REGULATING THE TRANSPIRATION
WATER that has entered the soil may be lost inthree ways. First, it may escape by downward seepage, whereby it passes beyond thereach of plant roots and often reaches the standing water. In dry-farm districtssuch loss is a rare occurrence, for the natural precipitation is not sufficientlylarge to connect with the country drainage, and it may, therefore, be eliminatedfrom consideration. Second, soil-water may be lost by direct evaporation from thesurface soil. The conditions prevailing in arid districts favor strongly this mannerof loss of soil-moisture. It has been shown, however, in the preceding chapter thatthe farmer, by proper and persistent cultivation of the topsoil, has it in his powerto 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 itis not generally understood, this source of loss is, in districts where dry-farmingis properly carried on, very much larger than that resulting either from seepageor from direct evaporation. While plants are growing, evaporation from plants, ordinarilycalled transpiration, continues. Experiments performed in various arid districtshave shown that one and a half to three times more water evaporates from the plantthan directly from well-tilled soil. To the present very little has been learnedconcerning the most effective methods of checking or controlling this continual lossof water. Transpiration, or the evaporation of water from the plants themselves andthe means of controlling this loss, are subjects of the deepest importance to thedry-farmer.
To understand the methods for reducing transpiration,as proposed in this chapter, it is necessary to review briefly the manner in whichplants take water from the soil. The roots are the organs of water absorption. Practicallyno water is taken into the plants by the stems or leaves, even under conditions ofheavy rainfall. Such small quantities as may enter the plant through the stems andleaves are of very little value in furthering the life and growth of the plant. Theroots alone are of real consequence in water absorption. All parts of the roots donot possess equal power of taking up soil-water. In the process of water absorptionthe 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 ofthe young growing roots are numerous fine hairs. These root-hairs, which clusterabout the growing point of the young roots, are the organs of the plant that absorbsoil-water. They are of value only for limited periods of time, for as they growolder, they lose their power of water absorption. In fact, they are active only whenthey are in actual process of growth. It follows, therefore, that water absorptionoccurs near the tips of the growing roots, and whenever a plant ceases to grow thewater absorption ceases also. The root-hairs are filled with a dilute solution ofvarious substances, as yet poorly understood, which plays an important tent partin the ab sorption of water and plant-food from the soil.
Owing to their minuteness, the root-hairs arein most cases immersed in the water film that surrounds the soil particles, and thesoil-water is taken directly into the roots from the soil-water film by the processknown as osmosis. The explanation of this inward movement is complicated and neednot be discussed here. It is sufficient to say that the concentration or strengthof 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 makethe solutions inside and outside of the root of the same concentration. If it shouldever occur that the soil-water and the water within the root-hair became the sameconcentration, that is to say, contained the same substances in the same proportionalamounts, there would be no further inward movement of water. Moreover, if it shouldhappen that the soil-water is stronger than the water within the root-hair, the waterwould tend to pass from the plant into the soil. This is the condition that prevailsin many alkali lands of the West, and is the cause of the death of plants growingon such lands.
It is clear that under these circumstances notonly water enters the root-hairs, but many of the substances found in solution inthe soil-water enter the plant also. Among these are the mineral substances whichare indispensable for the proper life and growth of plants. These plant nutrientsare so indispensable that if any one of them is absent, it is absolutely impossiblefor the plant to continue its life functions. The indispensable plant-foods gatheredfrom 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 plantuses these substances is yet poorly understood, but we are fairly certain that eachone has some particular function in the life of the plant. For instance, nitrogenand phosphorus are probably necessary in the formation of the protein or the flesh-formingportions of the plant, while potash is especially valuable in the formation of starch.
