THE WATER ECONOMY OF DRYLAND CROPS.

By THOMAS H. KEARNEY and H. L. SHANTZ.

Physiologists, Alkali and Drought Resistant Plant Breeding Investigations, Bureau of Plant Industry.

INTRODUCTION.

Success in dry-land agriculture—crop production without irriga- tion in regions of limited rainfall—depends largely upon two factors : (1) Maintenance of the largest possible supply of soil moisture and (2) growing crop plants which can adjust themselves to the occur- rence of drought. Such plants may be obtained by introduction from other countries having a similar climate or they may be de- veloped by plant-breeding methods. As a rule a combination of the two measures is necessary, since it rarely happens that an introduced variety is so well adapted to conditions in its new home that it can not be further adjusted by selection. Both lines of work can be carried on more effectively if we know just what habits of growth and what peculiarities of structure adapt plants to dry-land condi- tions. To point out some of these adaptations is the purjDOse of this paper.

The word " drought " is usually taken to mean " dry weather," but when used in connection with plant life it requires further defini- tion. Even when no rain falls and the air is excessively hot and dry, few, if any, crop plants are likely to suffer from drought if the soil is kept supplied with water by irrigation. From an agricultural point of view drought may be defined as a deficiency of soil moisture, usually accompanied by atmospheric conditions favorable to a high rate of evaporation. Dry-land farming is practiced in regions where drought is frequent but not continual, hence where the supply of soil moisture is alternately abundant and scanty. Crop plants are tiierefore required which can adjust themselves to such alternations.

Before we can discuss intelligently the means by which plants succeed in localities where drought occurs, we must have a clear conception of the relations of water to plant life in general. Every

1 This subject has been treated more fully In a paper by B. T. Galloway and A. F. Woods, entitled *' Water as a Factor in the Growth of Plants," in the Yearbook, U. S. Dept. of Agriculture, for 1894, pp. 165-176.

352 Yearbook of the Department of Agriculture.

South American companies and establishments producing refrigerated meats for export.

Name of company. Capital

stock (gold) Name of establishment. Location.

The River Plate Fresh Meat Co. (Ltd.).

Compañía Sansinena de Carnes Congeladas.

Do

$2,250,000

4,500,000

Campana

La Negra

Province of Buenos Aires.

Do.

Cuatreros Do.

Do Frieoríñca Unieuava.J Urueiiav. Las Palmas Produce Co. (Ltd.)—

Compañía Argentina de Cames Congeladas.

La Plata Cold Storage Co. (Ltd.).. Frigoríñco Montevideo

2,500,000

1,500,000

5,000,000

Las Palmas Province of Buenos Aires.

Do.

Do.

La Blanca

La Plata Montevideo Uruguay,

The Smithfleld and Argentine Meat Co. (Ltd.).

Sociedad ADÓnima Frigorífico Ar- gentino.

The New Patagonian Meat Pre- serving and Cold Storage Co. (Ltd.) (branch of La Plata).

Do

1,250,000

2,000,000

2,608,607

-Zarate Province of Buenœ

Argentino Aires.

Do.

Río Gallegos

San Julian

Patagonia.

Do.

Regarding the United States ownership in the above South American refrigerating companies, from our present informa- tion it may be stated that the two establishments. La Plata and Frigorífico Montevideo, the latter in Uruguay, with the two branches in Patagonia, are owned by the Swift Company; the La Blanca plant is owned by Morris & Co. and Armour & Co., and the Frigorífico Argentino has been leased by the Sulzberger Company.

The following table shows the exports of Argentine refrig- erated meat since the commencement of the trade. The in- crease in chilled beef with a corresponding decline in frozen beef exports in recent years shows a growing preference for the former. The great bulk of the exports has gone to Eng- land.

THE WATER ECONOMY OF DKY-LAND CROPS. 353

Different species and varieties of plants differ greatly in the activity of their transpiration. In regions of ample rainfall and in arid regions where irrigating water is available these differences are of relatively small agricultural importance, since there is usually moisture sufficient for the needs of the most wasteful plants. But where severe drought is of frequent occurrence, ability to control transpiration is an important factor in successful plant growth.

