REGULATING THE EVAPORATION
The demonstration in the last chapter that the water which falls as
rain or snow may be stored in the soil for the use of plants is of
first importance in dry-farming, for it makes the farmer
independent, in a large measure, of the distribution of the
rainfall. The dry-farmer who goes into the summer with a soil well
stored with water cares little whether summer rains come or not, for
he knows that his crops will mature in spite of external drouth. In
fact, as will be shown later, in many dry-farm sections where the
summer rains are light they are a positive detriment to the farmer
who by careful farming has stored his deep soil with an abundance of
water. Storing the soil with water is, however, only the first step
in making the rains of fall, winter, or the preceding year available
for plant growth. As soon as warm growing weather comes,
water-dissipating forces come into play, and water is lost by
evaporation. The farmer must, therefore, use all precautions to keep
the moisture in the soil until such time as the roots of the crop
may draw it into the plants to be used in plant production. That is,
as far as possible, direct evaporation of water from the soil must
be prevented.
Few farmers really realize the immense possible annual evaporation
in the dry-farm territory. It is always much larger than the total
annual rainfall. In fact, an arid region may be defined as one in
which under natural conditions several times more water evaporates
annually from a free water surface than falls as rain and snow. For
that reason many students of aridity pay little attention to
temperature, relative humidity, or winds, and simply measure the
evaporation from a free water surface in the locality in question.
In order to obtain a measure of the aridity, MacDougal has
constructed the following table, showing the annual precipitation
and the annual evaporation at several well-known localities in the
dry-farm territory.
True, the localities included in the following table are extreme,
but they illustrate the large possible evaporation, ranging from
about six to thirty-five times the precipitation. At the same time
it must be borne in mind that while such rates of evaporation may
occur from free water surfaces, the evaporation from agricultural
soils under like conditions is very much smaller.
Place Annual Precipitation Annual Evaporation Ratio
(In Inches) (In Inches)
El Paso, Texas 9.23 80 8.7
Fort Wingate,
New Mexico 14.00 80 5.7
Fort Yuma,
Arizona 2.84 100 35.2
Tucson, AZ 11.74 90 7.7
Mohave, CA 4.97 95 19.1
Hawthorne,
Nevada 4.50 80 17.5
Winnemucca,
Nevada 9.51 80 9.6
St. George, Utah 6.46 90 13.9
Fort Duchesne,
Utah 6.49 75 11.6
Pineville,
Oregon 9.01 70 7.8
Lost River,
Idaho 8.47 70 8.3
Laramie,
Wyoming 9.81 70 7.1
Torres, Mexico 16.97 100 6.0
To understand the methods employed for checking evaporation from the
soil, it is necessary to review briefly the conditions that
determine the evaporation of water into the air, and the manner in
which water moves in the soil.
The formation of water vapor
Whenever water is left freely exposed to the air, it evaporates;
that is, it passes into the gaseous state and mixes with the gases
of the air. Even snow and ice give off water vapor, though in very
small quantities. The quantity of water vapor which can enter a
given volume of air is definitely limited. For instance, at the
temperature of freezing water 2.126 grains of water vapor can enter
one cubic foot of air, but no more. When air contains all the water
possible, it is said to be saturated, and evaporation then ceases.
The practical effect of this is the well-known experience that on
the seashore, where the air is often very nearly fully saturated
with water vapor, the drying of clothes goes on very slowly, whereas
in the interior, like the dry-farming territory, away from the
ocean, where the air is far from being saturated, drying goes on
very rapidly.
The amount of water necessary to saturate air varies greatly with
the temperature. It is to be noted that as the temperature
increases, the amount of water that may be held by the air also
increases; and proportionately more rapidly than the increase in
temperature. This is generally well understood in common experience,
as in drying clothes rapidly by hanging them before a hot fire. At a
temperature of 100 deg F., which is often reached in portions of the
dry-farm territory during the growing season, a given volume of air
can hold more than nine times as much water vapor as at the
temperature of freezing water. This is an exceedingly important
principle in dry-farm practices, for it explains the relatively easy
possibility of storing water during the fall and winter when the
temperature is low and the moisture usually abundant, and the
greater difficulty of storing the rain that falls largely, as in the
Great Plains area, in the summer when water-dissipating forces are
very active. This law also emphasizes the truth that it is in times
of warm weather that every precaution must be taken to prevent the
evaporation of water from the soil surface.