There is a constant movement of the indispensableplant nutrients after they have entered the root-hairs, through the stems and intothe leaves. This constant movement of the plant-foods depends upon the fact thatthe plant consumes in its growth considerable quantities of these substances, andas the plant juices are diminished in their content of particular plant-foods, moreenters from the soil solution. The necessary plant-foods do not alone enter the plantbut 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 theunnecessary ones in solution, there is soon a cessation of the inward movement ofthe unimportant constituents of the soil solution. This process is often spoken ofas selective absorption; that is, the plant, because of its vital activity, appearsto 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 varietyof plant nutrients, passes from the root-hairs into the adjoining cells and graduallymoves from cell to cell throughout the whole plant. In many plants this stream ofwater does not simply pass from cell to cell, but moves through tubes that apparentlyhave been formed for the specific purpose of aiding the movement of water throughthe plant. The rapidity of this current is often considerable. Ordinarily, it variesfrom one foot to six feet per hour, though observations are on record showing thatthe 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 inthe soil to find its way to the uppermost parts of the plant.
The work of leaves
Whether water passes upward from cell to cellor through especially provided tubes, it reaches at last the leaves, where evaporationtakes place. It is necessary to consider in greater detail what takes place in leavesin order that we may more clearly understand the loss due to transpiration. One halfor more of every plant is made up of the element carbon. The remainder of the plantconsists of the mineral substances taken from the soil (not more than two to 10 percent of the dry plant) and water which has been combined with the carbon and thesemineral substances to form the characteristic products of plant life. The carbonwhich forms over half of the plant substance is gathered from the air by the leavesand it is evident that the leaves are very active agents of plant growth. The atmosphereconsists 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. Chiefamong the secondary constituents of the atmosphere is the gas carbon dioxid, whichis 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 separatingthe carbon from the oxygen. The oxygen is returned to the atmosphere while the carbonis retained to be used as the fundamental substance in the construction by the plantof oils, fats, starches, sugars, protein, and all the other products of plant growth.
This important process known as carbon assimilationis made possible by the aid of countless small openings which exist chicfly on thesurfaces of leaves and known as "stomata." The stomata are delicately balancedvalves, exceedingly sensitive to external influences. They are more numerous on thelower side than on the upper side of plants. In fact, there is often five times moreon the under side than on the upper side of a leaf. It has been estimated that 150,000stomata or more are often found per square inch on the under side of the leaves ofordinary cultivated plants. The stomata or breathing-pores are so constructed thatthey 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 usuallywide open. It is through the stomata that the gases of the air enter the plant throughwhich the discarded oxygen returns to the atmosphere.
It is also through the stomata that the waterwhich is drawn from the soil by the roots through the stems is evaporated into theair. There is some evaporation of water from the stems and branches of plants, butit is seldom more than a thirtieth or a fortieth of the total transpiration. Theevaporation of water from the leaves through the breathing-pores is the so-calledtranspiration, which is the greatest cause of the loss of soil-water under dry-farmconditions. It is to the prevention of this transpiration that much investigationmust be given by future students of dry-farming.
As water evaporates through the breathing-poresfrom the leaves it necessarily follows that a demand is made upon the lower portionsof the plant for more water. The effect of the loss of water is felt throughout thewhole plant and is, undoubtedly, one of the chief causes of the absorption of waterfrom the soil. As evaporation is diminished the amount of water that enters the plantsis also diminished. Yet transpiration appears to be a process wholly necessary forplant life. The question is, simply, to what extent it may be diminished withoutinjuring plant growth. Many students believe that the carbon assimilation of theplant, which is fundamentally important in plant growth, cannot be continued unlessthere is a steady stream of water passing through the plant and then evaporatingfrom the leaves.
Of one thing we are fairly sure: if the upwardstream of water is wholly stopped for even a few hours, the plant is likely to beso severely injured as to be greatly handicapped in its future growth.
Botanical authorities agree that transpirationis of value to plant growth, first, because it helps to distribute the mineral nutrientsnecessary for plant growth uniformly throughout the plant; secondly, because it permitsan active assimilation of the carbon by the leaves; thirdly, because it is not unlikelythat 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 transpirationis especially important in dry-farm districts, where, during the summer, the heatis often intense. Fourthly, transpiration apparently influences plant growth anddevelopment in a number of ways not yet clearly understood.