WATER REQUIREMENT.

The degree to which a plant is economical in its use of water is expressed in its water requirement, or the total quantity of water which it expends in producing a pound of dry matter. Water re- quirement can not readily be determined in the field, but a method has been devised for measuring it under such conditions that the relative efficiency of different species and varieties can be accurately determined.

The two essential features of this method are (1) growing the plants in large vessels with a volume of soil sufficient to permit the plants to behave normally and to mature a crop and (2) sealing the surface of the soil so as to prevent direct evaporation, thus avoiding loss of water except through the plants themselves. Conditions as to temperature and atmospheric moisture are the same as in the field. Provision is made for supplying daily or at frequent intervals the water lost by transpiration, a careful record being kept of the total quantity added during the course of the experiment. Knowing the quantity of water present in the soil at the outset and at the con- clusion of the experiment and the total quantity added during its progress, it is a simple matter to calculate the total quantity ex- pended by the plants. The total weight of dry matter produced is ascertained by weighing the plants as soon as they are harvested and thoroughly dried. With these data in hand the quantity of water which has been used in producing a pound of dry substance can be readily calculated.

Experience has shown that with proper care a great variety of crop plants can be made to grow thriftily and to yield large quan- tities of straw and of seed under these experimental conditions. Hence, there is every reason to believe that the comparative water requirements of different crop plants may be accurately determined under any given climatic conditions and that we may safely infer from the results what species and varieties are most economical in the.use of water. Other things being equal, the best plants for dry- land agriculture are those which produce the greatest weight of dry matter in proportion to the quantity of water transpired.

20139°—YBK1911 23

354 YEAEBOOK OF THE DEPAETMENT OF AGKICULTURE.

MEANS BY WHICH PLANTS SUCCEED UNDER DRY-LAND CONDITIONS.

The term "drought resistant" is usually applied to all plants which can grow without irrigation in regions where the soil is fre- quently deficient in moisture. Yet, as a matter of fact, many of these plants do not " resist " drought in any strict sense of the term. It is more correct to say that succe^ful plant growth under the conditions mentioned depends upon ability either to endure, to evade, or to escape drought.^

ADAPTATIONS FOE ENDURING DROUGHT.

Endurance of drought implies ability to maintain life when the soil is so dry that practically no absorption of water by the roots can take place. Plants may endure drought if they are able to store water in their own bodies or are able to remain alive in a dormant condition.

STORAGE OF WATER.—Many plants inhabiting deserts or other very arid situations are characterized by thick, fleshy leaves or by swollen stems and roots. These thickenings are due to the presence of numer- ous layers of cells which have the property of storing water during rainy periods and giving it up slowly, when the soil becomes dry, to the cells which carry on the work of assimilation. Long after the soil has become so dry that absorption of water by the roots has virtually ceased such plants may continue to grow, living upon the water accumulated in their tissues much as a camel can live for days with- out drinking by utilizing the water stored in its stomach. The va- rious cacti which are so characteristic of American deserts, some of which are useful forage plants, afford excellent examples of water storage. This adaptation is not developed to a noteworthy degree in any of the more important crop plants.

ABILITY TO BECOME DORMANT.—Many desert shrubs and trees pass the greater part of their lives in a leafless condition, putting forth leaves only during the brief and infrequent rainy periods. Such plants endure drought just as most northern trees and shrubs endure the rigors of winter, by virtue of their ability to remain alive with- out growing, i. e., in a dormant condition. Plants while in this condition make very little demand upon the soil moisture, their transpiration having practically ceased.

This adaptation reaches its highest development in the case of s^eds and of those mosses and lichens which can revive quickly when wetted even after having become so dry as to fall into powder when rubbed between the fingers. Some of the most important of the

1 That some such distinction should be drawn has been suggested by A. M. Ten Eyck in a paper entitled ** Drouth Resistant Crops " (Dry Farming Congress Bulletin, vol. 3, 1910, p. 369), and by C. R. Ball, "The Importance and Improvement of the Grain Sor- ghums" (Bulletin 203, Bureau of Plant Industry, 1911, p. 22).