Temperature Grains of Water held in
in Degrees F. One Cubic Foot of Air
32 2.126
40 2.862
50 4.089
60 5.756
70 7.992
80 10.949
90 14.810
100 19.790
It is of course well understood that the atmosphere as a whole is
never saturated with water vapor. Such saturation is at the best
only local, as, for instance, on the seashore during quiet days,
when the layer of air over the water may be fully saturated, or in a
field containing much water from which, on quiet warm days, enough
water may evaporate to saturate the layer of air immediately upon
the soil and around the plants. Whenever, in such cases, the air
begins to move and the wind blows, the saturated air is mixed with
the larger portion of unsaturated air, and evaporation is again
increased. Meanwhile, it must be borne in mind that into a layer of
saturated air resting upon a field of growing plants very little
water evaporates, and that the chief water-dissipating power of
winds lies in the removal of this saturated layer. Winds or air
movements of any kind, therefore, become enemies of the farmer who
depends upon a limited rainfall.
The amount of water actually found in a given volume of air at a
certain temperature, compared with the largest amount it can hold,
is called the relative humidity of the air. As shown in Chapter IV,
the relative humidity becomes smaller as the rainfall decreases. The
lower the relative humidity is at a given temperature, the more
rapidly will water evaporate into the air. There is no more striking
confirmation of this law than the fact that at a temperature of 90
deg sunstrokes and similar ailments are reported in great number
from New York, while the people of Salt Lake City are perfectly
comfortable. In New York the relative humidity in summer is about 73
per cent; in Salt Lake City, about 35 per cent. At a high summer
temperature evaporation from the skin goes on slowly in New York and
rapidly in Salt Lake City, with the resulting discomfort or comfort.
Similarly, evaporation from soils goes on rapidly under a low and
slowly under a high percentage of relative humidity.
Evaporation from water surfaces is hastened, therefore, by (1) an
increase in the temperature, (2) an increase in the air movements or
winds, and (3) a decrease in the relative humidity. The temperature
is higher; the relative humidity lower, and the winds usually more
abundant in arid than in humid regions. The dry-farmer must
consequently use all possible precautions to prevent evaporation
from the soil.
Conditions of evaporation from from soils
Evaporation does not alone occur from a surface of free water. All
wet or moist substances lose by evaporation most of the water that
they hold, providing the conditions of temperature and relative
humidity are favorable. Thus, from a wet soil, evaporation is
continually removing water. Yet, under ordinary conditions, it is
impossible to remove all the water, for a small quantity is
attracted so strongly by the soil particles that only a temperature
above the boiling point of water will drive it out. This part of the
soil is the hygroscopic moisture spoken of in the last chapter.
Moreover, it must be kept in mind that evaporation does not occur as
rapidly from wet soil as from a water surface, unless all the soil
pores are so completely filled with water that the soil surface is
practically a water surface. The reason for this reduced evaporation
from a wet soil is almost self-evident. There is a comparatively
strong attraction between soil and water, which enables the moisture
to cling as a thin capillary film around the soil particles, against
the force of gravity. Ordinarily, only capillary water is found in
well-tilled soil, and the force causing evaporation must be strong
enough to overcome this attraction besides changing the water into
vapor.
The less water there is in a soil, the thinner the water film, and
the more firmly is the water held. Hence, the rate of evaporation
decreases with the decrease in soil-moisture. This law is confirmed
by actual field tests. For instance, as an average of 274 trials
made at the Utah Station, it was found that three soils, otherwise
alike, that contained, respectively, 22.63 per cent, 17.14 per cent,
and 12.75 per cent of water lost in two weeks, to a depth of eight
feet, respectively 21.0, 17. 1, and 10.0 pounds of water per square
foot. Similar experiments conducted elsewhere also furnish proof of
the correctness of this principle. From this point of view the
dry-farmer does not want his soils to be unnecessarily moist. The
dry-farmer can reduce the per cent of water in the soil without
diminishing the total amount of water by so treating the soil that
the water will distribute itself to considerable depths. This brings
into prominence again the practices of fall plowing, deep plowing,
subsoiling, and the choice of deep soils for dry-farming.
Very much for the same reasons, evaporation goes on more slowly from
water in which salt or other substances have been dissolved. The
attraction between the water and the dissolved salt seems to be
strong enough to resist partially the force causing evaporation.
