CHAPTER VIII
REGULATING THE EVAPORATION


THE demonstration in the last chapter that thewater which falls as rain or snow may be stored in the soil for the use of plantsis of first importance in dry-farming, for it makes the farmer independent, in alarge measure, of the distribution of the rainfall. The dry-farmer who goes intothe summer with a soil well stored with water cares little whether summer rains comeor 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 lightthey are a positive detriment to the farmer who by careful farming has stored hisdeep soil with an abundance of water. Storing the soil with water is, however, onlythe first step in making the rains of fall, winter, or the preceding year availablefor plant growth. As soon as warm growing weather comes, water-dissipating forcescome into play, and water is lost by evaporation. The farmer must, therefore, useall precautions to keep the moisture in the soil until such time as the roots ofthe crop may draw it into the plants to be used in plant production. That is, asfar as possible, direct evaporation of water from the soil must be prevented.

Few farmers really realize the immense possibleannual evaporation in the dry-farm territory. It is always much larger than the totalannual rainfall. In fact, an arid region may be defined as one in which under naturalconditions several times more water evaporates annually from a free water surfacethan falls as rain and snow. For that reason many students of aridity pay littleattention to temperature, relative humidity, or winds, and simply measure the evaporationfrom a free water surface in the locality in question. In order to obtain a measureof the aridity, MacDougal has constructed the following table, showing the annualprecipitation and the annual evaporation at several well-known localities in thedry-farm territory.

True, the localities included in the followingtable are extreme, but they illustrate the large possible evaporation, ranging fromabout six to thirty-five times the precipitation. At the same time it must be bornein 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
(In Inches)
Annual Evaporation
(In Inches)
Ratio
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 checkingevaporation from the soil, it is necessary to review briefly the conditions thatdetermine the evaporation of water into the air, and the manner in which water movesin the soil.


The formation of water vapor

Whenever water is left freely exposed to theair, it evaporates; that is, it passes into the gaseous state and mixes with thegases 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 canenter 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 ofthis is the well-known experience that on the seashore, where the air is often verynearly 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, wherethe air is far from being saturated, drying goes on very rapidly.

The amount of water necessary to saturate airvaries 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 proportionatelymore rapidly than the increase in temperature. This is generally well understoodin common experience, as in drying clothes rapidly by hanging them before a hot fire.At a temperature of 100° F., which is often reached in portions of the dry-farmterritory during the growing season, a given volume of air can hold more than ninetimes as much water vapor as at the temperature of freezing water. This is an exceedinglyimportant principle in dry-farm practices, for it explains the relatively easy possibilityof storing water during the fall and winter when the temperature is low and the moistureusually 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 veryactive. This law also emphasizes the truth that it is in times of warm weather thatevery precaution must be taken to prevent the evaporation of water from the soilsurface.

Temperature in Degrees F. Grains of Water held in 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 atmosphereas a whole is never saturated with water vapor. Such saturation is at the best onlylocal, as, for instance, on the seashore during quiet days, when the layer of airover 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 immediatelyupon the soil and around the plants. Whenever, in such cases, the air begins to moveand the wind blows, the saturated air is mixed with the larger portion of unsaturatedair, and evaporation is again increased. Meanwhile, it must be borne in mind thatinto a layer of saturated air resting upon a field of growing plants very littlewater evaporates, and that the chief water-dissipating power of winds lies in theremoval of this saturated layer. Winds or air movements of any kind, therefore, becomeenemies of the farmer who depends upon a limited rainfall.

The amount of water actually found in a givenvolume 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 relativehumidity becomes smaller as the rainfall decreases. The lower the relative humidityis at a given temperature, the more rapidly will water evaporate into the air. Thereis no more striking confirmation of this law than the fact that at a temperatureof 90° sunstrokes and similar ailments are reported in great number from NewYork, while the people of Salt Lake City are perfectly comfortable. In New York therelative humidity in summer is about 73 per cent; in Salt Lake City, about 35 percent. At a high summer temperature evaporation from the skin goes on slowly in NewYork 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 percentageof relative humidity.