Conditions influencing transpiration
In general, the conditions that determine theevaporation of water from the leaves are the same as those that favor the directevaporation of water from soils, although there seems to be something in the lifeprocess of the plant, a physiological factor, which permits or prevents the ordinarywater-dissipating factors from exercising their full powers. That the evaporationof water from the soil or from a free water surface is not the same as that fromplant leaves may be shown in a general way from the fact that the amount of watertranspired from a given area of leaf surface may be very much larger or very muchsmaller than that evaporated from an equal surface of free water exposed to the sameconditions. It is further shown by the fact that whereas evaporation from a freewater surface goes on with little or no interruption throughout the twenty-four hoursof the day, transpiration is virtually at a standstill at night even though the conditionsfor the rapid evaporation from a free water surface are present.
Some of the conditions influencing the transpirationmay be enumerated as follows:--
First, transpiration is influenced by the relativehumidity. In dry air, under otherwise similar conditions, plants transpire more waterthan in moist air though it is to be noted that even when the atmosphere is fullysaturated, so that no water evaporates from a free water surface, the transpirationof plants still continues in a small degree. This is explained by the observationthat since the life process of a plant produces a certain amount of heat, the plantis always warmer than the surrounding air and that transpiration into an atmospherefully charged with water vapor is consequently made possible. The fact that transpirationis greater under a low relative humidity is of greatest importance to the dry-farmerwho has to contend with the dry atmosphere.
Second, transpiration increases with the increasein temperature; that is, under conditions otherwise the same, transpiration is morerapid 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 increaseof air currents, which is to say, that on a windy day transpiration is much morerapid than on a quiet day.
Fourth, transpiration increases with the increaseof direct sunlight. It is an interesting observation that even with the same relativehumidity, temperature, and wind, transpiration is reduced to a minimum during thenight and increases manyfold during the day when direct sunlight is available. Thiscondition is again to be noted by the dry-farmer, for the dry-farm districts arecharacterized by an abundance of sunshine.
Fifth, transpiration is decreased by the presencein the soil- water of large quantities of the substances which the plant needs forits food material. This will be discussed more fully in the next section.
Sixth, any mechanical vibration of the plantseems to have some effect upon the transpiration. At times it is increased and attimes it is decreased by such mechanical disturbance.
Seventh, transpiration varies also with the ageof the plant. In the young plant it is comparatively small. Just before bloomingit is very much larger and in time of bloom it is the largest in the history of theplant. As the plant grows older transpiration diminishes, and finally at the ripeningstage it almost ceases.
Eighth, transpiration varies greatly with thecrop. Not all plants take water from the soil at the same rate. Very little is asyet known about the relative water requirements of crops on the basis of transpiration.As an illustration, MacDougall has reported that sagebrush uses about one fourthas 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 engagedin the reclamation of dry-farm districts. Moreover, the same crop grown under differentconditions varies in its rate of transpiration. For instance, plants grown for sometime under arid conditions greatly modify their rate of transpiration, as shown bySpalding, who reports that a plant reared under humid conditions gave off 3.7 timesas much water as the same plant reared under arid conditions. This very interestingobservation tends to confirm the view commonly held that plants grown under aridconditions will gradually adapt themselves to the prevailing conditions, and in spiteof the greater water dissipating conditions will live with the expenditure of lesswater than would be the case under humid conditions. Further, Sorauer found, manyyears ago, that different varieties of the same crop possess very different ratesof transpiration. This also is an interesting subject that should be more fully investigatedin the future.
Ninth, the vigor of growth of a crop appearsto have a strong influence on transpiration. It does not follow, however, that themore vigorously a crop grows, the more rapidly does it transpire water, for it iswell known that the most luxuriant plant growth occurs in the tropics, where thetranspiration is exceedingly low. It seems to be true that under the same conditions,plants that grow most vigorously tend to use proportionately the smallest amountof water.
Tenth, the root system--its depth and mannerof growth--influences the rate of transpiration. The more vigorous and extensivethe root system, the more rapidly can water be secured from the soil by the plant.
The conditions above enumerated as influencingtranspiration are nearly all of a physical character, and it must not be forgottenthat they may all be annulled or changed by a physiological regulation. It must beadmitted that the subject of transpiration is yet poorly understood, though it isone of the most important subjects in its applications to plant production in localitieswhere water is scaree. It should also be noted that nearly all of the above conditionsinfluencing transpiration are beyond the control of the farmer. The one that seemsmost readily controlled in ordinary agricultural practice will be discussed in thefollowing section.
