REGULATING THE TRANSPIRATION
Water that has entered the soil may be lost in three ways. First, it
may escape by downward seepage, whereby it passes beyond the reach
of plant roots and often reaches the standing water. In dry-farm
districts such loss is a rare occurrence, for the natural
precipitation is not sufficiently large to connect with the country
drainage, and it may, therefore, be eliminated from consideration.
Second, soil-water may be lost by direct evaporation from the
surface soil. The conditions prevailing in arid districts favor
strongly this manner of loss of soil-moisture. It has been shown,
however, in the preceding chapter that the farmer, by proper and
persistent cultivation of the topsoil, has it in his power to reduce
this loss enough to be almost negligible in the farmer's
consideration. Third, soil-water may be lost by evaporation from the
plants themselves. While it is not generally understood, this source
of loss is, in districts where dry-farming is properly carried on,
very much larger than that resulting either from seepage or from
direct evaporation. While plants are growing, evaporation from
plants, ordinarily called transpiration, continues. Experiments
performed in various arid districts have shown that one and a half
to three times more water evaporates from the plant than directly
from well-tilled soil. To the present very little has been learned
concerning the most effective methods of checking or controlling
this continual loss of water. Transpiration, or the evaporation of
water from the plants themselves and the means of controlling this
loss, are subjects of the deepest importance to the dry-farmer.
Absorption
To understand the methods for reducing transpiration, as proposed in
this chapter, it is necessary to review briefly the manner in which
plants take water from the soil. The roots are the organs of water
absorption. Practically no water is taken into the plants by the
stems or leaves, even under conditions of heavy rainfall. Such small
quantities as may enter the plant through the stems and leaves are
of very little value in furthering the life and growth of the plant.
The roots alone are of real consequence in water absorption. All
parts of the roots do not possess equal power of taking up
soil-water. In the process of water absorption the younger roots are
most active and effective. Even of the young roots, however, only
certain parts are actively engaged in water absorption. At the very
tips of the young growing roots are numerous fine hairs. These
root-hairs, which cluster about the growing point of the young
roots, are the organs of the plant that absorb soil-water. They are
of value only for limited periods of time, for as they grow older,
they lose their power of water absorption. In fact, they are active
only when they are in actual process of growth. It follows,
therefore, that water absorption occurs near the tips of the growing
roots, and whenever a plant ceases to grow the water absorption
ceases also. The root-hairs are filled with a dilute solution of
various substances, as yet poorly understood, which plays an
important tent part in the ab sorption of water and plant-food from
the soil.
Owing to their minuteness, the root-hairs are in most cases immersed
in the water film that surrounds the soil particles, and the
soil-water is taken directly into the roots from the soil-water film
by the process known as osmosis. The explanation of this inward
movement is complicated and need not be discussed here. It is
sufficient to say that the concentration or strength of the solution
within the root-hair is of different degree from the soil-water
solution. The water tends, therefore, to move from the soil into the
root, in order to make the solutions inside and outside of the root
of the same concentration. If it should ever occur that the
soil-water and the water within the root-hair became the same
concentration, that is to say, contained the same substances in the
same proportional amounts, there would be no further inward movement
of water. Moreover, if it should happen that the soil-water is
stronger than the water within the root-hair, the water would tend
to pass from the plant into the soil. This is the condition that
prevails in many alkali lands of the West, and is the cause of the
death of plants growing on such lands.
It is clear that under these circumstances not only water enters the
root-hairs, but many of the substances found in solution in the
soil-water enter the plant also. Among these are the mineral
substances which are indispensable for the proper life and growth of
plants. These plant nutrients are so indispensable that if any one
of them is absent, it is absolutely impossible for the plant to
continue its life functions. The indispensable plant-foods gathered
from the soil by the root-hairs, in addition to water, are:
potassium, calcium, magnesium, iron, nitrogen, and phosphorus,--all
in their proper combinations. How the plant uses these substances is
yet poorly understood, but we are fairly certain that each one has
some particular function in the life of the plant. For instance,
nitrogen and phosphorus are probably necessary in the formation of
the protein or the flesh-forming portions of the plant, while potash
is especially valuable in the formation of starch.