Soil-water always contains some of the soil ingredients in solution,
and consequently under the given conditions evaporation occurs more
slowly from soil-water than from pure water. Now, the more fertile a
soil is, that is, the more soluble plant-food it contains, the more
material will be dissolved in the soil-water, and as a result the
more slowly will evaporation take place. Fallowing, cultivation,
thorough plowing and manuring, which increase the store of soluble
plant-food, all tend to diminish evaporation. While these conditions
may have little value in the eyes of the farmer who is under an
abundant rainfall, they are of great importance to the dry-farmer.
It is only by utilizing every possibility of conserving water and
fertility that dry-farming may be made a perfectly safe practice.
Loss by evaporation chiefly at the surface
Evaporation goes on from every wet substance. Water evaporates
therefore from the wet soil grains under the surface as well as from
those at the surface. In developing a system of practice which will
reduce evaporation to a minimum it must be learned whether the water
which evaporates from the soil particles far below the surface is
carried in large quantities into the atmosphere and thus lost to
plant use. Over forty years ago, Nessler subjected this question to
experiment and found that the loss by evaporation occurs almost
wholly at the soil surface, and that very little if any is lost
directly by evaporation from the lower soil layers. Other
experimenters have confirmed this conclusion, and very recently
Buckingham, examining the same subject, found that while there is a
very slow upward movement of the soil gases into the atmosphere, the
total quantity of the water thus lost by direct evaporation from
soil, a foot below the surface, amounted at most to one inch of
rainfall in six years. This is insignificant even under semiarid and
arid conditions. However, the rate of loss of water by direct
evaporation from the lower soil layers increases with the porosity
of the soil, that is, with the space not filled with soil particles
or water. Fine-grained soils, therefore, lose the least water in
this manner. Nevertheless, if coarse-grained soils are well filled
with water, by deep fall plowing and by proper summer fallowing for
the conservation of moisture, the loss of moisture by direct
evaporation from the lower soil layers need not be larger than from
finer grained soils
Thus again are emphasized the principles previously laid down that,
for the most successful dry-farming, the soil should always be kept
well filled with moisture, even if it means that the land, after
being broken, must lie fallow for one or two seasons, until a
sufficient amount of moisture has accumulated. Further, the
correlative principle is emphasized that the moisture in dry-farm
lands should be stored deeply, away from the immediate action of the
sun's rays upon the land surface. The necessity for deep soils is
thus again brought out.
The great loss of soil moisture due to an accumulation of water in
the upper twelve inches is well brought out in the experiments
conducted by the Utah Station. The following is selected from the
numerous data on the subject. Two soils, almost identical in
character, contained respectively 17.57 per cent and 16.55 per cent
of water on an average to a depth of eight feet; that is, the total
amount of water held by the two soils was practically identical.
Owing to varying cultural treatment, the distribution of the water
in the soil was not uniform; one contained 23.22 per cent and the
other 16.64 per cent of water in the first twelve inches. During the
first seven days the soil that contained the highest percentage of
water in the first foot lost 13.30 pounds of water, while the other
lost only 8.48 pounds per square foot. This great difference was due
no doubt to the fact that direct evaporation takes place in
considerable quantity only in the upper twelve inches of soil, where
the sun's heat has a full chance to act.
Any practice which enables the rains to sink quickly to considerable
depths should be adopted by the dry-farmer. This is perhaps one of
the great reasons for advocating the expensive but usually effective
subsoil plowing on dry-farms. It is a very common experience, in the
arid region, that great, deep cracks form during hot weather. From
the walls of these cracks evaporation goes on, as from the topsoil,
and the passing winds renew the air so that the evaporation may go
on rapidly. The dry-farmer must go over the land as often as needs
be with some implement that will destroy and fill up the cracks that
may have been formed. In a field of growing crops this is often
difficult to do; but it is not impossible that hand hoeing,
expensive as it is, would pay well in the saving of soil moisture
and the consequent increase in crop yield.