Plant-food and transpiration
It has been observed repeatedly by students oftranspiration that the amount of water which actually evaporates from the leavesis 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 avery long time. Woodward, in 1699, observed that the amount of water transpired bya plant growing in rain water was 192.3 grams; in spring water, 163.6 grams, andin water from the River Thames, 159.5 grams; that is, the amount of water transpiredby the plant in the comparatively pure rain water was nearly 20 per cent higher thanthat 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 inwhich he showed that the addition of potassium nitrate, ammonium sulphate or commonsalt to the solution in which plants grew reduced the transpiration; in fact, thereduction was large, varying from 10 to 75 per cent. This was confirmed by a numberof later workers, among them, for instance, Buergerstein, who, in 1875, showed thatwhenever acids were added to a soil or to water in which plants are growing, thetranspiration is increased greatly; but when alkalies of any kind are added, transpirationdecreases. This is of special interest in the development of dry-farming, since dry-farmsoils, as a rule, contain more substances that may be classed as alkalies than dosoils maintained under humid conditions. Sour soils are very characteristic of districtswhere the rainfall is abundant; the vegetation growing on such soils transpires excessivelyand the crops are consequently more subject to drouth.
The investigators of almost a generation agoalso determined beyond question that whenever a complete nutrient solution is presentedto plants, that is, a solution containing all the necessary plant-foods in the properproportions, the transpiration is reduced immensely. It is not necessary that theplant-foods should be presented in a water solution in order to effect this reductionin transpiration; if they are added to the soil on which plants are growing, thesame effect will result. The addition of commercial fertilizers to the soil willtherefore diminish transpiration. It was further discovered nearly half a centuryago that similar plants growing on different soils evaporate different amounts ofwater from their leaves; this difference, undoubtedly, is due to the conditions inthe fertility of the soils, for the more fertile a soil is, the richer will the soil-waterbe in the necessary plant-foods. The principle that transpiration or the evaporationof water from the plants depends on the nature and concentration of the soil solutionis 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 oftranspiration. If varying the transpiration varies the growth, there would be nospecial advantage in reducing the transpiration. From an economic point of view theimportant question is this: Does the plant when its rate of transpiration is reducedstill grow with the same vigor? If that be the case, then every effort should bemade 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 poundof dry matter if the soil solution were of the proper concentration and containedthe elements necessary for plant growth. Little more was done to answer the abovequestions for over one hundred and fifty years. Perhaps the question was not evenasked during this period, for scientific agriculture was just coming into being incountries where the rainfall was abundant. However, Tschaplowitz, in 1878, investigatedthe subject and found that the increase in dry matter is greatest when the transpirationis the smallest. Sorauer, in researches conducted from 1880 to 1882, determined withalmost absolute certainty that less water is required to produce a pound of dry matterwhen the soil is fertilized than when it is not fertilized. Moreover, he observedthat the enriching of the soil solution by the addition of artificial fertilizersenabled the plant to produce dry matter with less water. He further found that ifa soil is properly tilled so as to set free plant-food and in that way to enrichthe 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 increasesthe water-cost of every pound of dry matter produced. It was about this time thatthe Rothamsted Experiment Station reported that its experiments had shown that duringperiods 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 importantin 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 apoor 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 UnitedStates Department of Agriculture, Bureau of Soils, working in 1908, on the manuringof soils, came to the conclusion that the more fertile the soil the less water isrequired to produce a pound of dry matter. He incidentally called attention to thefact that in countries of limited rainfall this might be a very important principleto apply in crop production. Hopkins in his study of the soils of Illinois has repeatedlyobserved, in connection with certain soils, that where the land is kept fertile,injury from drouth is not common, implying thereby that fertile soils will producedry matter at a lower water-cost. The most recent experiments on this subject, conductedby the Utah Station, confirm these conclusions. The experiments, which covered severalyears, 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 solarge a reduction could be secured in practice, it would seem to justify the useof commercial fertilizers in years when the dry-farm year opens with little waterstored in the soil. Similar results, as will be shown below, were obtained by theuse of various cultural methods. It may therefore, be stated as a law, that any culturaltreatment which enables the soil-water to acquire larger quantities of plant-foodalso enables the plant to produce dry matter with the use of a smaller amount ofwater. In dry-farming, where the limiting factor is water, this principle must heemphasized in every cultural operation.