Evaporation from water surfaces is hastened,therefore, by (1) an increase in the temperature, (2) an increase in the air movementsor 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 inhumid regions. The dry-farmer must consequently use all possible precautions to preventevaporation from the soil.


Conditions of evaporation from from soils

Evaporation does not alone occur from a surfaceof free water. All wet or moist substances lose by evaporation most of the waterthat they hold, providing the conditions of temperature and relative humidity arefavorable. Thus, from a wet soil, evaporation is continually removing water. Yet,under ordinary conditions, it is impossible to remove all the water, for a smallquantity is attracted so strongly by the soil particles that only a temperature abovethe boiling point of water will drive it out. This part of the soil is the hygroscopicmoisture spoken of in the last chapter.

Moreover, it must be kept in mind that evaporationdoes not occur as rapidly from wet soil as from a water surface, unless all the soilpores are so completely filled with water that the soil surface is practically awater surface. The reason for this reduced evaporation from a wet soil is almostself-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-tilledsoil, and the force causing evaporation must be strong enough to overcome this attractionbesides changing the water into vapor.

The less water there is in a soil, the thinnerthe water film, and the more firmly is the water held. Hence, the rate of evaporationdecreases with the decrease in soil-moisture. This law is confirmed by actual fieldtests. For instance, as an average of 274 trials made at the Utah Station, it wasfound that three soils, otherwise alike, that contained, respectively, 22.63 percent, 17.14 per cent, and 12.75 per cent of water lost in two weeks, to a depth ofeight feet, respectively 21.0, 17. 1, and 10.0 pounds of water per square foot. Similarexperiments 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 unnecessarilymoist. The dry-farmer can reduce the per cent of water in the soil without diminishingthe total amount of water by so treating the soil that the water will distributeitself to considerable depths. This brings into prominence again the practices offall plowing, deep plowing, subsoiling, and the choice of deep soils for dry-farming.

Very much for the same reasons, evaporation goeson 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 enoughto resist partially the force causing evaporation. Soil-water always contains someof the soil ingredients in solution, and consequently under the given conditionsevaporation occurs more slowly from soil-water than from pure water. Now, the morefertile a soil is, that is, the more soluble plant-food it contains, the more materialwill be dissolved in the soil-water, and as a result the more slowly will evaporationtake place. Fallowing, cultivation, thorough plowing and manuring, which increasethe store of soluble plant-food, all tend to diminish evaporation. While these conditionsmay 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 possibilityof 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 asfrom those at the surface. In developing a system of practice which will reduce evaporationto a minimum it must be learned whether the water which evaporates from the soilparticles far below the surface is carried in large quantities into the atmosphereand thus lost to plant use. Over forty years ago, Nessler subjected this questionto experiment and found that the loss by evaporation occurs almost wholly at thesoil surface, and that very little if any is lost directly by evaporation from thelower soil layers. Other experimenters have confirmed this conclusion, and very recentlyBuckingham, examining the same subject, found that while there is a very slow upwardmovement of the soil gases into the atmosphere, the total quantity of the water thuslost by direct evaporation from soil, a foot below the surface, amounted at mostto one inch of rainfall in six years. This is insignificant even under semiarid andarid conditions. However, the rate of loss of water by direct evaporation from thelower soil layers increases with the porosity of the soil, that is, with the spacenot filled with soil particles or water. Fine-grained soils, therefore, lose theleast water in this manner. Nevertheless, if coarse-grained soils are well filledwith water, by deep fall plowing and by proper summer fallowing for the conservationof moisture, the loss of moisture by direct evaporation from the lower soil layersneed not be larger than from finer grained soils

Thus again are emphasized the principles previouslylaid down that, for the most successful dry-farming, the soil should always be keptwell filled with moisture, even if it means that the land, after being broken, mustlie 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 landsshould be stored deeply, away from the immediate action of the sun's rays upon theland surface. The necessity for deep soils is thus again brought out.