THE WATER ECONOMY OF DRY-LAND CROPS. 355

grasses which make up the range or native pasture in arid regions, such as the grama grass and buffalo grass of the Great Plains, are likewise able to remain alive in a very dry condition and to resume growth promptly when water again becomes available.

A tendency in this direction is shown by various orchard and forest trees, by alfalfa, and by the sorghums. These plants can withstand considerable drought by virtue of their ability to become partly dormant.

ADAPTATIONS FOE EVADING DROUGHT.

Evasion of drought implies ability to make economical use of a limited supply of water. The plant which can control or reduce its transpiration does not rapidly exhaust the available moisture and hence may avoid a condition of drought in the mass of soil from which its roots draw water, even at times when drought is prevalent in the locality. The same advantage may also be secured by an excep- tionally well-developed root system, which permits the plant to draw water from an unusually large mass of soil.

CONTROL OF TRANSPIRATION.—Plants differ greatly in the rapidity with which they lose water, the rate being inversely proportional to the ability of the plant to control its transpiration. Such control may be attained either by retarding the transpiration per unit area of leaf surface or by reducing the total area from which the loss of water takes place. Closing the breathing pores, or stomata, is a direct and very effective means of retarding transpiration. Various peculiarities of structure, especially marked in plants of arid regions, are regarded by botanists as additional means for hindering the loss of water. Among these may be mentioned: (1) Leaves so con- structed that the margins can roll in, forming a sheltering chamber into which the stomata open; (2) stomata confined to the lower sur- face of the leaf; (3) stomata situated at the bottom of pits or furrows instead of being level with the general surface of the leaf; (4) a dense feltlike or scaly covering of hairs; (5) a thick impervious cuticle or a coating of wax over the surface of the leaves and stems.

Many plants which inhabit deserts are characterized by dwarf growth or by the small number and size of their lea ^es. In such cases the loss of water is limited by the smallness of the lotal transpiring surface. A still further step in the reduction of transpiration is taken by such trees and shrubs as regularly shed all of their leaves during periods of drought.

With such extreme reduction of transpiration as we find in the native plants of arid regions, the rate of growth must be very slow and the annual production of vegetable matter must be small. It fol- lows that few cultivated plants show conspicuous adaptations for the control of transpiration, although such adaptations are found in a

356 YEABBOOK OF THE DEPARTMENT OF AGRICULTURE.

modified degree in those varieties of many crops which thrive best imder dry-land conditions. Wherever such adaptations exist, their tendency is to retard the exhaustion of the soil moisture within reach of the roots and hence to postpone the occurrence of drought.

EXCEPTIONAL ROOT DEVELOPI^IENT.—Plants differ greatly in the char- acters of their root systems. Some species are characterized by roots Avhich penetrate deeply into the soil, while others possess roots which lie near the surface. The root system* can be more or less modified by environment and is particularly susceptible to the influence of changes in soil moisture and soil texture. Yet there is always a limit to the amount of possible modification, and in most species the type is sufficiently fixed so that the best development of the roots and con- sequently the best growth of the plants are attained under some particular combination of soil conditions.

Ability to evade drought is often due to having roots developed in such manner as to absorb water from an unusually large mass of soil. Whether a shallow or a deep root system is most effective depends largely upon the character of the soil and upon the distribution of the rainfall.

If water penetrates readily to a considerable depth, plants having deep roots are obviously at an advantage as compared with shallow- rooting species. Such roots can push ahead into moist soil as fast as the surface layers dry out. In the virgin condition, soils which have this distribution of moisture are largely occupied by deep- rooting woody plants, such as the characteristic black sage {Arte- misia tridentata) of the Great Basin region. Alfalfa with its long taproot is a good example of this adaptation among cultivated plants.

On the other hand, if the soil is so shallow or the rainfall so scanty that moisture is ordinarily held in only a slight depth of soil, deep- rooted species can not thrive. Here the advantage is with plants like the surface-feeding trees, which extend their roots horizontally for long distances. The olive as described and figured by Mason ^ affords a good illustration. This tree, when growing without irrigation in regions of very limited rainfall, develops long roots which radiate almost horizontally from the crown, producing a great many fine branches at a depth of only a few inches below the surface of the ground. This is an ideal arrangement for taking up rapidly the water afforded by infrequent light showers.