There is a constant movement of the indispensable plant nutrients
after they have entered the root-hairs, through the stems and into
the leaves. This constant movement of the plant-foods depends upon
the fact that the plant consumes in its growth considerable
quantities of these substances, and as the plant juices are
diminished in their content of particular plant-foods, more enters
from the soil solution. The necessary plant-foods do not alone enter
the plant but whatever may be in solution in the soil-water enters
the plant in variable quantities. Nevertheless, since the plant uses
only a few definite substances and leaves the unnecessary ones in
solution, there is soon a cessation of the inward movement of the
unimportant constituents of the soil solution. This process is often
spoken of as selective absorption; that is, the plant, because of
its vital activity, appears to have the power of selecting from the
soil certain substances and rejecting others.
Movement of water through plant
The soil-water, holding in solution a great variety of plant
nutrients, passes from the root-hairs into the adjoining cells and
gradually moves from cell to cell throughout the whole plant. In
many plants this stream of water does not simply pass from cell to
cell, but moves through tubes that apparently have been formed for
the specific purpose of aiding the movement of water through the
plant. The rapidity of this current is often considerable.
Ordinarily, it varies from one foot to six feet per hour, though
observations are on record showing that the movement often reaches
the rate of eighteen feet per hour. It is evident, then, that in an
actively growing plant it does not take long for the water which is
in the soil to find its way to the uppermost parts of the plant.
The work of leaves
Whether water passes upward from cell to cell or through especially
provided tubes, it reaches at last the leaves, where evaporation
takes place. It is necessary to consider in greater detail what
takes place in leaves in order that we may more clearly understand
the loss due to transpiration. One half or more of every plant is
made up of the element carbon. The remainder of the plant consists
of the mineral substances taken from the soil (not more than two to
10 per cent of the dry plant) and water which has been combined with
the carbon and these mineral substances to form the characteristic
products of plant life. The carbon which forms over half of the
plant substance is gathered from the air by the leaves and it is
evident that the leaves are very active agents of plant growth. The
atmosphere consists chiefly of the gases oxygen and nitrogen in the
proportion of one to four, but associated with them are small
quantities of various other substances. Chief among the secondary
constituents of the atmosphere is the gas carbon dioxid, which is
formed when carbon burns, that is, when carbon unites with the
oxygen of the air. Whenever coal or wood or any carbonaceous
substance burns, carbon dioxid is formed. Leaves have the power of
absorbing the gas carbon dioxid from the air and separating the
carbon from the oxygen. The oxygen is returned to the atmosphere
while the carbon is retained to be used as the fundamental substance
in the construction by the plant of oils, fats, starches, sugars,
protein, and all the other products of plant growth.
This important process known as carbon assimilation is made possible
by the aid of countless small openings which exist chicfly on the
surfaces of leaves and known as "stomata." The stomata are
delicately balanced valves, exceedingly sensitive to external
influences. They are more numerous on the lower side than on the
upper side of plants. In fact, there is often five times more on the
under side than on the upper side of a leaf. It has been estimated
that 150,000 stomata or more are often found per square inch on the
under side of the leaves of ordinary cultivated plants. The stomata
or breathing-pores are so constructed that they may open and close
very readily. In wilted leaves they are practically closed; often
they also close immediately after a rain; but in strong sunlight
they are usually wide open. It is through the stomata that the gases
of the air enter the plant through which the discarded oxygen
returns to the atmosphere.
It is also through the stomata that the water which is drawn from
the soil by the roots through the stems is evaporated into the air.
There is some evaporation of water from the stems and branches of
plants, but it is seldom more than a thirtieth or a fortieth of the
total transpiration. The evaporation of water from the leaves
through the breathing-pores is the so-called transpiration, which is
the greatest cause of the loss of soil-water under dry-farm
conditions. It is to the prevention of this transpiration that much
investigation must be given by future students of dry-farming.
Transpiration
As water evaporates through the breathing-pores from the leaves it
necessarily follows that a demand is made upon the lower portions of
the plant for more water. The effect of the loss of water is felt
throughout the whole plant and is, undoubtedly, one of the chief
causes of the absorption of water from the soil. As evaporation is
diminished the amount of water that enters the plants is also
diminished. Yet transpiration appears to be a process wholly
necessary for plant life. The question is, simply, to what extent it
may be diminished without injuring plant growth. Many students
believe that the carbon assimilation of the plant, which is
fundamentally important in plant growth, cannot be continued unless
there is a steady stream of water passing through the plant and then
evaporating from the leaves.