How soil water reaches the surface
It may be accepted as an established truth that the direct
evaporation of water from wet soils occurs almost wholly at the
surface. Yet it is well known that evaporation from the soil surface
may continue until the soil-moisture to a depth of eight or ten feet
or more is depleted. This is shown by the following analyses of
dry-farm soil in early spring and midsummer. No attempt was made to
conserve the moisture in the soil:--
Per cent of water in Early spring Midsummer
1st foot 20.84 8.83
2nd foot 20.06 8.87
3rd foot 19.62 11.03
4th foot 18.28 9.59
5th foot 18.70 11.27
6th foot 14.29 11.03
7th foot 14.48 8.95
8th foot 13.83 9.47
Avg 17.51 9.88
In this case water had undoubtedly passed by capillary movement from
the depth of eight feet to a point near the surface where direct
evaporation could occur. As explained in the last chapter, water
which is held as a film around the soil particles is called
capillary water; and it is in the capillary form that water may be
stored in dry-farm soils. Moreover, it is the capillary
soil-moisture alone which is of real value in crop production. This
capillary water tends to distribute itself uniformly throughout the
soil, in accordance with the prevailing conditions and forces. If no
water is removed from the soil, in course of time the distribution
of the soil-water will be such that the thickness of the film at any
point in the soil mass is a direct resultant of the various forces
acting at that particular point. There will then be no appreciable
movement of the soil-moisture. Such a condition is approximated in
late winter or early spring before planting begins. During the
greater part of the year, however, no such quiescent state can
occur, for there are numerous disturbing elements that normally are
active, among which the three most effective are (l) the addition of
water to the soil by rains; (2) the evaporation of water from the
topsoil, due to the more active meteorological factors during
spring, summer, and fall; and (3) the abstraction of water from the
soil by plant roots.
Water, entering the soil, moves downward under the influence of
gravity as gravitational water, until under the attractive influence
of the soil it has been converted into capillary water and adheres
to the soil particles as a film. If the soil were dry, and the film
therefore thin, the rain water would move downward only a short
distance as gravitational water; if the soil were wet, and the film
therefore thick, the water would move down to a greater distance
before being exhausted. If, as is often the case in humid districts,
the soil is saturated, that is, the film is as thick as the
particles can hold, the water would pass right through the soil and
connect with the standing water below. This, of course, is seldom
the case in dry-farm districts. In any soil, excepting one already
saturated, the addition of water will produce a thickening of the
soil-water film to the full descent of the water. This immediately
destroys the conditions of equilibrium formerly existing, for the
moisture is not now uniformly distributed. Consequently a process of
redistribution begins which continues until the nearest approach to
equilibrium is restored. In this process water will pass in every
direction from the wet portion of the soil to the drier; it does not
necessarily mean that water will actually pass from the wet portion
to the drier portion; usually, at the driest point a little water is
drawn from the adjoining point, which in turn draws from the next,
and that from the next, until the redistribution is complete. The
process is very much like stuffing wool into a sack which already is
loosely filled. The new wool does not reach the bottom of the sack,
yet there is more wool in the bottom than there was before.
If a plant-root is actively feeding some distance under the soil
surface, the reverse process occurs. At the feeding point the root
continually abstracts water from the soil grains and thus makes the
film thinner in that locality. This causes a movement of moisture
similar to the one above described, from the wetter portions of the
soil to the portion being dried out by the action of the plant-root.
Soil many feet or even rods distant may assist in supplying such an
active root with moisture. When the thousands of tiny roots sent out
by each plant are recalled. it may well be understood what a
confusion of pulls and counter-pulls upon the soil-moisture exists
in any cultivated soil. In fact, the soil-water film may be viewed
as being in a state of trembling activity, tending to place itself
in full equilibrium with the surrounding contending forces which,
themselves, constantly change. Were it not that the water film held
closely around the soil particles is possessed of extreme mobility,
it would not be possible to meet the demands of the plants upon the
water at comparatively great distances. Even as it is, it frequently
happens that when crops are planted too thickly on dry-farms, the
soil-moisture cannot move quickly enough to the absorbing roots to
maintain plant growth, and crop failure results. Incidentally, this
points to planting that shall be proportional to the moisture
contained by the soil. See Chapter XI.
As the temperature rises in spring, with a decrease in the relative
humidity, and an increase in direct sunshine, evaporation from the
soil surface increases greatly. However, as the topsoil becomes
drier, that is, as the water fihn becomes thinner, there is an
attempt at readjustment, and water moves upward to take the place of
that lost by evaporation. As this continues throughout the season,
the moisture stored eight or ten feet or more below the surface is
gradually brought to the top and evaporated, and thus lost to plant
use.