The great loss of soil moisture due to an accumulationof water in the upper twelve inches is well brought out in the experiments conductedby 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 and16.55 per cent of water on an average to a depth of eight feet; that is, the totalamount of water held by the two soils was practically identical. Owing to varyingcultural treatment, the distribution of the water in the soil was not uniform; onecontained 23.22 per cent and the other 16.64 per cent of water in the first twelveinches. During the first seven days the soil that contained the highest percentageof water in the first foot lost 13.30 pounds of water, while the other lost only8.48 pounds per square foot. This great difference was due no doubt to the fact thatdirect evaporation takes place in considerable quantity only in the upper twelveinches of soil, where the sun's heat has a full chance to act.

Any practice which enables the rains to sinkquickly to considerable depths should be adopted by the dry-farmer. This is perhapsone of the great reasons for advocating the expensive but usually effective subsoilplowing 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 goeson, as from the topsoil, and the passing winds renew the air so that the evaporationmay go on rapidly. The dry-farmer must go over the land as often as needs be withsome 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 impossiblethat hand hoeing, expensive as it is, would pay well in the saving of soil moistureand the consequent increase in crop yield.


How soil water reaches the surface

It may be accepted as an established truth thatthe 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 thesoil-moisture to a depth of eight or ten feet or more is depleted. This is shownby the following analyses of dry-farm soil in early spring and midsummer. No attemptwas made to conserve the moisture in the soil:--

Per cent of water in 1st foot 2nd
foot
3rd
foot
4th
foot
5th
foot
6th
foot
7th
foot
8th
foot
Avg
Early spring 20.84 20.06 19.62 18.28 18.70 14.29 14.48 13.83 17.51
Midsummer 8.83 8.87 11.03 9.59 11.27 11.03 8.95 9.47 9.88



In this case water had undoubtedly passed bycapillary movement from the depth of eight feet to a point near the surface wheredirect evaporation could occur. As explained in the last chapter, water which isheld as a film around the soil particles is called capillary water; and it is inthe capillary form that water may be stored in dry-farm soils. Moreover, it is thecapillary soil-moisture alone which is of real value in crop production. This capillarywater tends to distribute itself uniformly throughout the soil, in accordance withthe prevailing conditions and forces. If no water is removed from the soil, in courseof time the distribution of the soil-water will be such that the thickness of thefilm at any point in the soil mass is a direct resultant of the various forces actingat 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 whichthe 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 meteorologicalfactors during spring, summer, and fall; and (3) the abstraction of water from thesoil by plant roots.

Water, entering the soil, moves downward underthe influence of gravity as gravitational water, until under the attractive influenceof the soil it has been converted into capillary water and adheres to the soil particlesas a film. If the soil were dry, and the film therefore thin, the rain water wouldmove 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 beforebeing 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 rightthrough the soil and connect with the standing water below. This, of course, is seldomthe case in dry-farm districts. In any soil, excepting one already saturated, theaddition of water will produce a thickening of the soil-water film to the full descentof the water. This immediately destroys the conditions of equilibrium formerly existing,for the moisture is not now uniformly distributed. Consequently a process of redistributionbegins which continues until the nearest approach to equilibrium is restored. Inthis process water will pass in every direction from the wet portion of the soilto the drier; it does not necessarily mean that water will actually pass from thewet portion to the drier portion; usually, at the driest point a little water isdrawn from the adjoining point, which in turn draws from the next, and that fromthe next, until the redistribution is complete. The process is very much like stuffingwool into a sack which already is loosely filled. The new wool does not reach thebottom of the sack, yet there is more wool in the bottom than there was before.