Of one thing we are fairly sure: if the upward stream of water is
wholly stopped for even a few hours, the plant is likely to be so
severely injured as to be greatly handicapped in its future growth.
Botanical authorities agree that transpiration is of value to plant
growth, first, because it helps to distribute the mineral nutrients
necessary for plant growth uniformly throughout the plant; secondly,
because it permits an active assimilation of the carbon by the
leaves; thirdly, because it is not unlikely that the heat required
to evaporate water, in large part taken from the plant itself,
prevents the plant from being overheated. This last mentioned value
of transpiration is especially important in dry-farm districts,
where, during the summer, the heat is often intense. Fourthly,
transpiration apparently influences plant growth and development in
a number of ways not yet clearly understood.
Conditions influencing transpiration
In general, the conditions that determine the evaporation of water
from the leaves are the same as those that favor the direct
evaporation of water from soils, although there seems to be
something in the life process of the plant, a physiological factor,
which permits or prevents the ordinary water-dissipating factors
from exercising their full powers. That the evaporation of water
from the soil or from a free water surface is not the same as that
from plant leaves may be shown in a general way from the fact that
the amount of water transpired from a given area of leaf surface may
be very much larger or very much smaller than that evaporated from
an equal surface of free water exposed to the same conditions. It is
further shown by the fact that whereas evaporation from a free water
surface goes on with little or no interruption throughout the
twenty-four hours of the day, transpiration is virtually at a
standstill at night even though the conditions for the rapid
evaporation from a free water surface are present.
Some of the conditions influencing the transpiration may be
enumerated as follows:--
First, transpiration is influenced by the relative humidity. In dry
air, under otherwise similar conditions, plants transpire more water
than in moist air though it is to be noted that even when the
atmosphere is fully saturated, so that no water evaporates from a
free water surface, the transpiration of plants still continues in a
small degree. This is explained by the observation that since the
life process of a plant produces a certain amount of heat, the plant
is always warmer than the surrounding air and that transpiration
into an atmosphere fully charged with water vapor is consequently
made possible. The fact that transpiration is greater under a low
relative humidity is of greatest importance to the dry-farmer who
has to contend with the dry atmosphere.
Second, transpiration increases with the increase in temperature;
that is, under conditions otherwise the same, transpiration is more
rapid on a warm day than on a cold one. The temperature increase of
itself, however, is not sufficient to cause transpiration.
Third, transpiration increases with the increase of air currents,
which is to say, that on a windy day transpiration is much more
rapid than on a quiet day.
Fourth, transpiration increases with the increase of direct
sunlight. It is an interesting observation that even with the same
relative humidity, temperature, and wind, transpiration is reduced
to a minimum during the night and increases manyfold during the day
when direct sunlight is available. This condition is again to be
noted by the dry-farmer, for the dry-farm districts are
characterized by an abundance of sunshine.
Fifth, transpiration is decreased by the presence in the soil-water
of large quantities of the substances which the plant needs for its
food material. This will be discussed more fully in the next
section.
Sixth, any mechanical vibration of the plant seems to have some
effect upon the transpiration. At times it is increased and at times
it is decreased by such mechanical disturbance.
Seventh, transpiration varies also with the age of the plant. In the
young plant it is comparatively small. Just before blooming it is
very much larger and in time of bloom it is the largest in the
history of the plant. As the plant grows older transpiration
diminishes, and finally at the ripening stage it almost ceases.
Eighth, transpiration varies greatly with the crop. Not all plants
take water from the soil at the same rate. Very little is as yet
known about the relative water requirements of crops on the basis of
transpiration. As an illustration, MacDougall has reported that
sagebrush uses about one fourth as much water as a tomato plant.