The effect of rapid top drying of soils
As the water held by soils diminishes, and the water film around the
soil grains becomes thinner, the capillary movement of the
soil-water is retarded. This is easily understood by recalling that
the soil particles have an attraction for water, which is of
definite value, and may be measured by the thickest film that may be
held against gravity. When the film is thinned, it does not diminish
the attraction of the soil for water; it simply results in a
stronger pull upon the water and a firmer holding of the film
against the surfaces of the soil grains. To move soil-water under
such conditions requires the expenditure of more energy than is
necessary for moving water in a saturated or nearly saturated soil.
Under like conditions, therefore, the thinner the soil-water film
the more difficult will be the upward movement of the soil-water and
the slower the evaporation from the topsoil.
As drying goes on, a point is reached at which the capillary
movement of the water wholly ceases. This is probably when little
more than the hygroscopic moisture remains. In fact, very dry soil
and water repel each other. This is shown in the common experience
of driving along a road in summer, immediately after a light shower.
The masses of dust are wetted only on the outside, and as the wheels
pass through them the dry dust is revealed. It is an important fact
that very dry soil furnishes a very effective protection against the
capillary movement of water.
In accordance with the principle above established if the surface
soil could be dried to the point where capillarity is very slow, the
evaporation would be diminished or almost wholly stopped. More than
a quarter of a century ago, Eser showed experimentally that
soil-water may be saved by drying the surface soil rapidly. Under
dry-farm conditions it frequently occurs that the draft upon the
water of the soil is so great that nearly all the water is quickly
and so completely abstracted from the upper few inches of soil that
they are left as an effective protection against further
evaporation. For instance, in localities where hot dry winds are of
common occurrence, the upper layer of soil is sometimes completely
dried before the water in the lower layers can by slow capillary
movement reach the top. The dry soil layer then prevents further
loss of water, and the wind because of its intensity has helped to
conserve the soil-moisture. Similarly in localities where the
relative humidity is low, the sunshine abundant, and the temperature
high, evaporation may go on so rapidly that the lower soil layers
cannot supply the demands made, and the topsoil then dries out so
completely as to form a protective covering against further
evaporation. It is on this principle that the native desert soils of
the United States, untouched by the plow, and the surfaces of which
are sun-baked, are often found to possess large percentages of water
at lower depths. Whitney recorded this observation with considerable
surprise, many years ago, and other observers have found the same
conditions at nearly all points of the arid region. This matter has
been subjected to further study by Buckingham, who placed a variety
of soils under artificially arid and humid conditions. It was found
in every case that, the initial evaporation was greater under arid
conditions, but as the process went on and the topsoil of the arid
soil became dry, more water was lost under humid conditions. For the
whole experimental period, also, more water was lost under humid
conditions. It was notable that the dry protective layer was formed
more slowly on alkali soils, which would point to the inadvisability
of using alkali lands for dry-farm purposes. All in all, however, it
appears "that under very arid conditions a soil automatically
protects itself from drying by the formation of a natural mulch on
the surface."
Naturally, dry-farm soils differ greatly in their power of forming
such a mulch. A heavy clay or a light sandy soil appears to have
less power of such automatic protection than a loamy soil. An
admixture of limestone seems to favor the formation of such a
natural protective mulch. Ordinarily, the farmer can further the
formation of a dry topsoil layer by stirring the soil thoroughly.
This assists the sunshine and the air to evaporate the water very
quickly. Such cultivation is very desirable for other reasons also,
as will soon be discussed. Meanwhile, the water-dissipating forces
of the dry-farm section are not wholly objectionable, for whether
the land be cultivated or not, they tend to hasten the formation of
dry surface layers of soil which guard against excessive
evaporation. It is in moist cloudy weather, when the drying process
is slow, that evaporation causes the greatest losses of
soil-moisture.
The effect of shading
Direct sunshine is, next to temperature, the most active cause of
rapid evaporation from moist soil surfaces. Whenever, therefore,
evaporation is not rapid enough to form a dry protective layer of
topsoil, shading helps materially in reducing surface losses of
soil-water. Under very arid conditions, however, it is questionable
whether in all cases shading has a really beneficial effect, though
under semiarid or sub-humid conditions the benefits derived from
shading are increased largely. Ebermayer showed in 1873 that the
shading due to the forest cover reduced evaporation 62 per cent, and
many experiments since that day have confirmed this conclusion. At
the Utah Station, under arid conditions, it was found that shading a
pot of soil, which otherwise was subjected to water-dissipating
influences, saved 29 per cent of the loss due to evaporation from a
pot which was not shaded. This principle cannot be applied very
greatly in practice, but it points to a somewhat thick planting,
proportioned to the water held by the soil. It also shows a possible
benefit to be derived from the high header straw which is allowed to
stand for several weeks in dry-farm sections where the harvest comes
early and the fall plowing is done late, as in the mountain states.
