter, or early spring, fall
plowing permits much of this precipitation to enter the soil and be
stored there until needed by plants.
A number of experiment stations have compared plowing done in the
early fall with plowing done late in the fall or in the spring, and
with almost no exception it has been found that early fall plowing
is water-conserving and in other ways advantageous. It was observed
on a Utah dry-farm that the fall-plowed land contained, to a depth
of 10 feet, 7.47 acre-inches more water than the adjoining
spring-plowed land--a saving of nearly one half of a year's
precipitation. The ground should be plowed in the early fall as soon
as possible after the crop is harvested. It should then be left in
the rough throughout the winter, so that it may be mellowed and
broken down by the elements. The rough lend further has a tendency
to catch and hold the snow that may be blown by the wind, thus
insuring a more even distribution of the water from the melting
snow.
A common objection to fall plowing is that the ground is so dry in
the fall that it does not plow up well, and that the great dry clods
of earth do much to injure the physical condition of the soil. It is
very doubtful if such an objection is generally valid, especially if
the soil is so cropped as to leave a fair margin of moisture in the
soil at harvest time. The atmospheric agencies will usually break
down the clods, and the physical result of the treatment will be
beneficial. Undoubtedly, the fall plowing of dry land is somewhat
difficult, but the good results more than pay the farmer for his
trouble. Late fall plowing, after the fall rains have softened the
land, is preferable to spring plowing. If for any reason the farmer
feels that he must practice spring plowing, he should do it as early
as possible in the spring. Of course, it is inadvisable to plow the
soil when it is so wet as to injure its tilth seriously, but as soon
as that danger period has passed, the plow should be placed in the
ground. The moisture in the soil will thereby be conserved, and
whatever water may fall during the spring months will be conserved
also. This is of especial importance in the Great Plains region and
in any district where the precipitation comes in the spring and
winter months.
Likewise, after fall plowing, the land must be well stirred in the
early spring with the disk harrow or a similar implement, to enable
the spring rains to enter the soil easily and to prevent the
evaporation of the water already stored. Where the rainfall is quite
abundant and the plowed land has been beaten down by the frequent
rains, the land should be plowed again in the spring. Where such
conditions do not exist, the treatment of the soil with the disk and
harrow in the spring is usually sufficient.
In recent dry-farm experience it has been fairly completely
demonstrated that, providing the soil is well stored with water,
crops will mature even if no rain falls during the growing season.
Naturally, under most circumstances, any rains that may fall on a
well-prepared soil during the season of crop growth will tend to
increase the crop yield, but some profitable yield is assured, in
spite of the season, if the soil is well stored with water at seed
time. This is an important principle in the system of dry-farming.