Even greater differences exist between other plants. This is one of
the interesting subjects yet to be investigated by those who are
engaged in the reclamation of dry-farm districts. Moreover, the same
crop grown under different conditions varies in its rate of
transpiration. For instance, plants grown for some time under arid
conditions greatly modify their rate of transpiration, as shown by
Spalding, who reports that a plant reared under humid conditions
gave off 3.7 times as much water as the same plant reared under arid
conditions. This very interesting observation tends to confirm the
view commonly held that plants grown under arid conditions will
gradually adapt themselves to the prevailing conditions, and in
spite of the greater water dissipating conditions will live with the
expenditure of less water than would be the case under humid
conditions. Further, Sorauer found, many years ago, that different
varieties of the same crop possess very different rates of
transpiration. This also is an interesting subject that should be
more fully investigated in the future.
Ninth, the vigor of growth of a crop appears to have a strong
influence on transpiration. It does not follow, however, that the
more vigorously a crop grows, the more rapidly does it transpire
water, for it is well known that the most luxuriant plant growth
occurs in the tropics, where the transpiration is exceedingly low.
It seems to be true that under the same conditions, plants that grow
most vigorously tend to use proportionately the smallest amount of
water.
Tenth, the root system--its depth and manner of growth--influences
the rate of transpiration. The more vigorous and extensive the root
system, the more rapidly can water be secured from the soil by the
plant.
The conditions above enumerated as influencing transpiration are
nearly all of a physical character, and it must not be forgotten
that they may all be annulled or changed by a physiological
regulation. It must be admitted that the subject of transpiration is
yet poorly understood, though it is one of the most important
subjects in its applications to plant production in localities where
water is scaree. It should also be noted that nearly all of the
above conditions influencing transpiration are beyond the control of
the farmer. The one that seems most readily controlled in ordinary
agricultural practice will be discussed in the following section.
Plant-food and transpiration
It has been observed repeatedly by students of transpiration that
the amount of water which actually evaporates from the leaves is
varied materially by the substances held in solution by the
soil-water. That is, transpiration depends upon the nature and
concentration of soil solution. This fact, though not commonly
applied even at the present time, has really been known for a very
long time. Woodward, in 1699, observed that the amount of water
transpired by a plant growing in rain water was 192.3 grams; in
spring water, 163.6 grams, and in water from the River Thames, 159.5
grams; that is, the amount of water transpired by the plant in the
comparatively pure rain water was nearly 20 per cent higher than
that used by the plant growing in the notoriously impure water of
the River Thames. Sachs, in 1859, carried on an elaborate series of
experiments on transpiration in which he showed that the addition of
potassium nitrate, ammonium sulphate or common salt to the solution
in which plants grew reduced the transpiration; in fact, the
reduction was large, varying from 10 to 75 per cent. This was
confirmed by a number of later workers, among them, for instance,
Buergerstein, who, in 1875, showed that whenever acids were added to
a soil or to water in which plants are growing, the transpiration is
increased greatly; but when alkalies of any kind are added,
transpiration decreases. This is of special interest in the
development of dry-farming, since dry-farm soils, as a rule, contain
more substances that may be classed as alkalies than do soils
maintained under humid conditions. Sour soils are very
characteristic of districts where the rainfall is abundant; the
vegetation growing on such soils transpires excessively and the
crops are consequently more subject to drouth.
The investigators of almost a generation ago also determined beyond
question that whenever a complete nutrient solution is presented to
plants, that is, a solution containing all the necessary plant-foods
in the proper proportions, the transpiration is reduced immensely.
It is not necessary that the plant-foods should be presented in a
water solution in order to effect this reduction in transpiration;
if they are added to the soil on which plants are growing, the same
effect will result. The addition of commercial fertilizers to the
soil will therefore diminish transpiration. It was further
discovered nearly half a century ago that similar plants growing on
different soils evaporate different amounts of water from their
leaves; this difference, undoubtedly, is due to the conditions in
the fertility of the soils, for the more fertile a soil is, the
richer will the soil-water be in the necessary plant-foods. The
principle that transpiration or the evaporation of water from the
plants depends on the nature and concentration of the soil solution
is of far-reaching importance in the development of a rational
practice of dry-farming.