The high header stubble shades the ground very thoroughly. Thus the
stubble may be made to conserve the soil-moisture in dry-farm
sections, where grain is harvested by the "header" method.
A special case of shading is the mulching of land with straw or
other barnyard litter, or with leaves, as in the forest. Such
mulching reduces evaporation, but only in part, because of its
shading action, since it acts also as a loose top layer of soil
matter breaking communication with the lower soil layers.
Whenever the soil is carefully stirred, as will be described, the
value of shading as a means or checking evaporation disappears
almost entirely. It is only with soils which are tolerably moist at
the surface that shading acts beneficially.
Alfalfa in cultivated rows. This practice is employed to make
possible the growth of alfalfa and other perennial crops on arid
lands without irrigation.
The effect of tillage
Capillary soil-moisture moves from particle to particle until the
surface is reached. The closer the soil grains are packed together,
the greater the number of points or contact, and the more easily
will the movement of the soil-moisture proceed. If by any means a
layer of the soil is so loosened as to reduce the number of points
of contact, the movement of the soil-moisture is correspondingly
hindered. The process is somewhat similar to the experience in large
r airway stations. Just before train time a great crowd of people is
gathered outside or the gates ready to show their tickets. If one
gate is opened, a certain number of passengers can pass through each
minute; if two are opened, nearly twice as many may be admitted in
the same time; if more gates are opened, the passengers will be able
to enter the train more rapidly. The water in the lower layers of
the soil is ready to move upward whenever a call is made upon it. To
reach the surface it must pass from soil grain to soil grain, and
the larger the number of grains that touch, the more quickly and
easily will the water reach the surface, for the points of contact
of the soil particles may be likened to the gates of the railway
station. Now if, by a thorough stirring and loosening of the
topsoil, the number of points of contact between the top and subsoil
is greatly reduced, the upward flow of water is thereby largely
checked. Such a loosening of the topsoil for the purpose of reducing
evaporation from the topsoil has come to be called cultivation, and
includes plowing, harrowing, disking, hoeing, and other cultural
operations by which the topsoil is stirred. The breaking of the
points of contact between the top and subsoil is undoubtedly the
main reason for the efficiency of cultivation, but it is also to be
remembered that such stirring helps to dry the top soil very
thoroughly, and as has been explained a layer of dry soil of itself
is a very effective check upon surface evaporation.
That the stirring or cultivation of the topsoil really does diminish
evaporation of water from the soil has been shown by numerous
investigations. In 1868, Nessler found that during six weeks of an
ordinary German summer a stirred soil lost 510 grams of water per
square foot, while the adjoining compacted soil lost 1680 grams,--a
saving due to cultivation of nearly 60 per cent. Wagner, testing the
correctness of Nessler's work, found, in 1874, that cultivation
reduced the evaporation a little more than 60 per cent; Johnson, in
1878, confirmed the truth of the principle on American soils, and
Levi Stockbridge, working about the same time, also on American
soils, found that cultivation diminished evaporation on a clay soil
about 23 per cent, on a sandy loam 55 per cent, and on a heavy loam
nearly 13 per cent. All the early work done on this subject was done
under humid conditions, and it is only in recent years that
confirmation of this important principle has been obtained for the
soils of the dry-farm region. Fortier, working under California
conditions, determined that cultivation reduced the evaporation from
the soil surface over 55 per cent. At the Utah Station similar
experiments have shown that the saving of soil-moisture by
cultivation was 63 per cent for a clay soil, 34 per cent for a
coarse sand, and 13 per cent for a clay loam. Further, practical
experience has demonstrated time and time again that in cultivation
the dry-farmer has a powerful means of preventing evaporation from
agricultural soils.
Closely connected with cultivation is the practice of scattering
straw or other litter over the ground. Such artificial mulches are
very effective in reducing evaporation. Ebermayer found that by
spreading straw on the land, the evaporation was reduced 22 per
cent; Wagner found under similar conditions a saving of 38 per cent,
and these results have been confirmed by many other investigators.