If a plant-root is actively feeding some distanceunder the soil surface, the reverse process occurs. At the feeding point the rootcontinually abstracts water from the soil grains and thus makes the film thinnerin 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 actionof the plant-root. Soil many feet or even rods distant may assist in supplying suchan active root with moisture. When the thousands of tiny roots sent out by each plantare recalled. it may well be understood what a confusion of pulls and counter-pullsupon the soil- moisture exists in any cultivated soil. In fact, the soil-water filmmay be viewed as being in a state of trembling activity, tending to place itselfin full equilibrium with the surrounding contending forces which, themselves, constantlychange. Were it not that the water film held closely around the soil particles ispossessed of extreme mobility, it would not be possible to meet the demands of theplants upon the water at comparatively great distances. Even as it is, it frequentlyhappens that when crops are planted too thickly on dry-farms, the soil-moisture cannotmove quickly enough to the absorbing roots to maintain plant growth, and crop failureresults. Incidentally, this points to planting that shall be proportional to themoisture contained by the soil. See Chapter XI.

As the temperature rises in spring, with a decreasein the relative humidity, and an increase in direct sunshine, evaporation from thesoil surface increases greatly. However, as the topsoil becomes drier, that is, asthe water fihn becomes thinner, there is an attempt at readjustment, and water movesupward to take the place of that lost by evaporation. As this continues throughoutthe season, the moisture stored eight or ten feet or more below the surface is graduallybrought 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 thewater film around the soil grains becomes thinner, the capillary movement of thesoil-water is retarded. This is easily understood by recalling that the soil particleshave an attraction for water, which is of definite value, and may be measured bythe thickest film that may be held against gravity. When the film is thinned, itdoes not diminish the attraction of the soil for water; it simply results in a strongerpull upon the water and a firmer holding of the film against the surfaces of thesoil grains. To move soil-water under such conditions requires the expenditure ofmore energy than is necessary for moving water in a saturated or nearly saturatedsoil. Under like conditions, therefore, the thinner the soil-water film the moredifficult will be the upward movement of the soil-water and the slower the evaporationfrom the topsoil.

As drying goes on, a point is reached at whichthe capillary movement of the water wholly ceases. This is probably when little morethan the hygroscopic moisture remains. In fact, very dry soil and water repel eachother. 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 importantfact that very dry soil furnishes a very effective protection against the capillarymovement of water.

In accordance with the principle above establishedif the surface soil could be dried to the point where capillarity is very slow, theevaporation would be diminished or almost wholly stopped. More than a quarter ofa century ago, Eser showed experimentally that soil-water may be saved by dryingthe surface soil rapidly. Under dry-farm conditions it frequently occurs that thedraft upon the water of the soil is so great that nearly all the water is quicklyand so completely abstracted from the upper few inches of soil that they are leftas an effective protection against further evaporation. For instance, in localitieswhere hot dry winds are of common occurrence, the upper layer of soil is sometimescompletely dried before the water in the lower layers can by slow capillary movementreach the top. The dry soil layer then prevents further loss of water, and the windbecause of its intensity has helped to conserve the soil-moisture. Similarly in localitieswhere 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 demandsmade, and the topsoil then dries out so completely as to form a protective coveringagainst further evaporation. It is on this principle that the native desert soilsof 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 recordedthis observation with considerable surprise, many years ago, and other observershave found the same conditions at nearly all points of the arid region. This matterhas been subjected to further study by Buckingham, who placed a variety of soilsunder artificially arid and humid conditions. It was found in every case that, theinitial evaporation was greater under arid conditions, but as the process went onand 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 automaticallyprotects itself from drying by the formation of a natural mulch on the surface."