Transpiration for a pound of dry matter
Is plant growth proportional to transpiration? Do plants that
evaporate much water grow more rapidly than those that evaporate
less? These questions arose very early in the period characterized
by an active study of transpiration. If varying the transpiration
varies the growth, there would be no special advantage in reducing
the transpiration. From an economic point of view the important
question is this: Does the plant when its rate of transpiration is
reduced still grow with the same vigor? If that be the case, then
every effort should be made by the farmer to control and to diminish
the rate of transpiration.
One of the very earliest experiments on transpiration, conducted by
Woodward in 1699, showed that it required less water to produce a
pound of dry matter if the soil solution were of the proper
concentration and contained the elements necessary for plant growth.
Little more was done to answer the above questions for over one
hundred and fifty years. Perhaps the question was not even asked
during this period, for scientific agriculture was just coming into
being in countries where the rainfall was abundant. However,
Tschaplowitz, in 1878, investigated the subject and found that the
increase in dry matter is greatest when the transpiration is the
smallest. Sorauer, in researches conducted from 1880 to 1882,
determined with almost absolute certainty that less water is
required to produce a pound of dry matter when the soil is
fertilized than when it is not fertilized. Moreover, he observed
that the enriching of the soil solution by the addition of
artificial fertilizers enabled the plant to produce dry matter with
less water. He further found that if a soil is properly tilled so as
to set free plant-food and in that way to enrich the soil solution
the water-cost of dry plant substance is decreased. Hellriegel, in
1883, confirmed this law and laid down the law that poor plant
nutrition increases the water-cost of every pound of dry matter
produced. It was about this time that the Rothamsted Experiment
Station reported that its experiments had shown that during periods
of drouth the well-tilled and well-fertilized fields yielded good
crops, while the unfertilized fields yielded poor crops or crop
failures--indicating thereby, since rainfall was the critical
factor, that the fertility of the soil is important in determining
whether or not with a small amount of water a good crop can be
produced. Pagnoul, working in 1895 with fescue grass, arrived at the
same conclusion. On a poor clay soil it required 1109 pounds of
water to produce one pound of dry matter, while on a rich calcareous
soil only 574 pounds were required. Gardner of the United States
Department of Agriculture, Bureau of Soils, working in 1908, on the
manuring of soils, came to the conclusion that the more fertile the
soil the less water is required to produce a pound of dry matter. He
incidentally called attention to the fact that in countries of
limited rainfall this might be a very important principle to apply
in crop production. Hopkins in his study of the soils of Illinois
has repeatedly observed, in connection with certain soils, that
where the land is kept fertile, injury from drouth is not common,
implying thereby that fertile soils will produce dry matter at a
lower water-cost. The most recent experiments on this subject,
conducted by the Utah Station, confirm these conclusions. The
experiments, which covered several years, were conducted in pots
filled with different soils. On a soil, naturally fertile, 908
pounds of water were transpired for each pound of dry matter (corn)
produced; by adding to this soil an ordinary dressing of manure'
this was reduced to 613 pounds, and by adding a small amount of
sodium nitrate it was reduced to 585 pounds. If so large a reduction
could be secured in practice, it would seem to justify the use of
commercial fertilizers in years when the dry-farm year opens with
little water stored in the soil. Similar results, as will be shown
below, were obtained by the use of various cultural methods. It may
therefore, be stated as a law, that any cultural treatment which
enables the soil-water to acquire larger quantities of plant-food
also enables the plant to produce dry matter with the use of a
smaller amount of water. In dry-farming, where the limiting factor
is water, this principle must he emphasized in every cultural
operation.
Methods of controlling transpiration
It would appear that at present the only means possessed by the
farmer for controlling transpiration and making possible maximum
crops with the minimum amount of water in a properly tilled soil is
to keep the soil as fertile as is possible. In the light of this
principle the practices already recommended for the storing of water
and for the prevention of the direct evaporation of water from the
soil are again emphasized. Deep and frequent plowing, preferably in
the fall so that the weathering of the winter may be felt deeply and
strongly, is of first importance in liberating plant-food.