On the modern dry-farms, which are large in area, the artificial
mulching of soils cannot become a very extensive practice, yet it is
well to bear the principle in mind. The practice of harvesting
dry-farm grain with the header and plowing under the high stubble in
the fall is a phase of cultivation for water conservation that
deserves special notice. The straw, thus incorporated into the soil,
decomposes quite readily in spite of the popular notion to the
contrary, and makes the soil more porous, and, therefore, more
effectively worked for the prevention of evaporation. When this
practice is continued for considerable periods, the topsoil becomes
rich in organic matter, which assists in retarding evaporation,
besides increasing the fertility of the land. When straw cannot be
fed to advantage, as is yet the case on many of the western
dry-farms, it would be better to scatter it over the land than to
burn it, as is often done. Anything that covers the ground or
loosens the topsoil prevents in a measure the evaporation of the
water stored in lower soil depths for the use of crops.
Depth of cultivation
The all-important practice for the dry-farmer who is entering upon
the growing season is cultivation. The soil must be covered
continually with a deep layer of dry loose soil, which because of
its looseness and dryness makes evaporation difficult. A leading
question in connection with cultivation is the depth to which the
soil should be stirred for the best results. Many of the early
students of the subject found that a soil mulch only one half inch
in depth was effective in retaining a large part of the
soil-moisture which noncultivated soils would lose by evaporation.
Soils differ greatly in the rate of evaporation from their surfaces.
Some form a natural mulch when dried, which prevents further water
loss. Others form only a thin hard crust, below which lies an active
evaporating surface of wet soil. Soils which dry out readily and
crumble on top into a natural mulch should be cultivated deeply, for
a shallow cultivation does not extend beyond the naturally formed
mulch. In fact, on certain calcareous soils, the surfaces of which
dry out quickly and form a good protection against evaporation,
shallow cultivations often cause a greater evaporation by disturbing
the almost perfect natural mulch. Clay or sand soils, which do not
so well form a natural mulch, will respond much better to shallow
cultivations. In general, however, the deeper the cultivation, the
more effective it is in reducing evaporation. Fortier, in the
experiments in California to which allusion has already been made,
showed the greater value of deep cultivation. During a period of
fifteen days, beginning immediately after an irrigation, the soil
which had not been mulched lost by evaporation nearly one fourth of
the total amount of water that had been added. A mulch 4 inches deep
saved about 72 per cent of the evaporation; a mulch 8 inches deep
saved about 88 per cent, and a mulch 10 inches deep stopped
evaporation almost wholly. It is a most serious mistake for the
dry-farmer, who attempts cultivation for soil-moisture conservation,
to fail to get the best results simply to save a few cents per acre
in added labor.
When to cultivate or till
It has already been shown that the rate of evaporation is greater
from a wet than from a dry surface. It follows, therefore, that the
critical time for preventing evaporation is when the soil is
wettest. After the soil is tolerably dry, a very large portion of
the soil-moisture has been lost, which possibly might have been
saved by earlier cultivation. The truth of this statement is well
shown by experiments conducted by the Utah Station. In one case on a
soil well filled with water, during a three weeks' period, nearly
one half of the total loss occurred the first, while only one fifth
fell on the third week. Of the amount lost during the first week,
over 60 per cent occurred during the first three days. Cultivation
should, therefore, be practiced as soon as possible after conditions
favorable for evaporation have been established. This means, first,
that in early spring, just as soon as the land is dry enough to be
worked without causing puddling, the soil should be deeply and
thoroughly stirred. Spring plowing, done as early as possible, is an
excellent practice for forming a mulch against evaporation. Even
when the land has been fall-plowed, spring plowing is very
beneficial, though on fall-plowed land the disk harrow is usually
used in early spring, and if it is set at rather a sharp angle, and
properly weighted, so that it cuts deeply into the ground, it is
practically as effective as spring plowing. The chief danger to the
dry-farmer is that he will permit the early spring days to slip by
until, when at last he begins spring cultivation, a large portion of
the stored soil-water has been evaporated. It may be said that deep
fall plowing, by permitting the moisture to sink quickly into the
lower layers of soil, makes it possible to get upon the ground
earlier in the spring. In fact, unplowed land cannot be cultivated
as early as that which has gone through the winter in a plowed
condition
If the land carries a fall-sown crop, early spring cultivation is
doubly important. As soon as the plants are well up in spring the
land should be gone over thoroughly several times if necessary, with
an iron tooth harrow, the teeth of which are set to slant backward
in order not to tear up the plants. The loose earth mulch thus
formed is very effective in conserving moisture; and the few plants
torn up are more than paid for by the increased water supply for the
remaining plants. The wise dry-fanner cultivates his land, whether
fallow or cropped, as early as possible in the spring.