Naturally, dry-farm soils differ greatly in theirpower of forming such a mulch. A heavy clay or a light sandy soil appears to haveless power of such automatic protection than a loamy soil. An admixture of limestoneseems to favor the formation of such a natural protective mulch. Ordinarily, thefarmer 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 cultivationis very desirable for other reasons also, as will soon be discussed. Meanwhile, thewater-dissipating forces of the dry-farm section are not wholly objectionable, forwhether the land be cultivated or not, they tend to hasten the formation of dry surfacelayers 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, themost active cause of rapid evaporation from moist soil surfaces. Whenever, therefore,evaporation is not rapid enough to form a dry protective layer of topsoil, shadinghelps materially in reducing surface losses of soil-water. Under very arid conditions,however, it is questionable whether in all cases shading has a really beneficialeffect, though under semiarid or sub-humid conditions the benefits derived from shadingare increased largely. Ebermayer showed in 1873 that the shading due to the forestcover reduced evaporation 62 per cent, and many experiments since that day have confirmedthis conclusion. At the Utah Station, under arid conditions, it was found that shadinga pot of soil, which otherwise was subjected to water-dissipating influences, saved29 per cent of the loss due to evaporation from a pot which was not shaded. Thisprinciple cannot be applied very greatly in practice, but it points to a somewhatthick planting, proportioned to the water held by the soil. It also shows a possiblebenefit to be derived from the high header straw which is allowed to stand for severalweeks in dry-farm sections where the harvest comes early and the fall plowing isdone late, as in the mountain states. The high header stubble shades the ground verythoroughly. Thus the stubble may be made to conserve the soil-moisture in dry-farmsections, where grain is harvested by the "header" method.

A special case of shading is the mulching ofland with straw or other barnyard litter, or with leaves, as in the forest. Suchmulching reduces evaporation, but only in part, because of its shading action, sinceit acts also as a loose top layer of soil matter breaking communication with thelower soil layers.

Whenever the soil is carefully stirred, as willbe described, the value of shading as a means or checking evaporation disappearsalmost entirely. It is only with soils which are tolerably moist at the surface thatshading acts beneficially.


Alfalfa in cultivated rows. This practice is employed to make possiblethe growth of alfalfa and other perennial crops on arid lands without irrigation.



The effect of tillage

Capillary soil-moisture moves from particle toparticle 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 movementof the soil- moisture proceed. If by any means a layer of the soil is so loosenedas to reduce the number of points of contact, the movement of the soil-moisture iscorrespondingly hindered. The process is somewhat similar to the experience in larger airway stations. Just before train time a great crowd of people is gathered outsideor the gates ready to show their tickets. If one gate is opened, a certain numberof passengers can pass through each minute;

if two are opened, nearly twice as many may beadmitted in the same time; if more gates are opened, the passengers will be ableto enter the train more rapidly. The water in the lower layers of the soil is readyto move upward whenever a call is made upon it. To reach the surface it must passfrom soil grain to soil grain, and the larger the number of grains that touch, themore quickly and easily will the water reach the surface, for the points of contactof 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 contactbetween the top and subsoil is greatly reduced, the upward flow of water is therebylargely checked. Such a loosening of the topsoil for the purpose of reducing evaporationfrom the topsoil has come to be called cultivation, and includes plowing, harrowing,disking, hoeing, and other cultural operations by which the topsoil is stirred. Thebreaking of the points of contact between the top and subsoil is undoubtedly themain reason for the efficiency of cultivation, but it is also to be remembered thatsuch stirring helps to dry the top soil very thoroughly, and as has been explaineda layer of dry soil of itself is a very effective check upon surface evaporation.

That the stirring or cultivation of the topsoilreally does diminish evaporation of water from the soil has been shown by numerousinvestigations. In 1868, Nessler found that during six weeks of an ordinary Germansummer a stirred soil lost 510 grams of water per square foot, while the adjoiningcompacted 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 cultivationreduced the evaporation a little more than 60 per cent; Johnson, in 1878, confirmedthe truth of the principle on American soils, and Levi Stockbridge, working aboutthe same time, also on American soils, found that cultivation diminished evaporationon a clay soil about 23 per cent, on a sandy loam 55 per cent, and on a heavy loamnearly 13 per cent. All the early work done on this subject was done under humidconditions, and it is only in recent years that confirmation of this important principlehas been obtained for the soils of the dry-farm region. Fortier, working under Californiaconditions, determined that cultivation reduced the evaporation from the soil surfaceover 55 per cent. At the Utah Station similar experiments have shown that the savingof soil-moisture by cultivation was 63 per cent for a clay soil, 34 per cent fora coarse sand, and 13 per cent for a clay loam. Further, practical experience hasdemonstrated time and time again that in cultivation the dry-farmer has a powerfulmeans of preventing evaporation from agricultural soils.