ADAPTATION FOR ESCAPING DEOUGHT.

Plants which have so short a growing period that they can mature before the season of drought begins may be said to escape drought. It is one of the striking features of arid regions that when the rains

1 Mason, S. C. Drought Resistance of the Olive in the Southwestern States. Bulletin 192, Bureau of Plant Industry, 1911. See especially figures 7 and 8, p. 29.

THE WATER ECONOMY OF DRY-LAND CROPS. 357

commence a host of small annual plants spring into life. These so-called " ephemeral " plants are able to germinate, complete their growth, and ripen seed in a very few weeks, so that their chances of reaching maturity before the soil dries out again are exceptionally good. No cultivated plants have so brief a span of existence as these desert " ephemerals," but we may properly compare with them cer- tain quickly maturing varieties of " small grains " which are well adapted to dry-land agriculture in regions of summer rainfall. In crop plants which have this habit of life, marked eiBSciency in the absorption and conservation of water is of comparatively little im- portance. They owe their success to their rapid growth, which enables them to ripen seed in a comparatively brief time. Often they are no more exposed to actual drought than are crops grown under irrigation in the same region.

COMBINATION OF ADAPTATIONS.

Most of the crop plants which succeed under dry-land conditions possess more than one of the adaptations just described. Thus, alfalfa is in a measure drought enduring, owing to the ability of the crowns to become partly dormant, and it is also well adapted to evade drought by means of its deeply penetrating roots. The varieties of small grains which are best adapted to growing without irrigation in semiarid regions not only have a short growing season, which enables them often to escape drought, but have also a relatively small total leaf surface, a character which favors drought evasion by reducing transpiration and thus diminishing the danger of exhausting the soil moisture.

The sorghums afford what is perhaps the best example among crop plants of a combination of adaptations for meeting drought. They are typically drought evading, but have also considerable power to endure drought. Sorghums are exceptionally well adapted to dry- land agriculture in regions of summer rainfall like the Great Plains.

The plants grow slowly during the first few weeks after germi- nation. This might be regarded as a disadvantage, since in the Great Plains the moisture content of the soil is usually greater at this time than at any other season of the year. The atmospheric humidity is also usually greater, and consequently the plants trans- pire less and have a lower water requirement than is the case later in the summer. But this disadvantage is compensated by the fact that the slow early growth results in conserving the soil moisture and makes it possible for the plants to evade drought during the remainder of the season. Such crop plants as grow rapidly from the start exhaust the available soil moisture much earlier in the season and if a drought occurs may fail to mature, while sorghum will continue its growth for a considerably longer period. If the drought

358 YEARBOOK OF THE DEPARTMENT OF AGKICULTUEE.

continues, their ability to become partly dormant enables the sor- ghum plants to remain alive in a wilted condition for several weeks, resuming growth when rain comes. Consequently this crop is often in a position to utilize the rainfall of the entire growing season. The sorghums have considerable ability to control transpiration, as is shown by the fact that the plants wilt but slowly when the water supply is cut off. This contributes to their ability to evade and, in extreme cases, to endure drought. Finally, they have a low water requirement, being exceptionally efficient in the production of a crop with a relatively slight expenditure of water.

ADAPTATIONS TO DROUGHT AS AFFECTING CROP PRODUCTION.

The typical drought-enduring plants, those which have a marked ability to store water or to become dormant, are admirably equipped for maintaining life under arid conditions, but because of their very slow growth are not as a rule suited for crop production. It is even doubtful whether such plants are as efficient in their use of water when available as are many plants which have no marked capacity to withstand drought.

On the other hand, a limited degree of drought endurance, such as is shown by the sorghums, by the crowns of alfalfa plants, and by many trees, affords insurance against total loss of crop during briefer periods of drought and need not prevent profitable production. In seasons when the rainfall is normal as to total quantity but very irregular in its distribution these crops may finally give good yields, while corn and potatoes, for example, which have less ability to become dormant, may fail utterly. In fact, no perennial plant and no annual having a long growth period can be considered a safe crop for dry-land agriculture unless they are in some degree drought enduring.