Methods of controlling transpiration
It would appear that at present the only meanspossessed by the farmer for controlling transpiration and making possible maximumcrops with the minimum amount of water in a properly tilled soil is to keep the soilas fertile as is possible. In the light of this principle the practices already recommendedfor the storing of water and for the prevention of the direct evaporation of waterfrom the soil are again emphasized. Deep and frequent plowing, preferably in thefall so that the weathering of the winter may be felt deeply and strongly, is offirst importance in liberating plant-food. Cultivation which has been recommendedfor the prevention of the direct evaporation of water is of itself an effective factorin 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 strikinglythe value of cultivation in reducing the transpiration. For instance, in a seriesof 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; onthe 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 thecultivated soil only 428 pounds were necessary. On a clay soil, not cultivated, 753pounds 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 summerand after every rain has therefore the satisfaction of knowing that he is accomplishingtwo very important things: he is keeping the moisture in the soil, and he is makingit possible for good crops to be grown with much less water than would otherwisebe required. Even in the case of a peculiar soil on which ordinary cultivation didnot 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 produceone pound of dry matter (corn), while on the cultivated soils, though the directevaporation was no smaller, the number of pounds of water for each pound of dry substancewas as low as 265.
One of the chief values of fallowing lies inthe liberation of the plant-food during the fallow year, which reduces the quantityof water required the next year for the full growth of crops. The Utah experimentsto which reference has already been made show the effect of the previous soil treatmentupon the water requirements of crops. One half of the three types of soil had beencropped for three successive years, while the other half had been left bare. Duringthe fourth year both halves were planted to corn. For the sandy loam it was foundthat, on the part that had been cropped previously, 659 pounds of water were requiredfor each pound of dry matter produced, while on the part that had been bare only573 pounds were required. For the clay loam 889 pounds on the cropped part and 550on the previously bare part were required for each pound of dry matter. For the clay7466 pounds on the cropped part and 1739 pounds on the previously bare part wererequired for each pound of dry matter. These results teach clearly and emphaticallythat the fertile condition of the soil induced by fallowing makes it possible toproduce dry matter with a smaller amount of water than can be done on soils thatare cropped continuously. The beneficial effects of fallowing are therefore clearlytwofold: to store the moisture of two seasons for the use of one crop; and to setfree fertility to enable the plant to grow with the least amount of water. It isnot yet fully understood what changes occur in fallowing to give the soil the fertilitywhich 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 thatthe formation of nitrates plays an important part in the whole process. If a soilis of such a nature that neither careful, deep plowing at the right time nor constantcrust cultivation are sufficient to set free an abundance of plant-food, it may benecessary to apply manures or commercial fertilizers to the soil. While the questionof 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 timewill come when the farmers must give primary attention to soil fertility in additionto the storing and conservation of soil-moisture. The fertilizing of lands with properplant-foods, as shown in the last sections, tends to check transpiration and makespossible the production of dry matter at the lowest water-cost.
The recent practice in practically all dry-farmdistricts, at least in the intermountain and far West, to use the header for harvestingbears directly upon the subject considered in this chapter. The high stubble whichremains contains much valuable plant-food, often gathered many feet below the surfaceby the plant roots. When this stubble is plowed under there is a valuable additionof the plant-food to the upper soil. Further, as the stubble decays, acid substancesare produced that act upon the soil grains to set free the plant-food locked up inthem. 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 casesfertility is concentrated near the surface, which dissolves in the soil-water andenables the crop to mature with the Ieast quantity of water.
The lesson then to be learned from this chapteris, that it is not aufficient for the dry-farmer to store an abundance of water inthe soil and to prevent that water from evaporating directly from the soil; but thesoil must be kept in such a state of high fertility that plants are enabled to utilizethe stored moisture in the most economical manner. Water storage, the preventionof evaporation, and the maintenance of soil fertility go hand in hand in the developmentof a successful system of farming without irrigation.