Closely connected with cultivation is the practiceof scattering straw or other litter over the ground. Such artificial mulches arevery effective in reducing evaporation. Ebermayer found that by spreading straw onthe land, the evaporation was reduced 22 per cent; Wagner found under similar conditionsa 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 soilscannot become a very extensive practice, yet it is well to bear the principle inmind. The practice of harvesting dry-farm grain with the header and plowing underthe high stubble in the fall is a phase of cultivation for water conservation thatdeserves special notice. The straw, thus incorporated into the soil, decomposes quitereadily 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 thispractice is continued for considerable periods, the topsoil becomes rich in organicmatter, which assists in retarding evaporation, besides increasing the fertilityof the land. When straw cannot be fed to advantage, as is yet the case on many ofthe western dry-farms, it would be better to scatter it over the land than to burnit, as is often done. Anything that covers the ground or loosens the topsoil preventsin a measure the evaporation of the water stored in lower soil depths for the useof crops.


Depth of cultivation

The all-important practice for the dry-farmerwho is entering upon the growing season is cultivation. The soil must be coveredcontinually with a deep layer of dry loose soil, which because of its looseness anddryness makes evaporation difficult. A leading question in connection with cultivationis the depth to which the soil should be stirred for the best results. Many of theearly students of the subject found that a soil mulch only one half inch in depthwas effective in retaining a large part of the soil-moisture which noncultivatedsoils would lose by evaporation. Soils differ greatly in the rate of evaporationfrom their surfaces. Some form a natural mulch when dried, which prevents furtherwater loss. Others form only a thin hard crust, below which lies an active evaporatingsurface of wet soil. Soils which dry out readily and crumble on top into a naturalmulch should be cultivated deeply, for a shallow cultivation does not extend beyondthe naturally formed mulch. In fact, on certain calcareous soils, the surfaces ofwhich dry out quickly and form a good protection against evaporation, shallow cultivationsoften 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 muchbetter to shallow cultivations. In general, however, the deeper the cultivation,the more effective it is in reducing evaporation. Fortier, in the experiments inCalifornia to which allusion has already been made, showed the greater value of deepcultivation. 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 thetotal amount of water that had been added. A mulch 4 inches deep saved about 72 percent of the evaporation; a mulch 8 inches deep saved about 88 per cent, and a mulch10 inches deep stopped evaporation almost wholly. It is a most serious mistake forthe dry-farmer, who attempts cultivation for soil-moisture conservation, to failto 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 evaporationis greater from a wet than from a dry surface. It follows, therefore, that the criticaltime for preventing evaporation is when the soil is wettest. After the soil is tolerablydry, a very large portion of the soil-moisture has been lost, which possibly mighthave been saved by earlier cultivation. The truth of this statement is well shownby experiments conducted by the Utah Station. In one case on a soil well filled withwater, during a three weeks' period, nearly one half of the total loss occurred thefirst, while only one fifth fell on the third week. Of the amount lost during thefirst week, over 60 per cent occurred during the first three days. Cultivation should,therefore, be practiced as soon as possible after conditions favorable for evaporationhave been established. This means, first, that in early spring, just as soon as theland is dry enough to be worked without causing puddling, the soil should be deeplyand thoroughly stirred. Spring plowing, done as early as possible, is an excellentpractice 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 isusually used in early spring, and if it is set at rather a sharp angle, and properlyweighted, so that it cuts deeply into the ground, it is practically as effectiveas spring plowing. The chief danger to the dry-farmer is that he will permit theearly spring days to slip by until, when at last he begins spring cultivation, alarge portion of the stored soil-water has been evaporated. It may be said that deepfall plowing, by permitting the moisture to sink quickly into the lower layers ofsoil, makes it possible to get upon the ground earlier in the spring. In fact, unplowedland cannot be cultivated as early as that which has gone through the winter in aplowed condition