Bui in the main the success of dry-land crop plants is due rather to ability to evade or to escape drought than to drought endurance. Control of transpiration, unless carried to an extreme, and excep- tional root development, the two most important means for evading drought, may be quite compatible with satisfactory yields. Like- wise, plants which escape drought by having a short growing season and completing their development while moisture is abundant may be well adapted to profitable crop production under dry-land conditions.

The degree to which a plant is successful in enduring, evading, or escaping drought measures only its ability to maintain life and to reproduce itself. To insure profitable crop production the plant must also be able to produce a large quantity of organic substance. In regions where the quantity of available soil moisture is often small this can be attained only by plants which produce a satisfactory

THE WATER ECONOMY OF DRY-LAND CROPS. 359

yield with a relatively small expenditure of water; in other words, by plants which have a low water requirement. This is especially true in the case of drought-evading crops, which make their growth during the season when drought is likely to occur and which depend for success upon their ability to use water so economically as to avoid a deficiency of moisture in the soil around their roots.

AGRICULTURAL PRACTICE AND ADAPTATION TO DROUGHT.

By proper cultural methods it is possible to imitate some of the natural adaptations to drought. This applies only to adaptations for evading and for escaping drought, since endurance of drought is due to inherent properties of the plant alone and is entirely independent of agricultural practice.

EVASION OF DROUGHT.

Plants can be greatly aided in evading drought by reducing the amount of growth per unit of soil mass. Other things being equal, the amount of growth made under dry-land conditions will be lim- ited by the quantity of water available. Hence, if measures are taken to avoid having the stand so thick or the growth so heavy that the soil moisture will be rapidly exhausted by excessive transpira- tion, there is much more likelihood that the remaining plants will reach maturity. Various methods are available for reducing the amount of plant growth to the limit of safety.

THIN STAND.—In extremely arid regions the native shrubs and trees generally stand far apart, so that each individual has a large mass of soil from which to draw water. The same advantage may be secured artificially by thin planting. Many crop failures in arid regions are caused by a too thick stand. The ideal should be to have no greater number of plants per acre than can be brought to maturity by the moisture supply which may reasonably be expected.

Perennial crops like alfalfa and trees should be planted so thin that even during the most unfavorable years the plants may success- fully evade drought. An admirable example of adjustment of planting distance to moisture supply is afforded by the dry-land olive orchards of southern Tunis, in a district where the annual rain- fall averages about 9 inches.^ Although only from 7 to 11 trees are planted to the acre, profitable crops of fruit are obtained with much regularity.

With annual crops the rate of seeding should be governed as far as possible by the moisture supply of average years. Yet the stand, especially of grain crops, is not entirely controllable by the rate of

1 Kearney, T. H. Dry-Land Olive Culture in Northern Africa. Bulletin 125, Bureau of Plant Industry, 1908.

360 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE.

seeding. Too abundant moisture during the early period of growth, even though thin seeding is practiced, may result in failure of the crop by stimulating the plants to unusually heavy stooling. The excessive loss of water by transpiration from the dense growth thus produced may lead to a fatal deficiency of soil moisture before the grain is ripe.^

The crop can obtain the full benefit from thin planting only where the land is free from weeds. With intertilled crops this condition may be met by clean cultivation. When intertillage is not practiced, as is usually the case with small grains, little or no advantage is likely to be obtained by thin seeding in case the land is foul. The more favorable moisture condition thus created leads to a vigorous growth of weeds and a correspondingly rapid exhaustion of the water supply. As a result the crop may not have as good a chance to ripen as would have been the case had the grain been sown thickly.

CUTTING OR PRUNING.—In the case of tree crops, the top should be kept small as a protection against total exhaustion of the soil moisture during periods of drought. As a general rule the older and larger a tree becomes, the greater is the danger of its death from drought. This danger may often be averted by judicious pruning. Clean intertillage is recommended for an orchard or woodlot thus managed, since otherwise the water economized by reducing the total leaf surface of the trees would merely be diverted to the use of the grass and weeds.