Following the first spring plowing, disking, or cultivation, must
come more cultivation. Soon after the spring plowing, the land
should be disked and. then harrowed. Every device should be used to
secure the formation of a layer of loose drying soil over the land
surface. The season's crop will depend largely upon the
effectiveness of this spring treatment.
As the season advances, three causes combine to permit the
evaporation of soil-moisture.
First, there is a natural tendency, under the somewhat moist
conditions of spring, for the soil to settle compactly and thus to
restore the numerous capillary connections with the lower soil
layers through which water escapes. Careful watch should therefore
be kept upon the soil surface, and whenever the mulch is not loose,
the disk or harrow should be run over the land.
Secondly, every rain of spring or summer tends to establish
connections with the store of moisture in the soil. In fact, late
spring and summer rains are often a disadvantage on dry-farms, which
by cultural treatment have been made to contain a large store of
moisture. It has been shown repeatedly that light rains draw
moisture very quickly from soil layers many feet below the surface.
The rainless summer is not feared by the dry-farmer whose soils are
fertile and rich in moisture. It is imperative that at the very
earliest moment after a spring or summer rain the topsoil be well
stirred to prevent evaporation. It thus happens that in sections of
frequent summer rains, as in the Great Plains area, the farmer has
to harrow his land many times in succession, but the increased crop
yields invariably justify the added expenditure of effort.
Thirdly, on the summer-fallowed ground weeds start vigorously in the
spring and draw upon the soil-moisture, if allowed to grow, fully as
heavily as a crop of wheat or corn. The dry-farmer must not allow a
weed upon his land. Cultivation must he so continuous as to make
weeds an impossibility. The belief that the elements added to the
soil by weeds offset the loss of soil-moisture is wholly erroneous.
The growth of weeds on a fallow dry-farm is more dangerous than the
packed uncared-for topsoil. Many implements have been devised for
the easy killing of weeds, but none appear to be better than the
plow and the disk which are found on every farm. (See Chapter XV.)
When crops are growing on the land, thorough summer cultivation is
somewhat more difficult, but must be practiced for the greatest
certainty of crop yields. Potatoes, corn, and similar crops may be
cultivated with comparative ease, by the use of ordinary
cultivators. With wheat and the other small grains, generally, the
damage done to the crop by harrowing late in the season is too
great, and reliance is therefore placed on the shading power of the
plants to prevent undue evaporation. However, until the wheat and
other grains are ten to twelve inches high, it is perfectly safe to
harrow them. The teeth should be set backward to diminish the
tearing up of the plants, and the implement weighted enough to break
the soil crust thoroughly. This practice has been fully tried out
over the larger part of the dry-farm territory and found
satisfactory.
So vitally important is a permanent soil mulch for the conservation
for plant use of the water stored in the soil that many attempts
have been made to devise means for the effective cultivation of land
on which small grains and grasses are growing. In many places plants
have been grown in rows so far apart that a man with a hoe could
pass between them. Scofield has described this method as practiced
successfully in Tunis. Campbell and others in America have proposed
that a drill hole be closed every three feet to form a path wide
enough for a horse to travel in and to pull a large spring tooth
cultivator' with teeth so spaced as to strike between the rows of
wheat. It is yet doubtful whether, under average conditions, such
careful cultivation, at least of grain crops, is justified by the
returns. Under conditions of high aridity, or where the store of
soil-moisture is low, such treatment frequently stands between crop
success and failure, and it is not unlikely that methods will be
devised which will permit of the cheap and rapid cultivation between
the rows of growing wheat. Meanwhile, the dry-farmer must always
remember that the margin under which he works is small, and that his
success depends upon the degree to which he prevents small wastes.
Dry-farm potatoes, Rosebud Co., Montana, 1909. Yield, 282 bushels
per acre.
The conservation of soil-moisture depends upon the vigorous,
unremitting, continuous stirring of the topsoil. Cultivation!
cultivation! and more cultivation! must be the war-cry of the
dry-farmer who battles against the water thieves of an arid climate.