Cultivation which has been recommended for the prevention of the
direct evaporation of water is of itself an effective factor in
setting free plant-food and thus in reducing the amount of water
required by plants. The experiments at the Utah Station, already
referred to, bring out very strikingly the value of cultivation in
reducing the transpiration. For instance, in a series of experiments
the following results were obtained. On a sandy loam, not
cultivated, 603 pounds of water were transpired to produce one pound
of dry matter of corn; on the same soil, cultivated, only 252 pounds
were required. On a clay loam, not cultivated, 535 pounds of water
were transpired for each pound of dry matter, whereas on the
cultivated soil only 428 pounds were necessary. On a clay soil, not
cultivated, 753 pounds of water were transpired for each pound of
dry matter; on the cultivated soil, only 582 pounds. The farmer who
faithfully cultivates the soil throughout the summer and after every
rain has therefore the satisfaction of knowing that he is
accomplishing two very important things: he is keeping the moisture
in the soil, and he is making it possible for good crops to be grown
with much less water than would otherwise be required. Even in the
case of a peculiar soil on which ordinary cultivation did not reduce
the direct evaporation, the effect upon the transpiration was very
marked. On the soil which was not cultivated, 451 pounds of water
were required to produce one pound of dry matter (corn), while on
the cultivated soils, though the direct evaporation was no smaller,
the number of pounds of water for each pound of dry substance was as
low as 265.
One of the chief values of fallowing lies in the liberation of the
plant-food during the fallow year, which reduces the quantity of
water required the next year for the full growth of crops. The Utah
experiments to which reference has already been made show the effect
of the previous soil treatment upon the water requirements of crops.
One half of the three types of soil had been cropped for three
successive years, while the other half had been left bare. During
the fourth year both halves were planted to corn. For the sandy loam
it was found that, on the part that had been cropped previously, 659
pounds of water were required for each pound of dry matter produced,
while on the part that had been bare only 573 pounds were required.
For the clay loam 889 pounds on the cropped part and 550 on the
previously bare part were required for each pound of dry matter. For
the clay 7466 pounds on the cropped part and 1739 pounds on the
previously bare part were required for each pound of dry matter.
These results teach clearly and emphatically that the fertile
condition of the soil induced by fallowing makes it possible to
produce dry matter with a smaller amount of water than can be done
on soils that are cropped continuously. The beneficial effects of
fallowing are therefore clearly twofold: to store the moisture of
two seasons for the use of one crop; and to set free fertility to
enable the plant to grow with the least amount of water. It is not
yet fully understood what changes occur in fallowing to give the
soil the fertility which reduces the water needs of the plant. The
researches of Atkinson in Montana, Stewart and Graves in Utah, and
Jensen in South Dakota make it seem probable that the formation of
nitrates plays an important part in the whole process. If a soil is
of such a nature that neither careful, deep plowing at the right
time nor constant crust cultivation are sufficient to set free an
abundance of plant-food, it may be necessary to apply manures or
commercial fertilizers to the soil. While the question of restoring
soil fertility has not yet come to be a leading one in dry-farming,
yet in view of what has been said in this chapter it is not
impossible that the time will come when the farmers must give
primary attention to soil fertility in addition to the storing and
conservation of soil-moisture. The fertilizing of lands with proper
plant-foods, as shown in the last sections, tends to check
transpiration and makes possible the production of dry matter at the
lowest water-cost.
The recent practice in practically all dry-farm districts, at least
in the intermountain and far West, to use the header for harvesting
bears directly upon the subject considered in this chapter. The high
stubble which remains contains much valuable plant-food, often
gathered many feet below the surface by the plant roots. When this
stubble is plowed under there is a valuable addition of the
plant-food to the upper soil. Further, as the stubble decays, acid
substances are produced that act upon the soil grains to set free
the plant-food locked up in them. The plowing under of stubble is
therefore of great value to the dry-farmer. The plowing under of any
other organic substance has the same effect. In both cases fertility
is concentrated near the surface, which dissolves in the soil-water
and enables the crop to mature with the Ieast quantity of water.
The lesson then to be learned from this chapter is, that it is not
aufficient for the dry-farmer to store an abundance of water in the
soil and to prevent that water from evaporating directly from the
soil; but the soil must be kept in such a state of high fertility
that plants are enabled to utilize the stored moisture in the most
economical manner. Water storage, the prevention of evaporation, and
the maintenance of soil fertility go hand in hand in the development
of a successful system of farming without irrigation.