Perennial forage crops, such as alfalfa, may be safeguarded against drought by cutting when a serious shortage in moisture supply threatens. If the greater part of the transpiring surface is removed, the plants will be able to continue growing for a much longer period than if the crop were left uncut, and, if there is suffi- cient rainfall later in the season, they may produce a good second or third crop.

GROWING DWARF VARIETIES.—There are certain varieties of crop plants which have an inherent tendency to limited growth. By the choice of such dwarf varieties it may be possible to prevent over- production and thus bring the crop to maturity, where a larger variety having a greater transpiring surface might exhaust the soil moisture long before growth is completed. Some of the most success- ful crop varieties of the Great Plains area, such as Dwarf milo and other dwarf sorghums and the smaller growing varieties of oats and barley, are cases in point.

1 The disastrous effects sometimes observed when manure is applied in dry-land farm- ing may also be due to overstimulation of the plants and consequent premature ex- haustion of the soil moisture.

THE WATEB ECONOMY OF DRY-LAND CEOPS. 361

ESCAPE FEOM DROUGHT.

Annual crops which have a comparatively brief growing season, such as the spring grains, may be enabled to escape drought if seed- ing is so timed that the maximum growth will take place during the period when rainfall is heaviest and evaporation is least. Sometimes the crop may be grown so as to evade drought at one season and escape it at another. A good example is winter wheat. In regions of summer rainfall, such as the Great Plains area, water is conserved in the soil by clean cultivation throughout the summer and the seed is sown in the fall. The stored moisture is sufficient to enable the crop to evade drought during the fall and winter and to start growth early enough in the spring to escape the severe droughts of the middle and late summer. On the other hand, where rain comes mostly in winter the plants escape drought during the wet winter months and by economical use of the water left in the soil at the close of the rainy period are able to evade drought during the spring and summer.

CONCLUSIONS.

Plant introduction and plant breeding in connection with dry-land agriculture should be guided by the nature of the adaptations to drought possessed by the crop in question. The following points should always be considered :

(1) Season of growth and length of the growing period. (2) Ability to endure drought by becoming more or less dormant. (3) Character of the root system as determining the distribution

of soil moisture to which the plant is best adapted. (4) Adaptations for controlling transpiration. (5) Smallness of the water requirement as determining efficiency

in crop production. In dealing with a plant which has some ability to endure drought,

whether a perennial like alfalfa or an annual having a rather long growing season, such as sorghum, the comparison should be made under conditions of actual drought, selection being based upon the ability of the plants to survive and to give a profitable yield after having experienced a deficiency of soil moisture. Since adaptations for postponing or evading drought are equally important in crops of this character, the nature of the root system and the means for controlling transpiration should also be studied. In the case of rapidly matur- ing annuals like the small grains, attention should be given to the length of the growing season, upon which depends the ability of the plants to escape drought. In all cases the water requirement should be investigated, a low ratio of water expenditure to crop production being one of the best indications of adaptability to dry-land con- ditions.

36-2 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE.

Much of the plant breeding and variety testing which has been carried on in semiarid regions with a view to obtaining " drought- resistant " crops is open to criticism on the score that too much attention has been paid to the highest yielding plants. Often the work has been done in seasons of unusually favorable rainfall. Often the largest and most productive plants in a field have been selected without attempting to determine whether these have not been favored by a larger supply of moisture or of plant food in the soil imme- diately around their roots. Varieties thus developed are not likely to prove safe for crop production under average conditions. Allow- ance must be made for the fact that nature has set limits to the yields attainable in dry-land agriculture. In the last analysis these are determined by the quantity of water available. The varieties which prove safest for growing in regions where drought is frequent will almost inevitably be less productive than varieties of the same crop which are more exacting in their water requirement.

On the other hand, the agricultural value of dry-land plants is not determined by their water economy alone. Since the ultimate object is profitable crop production, the yield and the quality of the product must not be overlooked. If selection is focused exclusively upon adaptability to drought conditions there is danger that the variety obtained will prove so inferior that it can not be profitably grown.

A compromise between the two ideals will generally give the best results. The plants to be selected are those which in one way or another are reasonably secure from destruction by drought and which also, when grown under conditions of moisture supply normal to the region, can give a product that will be remunerative to the grower.