If the land carries a fall-sown crop, early springcultivation is doubly important. As soon as the plants are well up in spring theland should be gone over thoroughly several times if necessary, with an iron toothharrow, the teeth of which are set to slant backward in order not to tear up theplants. 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 forthe remaining plants. The wise dry-fanner cultivates his land, whether fallow orcropped, 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 landshould be disked and. then harrowed. Every device should be used to secure the formationof a layer of loose drying soil over the land surface. The season's crop will dependlargely upon the effectiveness of this spring treatment.

As the season advances, three causes combineto permit the evaporation of soil-moisture.

First, there is a natural tendency, under thesomewhat moist conditions of spring, for the soil to settle compactly and thus torestore the numerous capillary connections with the lower soil layers through whichwater escapes. Careful watch should therefore be kept upon the soil surface, andwhenever the mulch is not loose, the disk or harrow should be run over the land.

Secondly, every rain of spring or summer tendsto establish connections with the store of moisture in the soil. In fact, late springand summer rains are often a disadvantage on dry-farms, which by cultural treatmenthave been made to contain a large store of moisture. It has been shown repeatedlythat light rains draw moisture very quickly from soil layers many feet below thesurface. The rainless summer is not feared by the dry-farmer whose soils are fertileand rich in moisture. It is imperative that at the very earliest moment after a springor summer rain the topsoil be well stirred to prevent evaporation. It thus happensthat in sections of frequent summer rains, as in the Great Plains area, the farmerhas to harrow his land many times in succession, but the increased crop yields invariablyjustify the added expenditure of effort.

Thirdly, on the summer-fallowed ground weedsstart 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 weedupon 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-moistureis wholly erroneous. The growth of weeds on a fallow dry-farm is more dangerous thanthe packed uncared-for topsoil. Many implements have been devised for the easy killingof weeds, but none appear to be better than the plow and the disk which are foundon every farm. (See Chapter XV.)

When crops are growing on the land, thoroughsummer cultivation is somewhat more difficult, but must be practiced for the greatestcertainty of crop yields. Potatoes, corn, and similar crops may be cultivated withcomparative ease, by the use of ordinary cultivators. With wheat and the other smallgrains, generally, the damage done to the crop by harrowing late in the season istoo great, and reliance is therefore placed on the shading power of the plants toprevent undue evaporation. However, until the wheat and other grains are ten to twelveinches high, it is perfectly safe to harrow them. The teeth should be set backwardto diminish the tearing up of the plants, and the implement weighted enough to breakthe soil crust thoroughly. This practice has been fully tried out over the largerpart of the dry-farm territory and found satisfactory.

So vitally important is a permanent soil mulchfor the conservation for plant use of the water stored in the soil that many attemptshave been made to devise means for the effective cultivation of land on which smallgrains and grasses are growing. In many places plants have been grown in rows sofar apart that a man with a hoe could pass between them. Scofield has described thismethod as practiced successfully in Tunis. Campbell and others in America have proposedthat a drill hole be closed every three feet to form a path wide enough for a horseto travel in and to pull a large spring tooth cultivator' with teeth so spaced asto 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. Underconditions of high aridity, or where the store of soil-moisture is low, such treatmentfrequently stands between crop success and failure, and it is not unlikely that methodswill be devised which will permit of the cheap and rapid cultivation between therows of growing wheat. Meanwhile, the dry-farmer must always remember that the marginunder which he works is small, and that his success depends upon the degree to whichhe prevents small wastes.


Dry-farm potatoes, Rosebud Co., Montana, 1909. Yield, 282 bushelsper acre.


The conservation of soil-moisture depends uponthe vigorous, unremitting, continuous stirring of the topsoil. Cultivation! cultivation!and more cultivation! must be the war-cry of the dry-farmer who battles against thewater thieves of an arid climate.