UNIVERSITY OF CALIFORNIA AGRICULTURAL EXPERIMENT STATION E. W. HILGARD, DIRECTOR THE CONSERVATION OF SOIL MOISTURE AND ECONOMY IN THE USE OF IRRIGATION WATER BULLETIN 121 BERKELEY £be TUniversitE press AUGUST, 1898 THE CONSERVATION OF SOIL MOISTURE AND ECONOMY IN THE USE OF IRRIGATION WATER. By E. W. Hilgard and R. H. Loughridge. The exceptionally dry season of 1897-8, coupled with the early cessation of rains in the spring of 1897, have brought about in Cali- fornia a more extended failure of cereals and pasturage, and shallow- rooted crops generally, than in any year since the State became a prominently agricultural one the season of 1876-7 being the nearest to carry with it a similar deficiency in crop production. It has been the effort of the Experiment Station to utilize the present unusual season for the study of the limits of endurance of drought on the part of the several crop plants, and with it to determine the minimum of water that will suffice for their satisfactory growth in the several soils. While far from completed, this work (involving many hundreds of determinations of moisture in soils) has already yielded some results which render it desirable that they should be placed before the farmers and discussed at once, in order to provide against a recurrence of avoidable injury in the future. Amount of water required by crops. — It is not very generally understood how large an amount of water is required for the production even of fair crops; for the maximum of possible product is rarely obtained on the large scale, because it is not often that all conditions are at their best at anyone time and locality. But from numerous observations, made both in Europe and in the Eastern United States, it has been found that from 300 to over 500 tons of water are on the average required to produce one ton of dry vegetable matter. In Wisconsin, King found that a two- ton crop of oat hay required over one thousand tons of water per acre, equal to about nine inches of rainfall. The average rate for field crops at large is given by European observers at 325 times the weight of dry matter produced, being at the rate of about three inches of rainfall actually evaporated through the plant. These data should enable us to estimate the adequacy of the moisture contained in the soil at the beginning of the dry season to mature the crop, provided we make due allowance for any growth already made at the time, and provided also that the estimates as to the water-requirements derived from the experience of the countries of summer rains (the humid regions) hold good for the arid region also. Whether or not this can be assumed, is among the points our experiments are designed to determine. The surprisingly successful growth and bearing especially of deciduous trees, without irrigation, despite a drought of five or six months in the "Franciscan climate,"* has led to an impression that a less amount of water may suffice under arid conditions. For in the East, as many weeks of drought and intense heat would frequently suffice to destroy the crop. Probable causes of this endurance of drought. — Doubtless the main cause of this remarkable endurance is to be found in the much deeper rooting of all plants in arid climates; whereby not only a much larger bulk of moist soil is at their command, but the roots are withdrawn from the injurious effects of the hot, dry surface and air. This deeper range of the roots is not the result of foresight on the part of the plant. It could not occur on Eastern soils, because of the intervention, in the great majority of cases, of difficultly pene- trable subsoils; from which, moreover, plants could draw but little nourishment on account of their "rawness." In the arid region, as a rule, subsoils in the Eastern sense do not exist; the soil mass is practically the same for several feet, and in the prevalent soils is very readily penetrable to great depths. This, summarily speaking, is due to the slight formation of clay, and the rarity of heavy rains, in the arid region. And this easy penetrability of the soil implies, moreover, that being well aerated, the depths of the soil are not "raw," as in the East; and therefore that the "subsoil," such as it is, may fearlessly be turned up as deeply as the farmer is willing to go with the plow, without danger of injuring the next season's crop, in all lands that are well drained; as, by reason of their depth and perviousness, is the case with most California soils. The accompanying plate illustrates from nature the deep penetration of a peach root developing in a normally deep, well aerated "bench" soil, in a manner quite impossible to the same root when growing in land underlaid, as are most Eastern ones, by a subsoil which either is too dense or too wet to be penetrated and utilized by the tree. A glance at the figures suffices to show that while a root system like plate 1, a typical Eastern tree root (as given by Thomas' Fruit Cul- turist, page 82) will stand in absolute need of frequent rains or irrigation to sustain its vitality, such a one as plate 2 may brave prolonged drought with impunity, being independent of surface con- ditions, and able to perform all its functions out of reach of stress from lack of moisture. t It is equally clear that it is to the farmer's *This name has been felicitously applied by Powell to the climate of middle and southern California, which is characterized by the concentration of rains within a winter which is mild enough to constitute a growing season, while the summer is practically rainless. tThe moisture determination under this tree gave, to the depth of eight feet, an aggregate amount of water of 1,058 tons per acre. Plate 2. PRUNE TREE ON PEACH ROOT. NILES, CAL. interest to favor, to the utmost, this deep penetration of the roots, both in the preparation and tillage of the ground, and in the use of — ^-^AJ^yfitfi Plate 1. ROOT OF AN EASTERN FRUIT TREE. irrigation water. For if the latter is used too frequently or too abundantly, the salutary habit of deep rooting will be abandoned by the plant, and it will, as in the East, be dependent upon frequent rain or irrigation; and also, owing to the small bulk of soil upon which it can draw for its nourishment, upon frequent and abundant fertilization. Eastern immigrants as well as a large proportion of California farmers do not realize the privilege they possess of having a triple and quadruple acreage of arable soil under their feet, over and above the area for which their deeds call; and they tenaciously continue to adhere to precautions and practices which, however salutary and necessary in the region of summer rains, do not apply to this climate. The shallow plowing so persistently practiced results in the formation of a " plowsole" that plays the part of the Eastern subsoil in preventing root penetration; limiting their range for moisture and plant food, and thus naturally causing crops to succumb to a slight stress of season which ought to have passed without injury, had the natural conditions been taken into proper consideration. Roots follow moisture. — Very striking examples of deep rooting as the result of vertical moisture penetration can be observed in some of our native trees, which, while naturally at home on moist ground, are nevertheless sometimes found forming luxuriant clumps on the slopes and even summits of our coast ranges and foothills. If we examine the ground where this occurs in the case of California laurel, we will generally find that the soil in which they grow is underlaid by slate or shale standing on edge, into the crevices of which the roots penetrate, wedging them open; while themselves flattening out, and thus pene- trating to moisture at considerable depths. The same may be observed in the case of the erect "bedrock" or foothill slates of the Sierra, on which native as well as fruit trees flourish in very shallow soils, some- times reaching permanent moisture at the depth of ten or more feet below the surface. It can readily be observed during rains that there is comparatively little run- off from the surface of these lands underlaid by vertical shales. 8 On the same principle, the grape vines which bear some of the choicest raisins of Malaga on the arid coastward slopes, are made to supply themselves with moisture, without irrigation, by opening around them large, funnel-shaped pits, which remain open in winter so as to catch the rain, causing it to penetrate downward along the tap-root of the vine, in clay shale quite similar to that of the California Coast Ranges, and like this latter, almost vertically on edge. Yet on these same slopes scarcely any natural vegetation now finds a foothold. Similarly the "ryots" of parts of India water their crops by applying to each plant immediately around the stem such scanty measure of the precious fluid as they have taken from wells, often of considerable depth, which form their only source of water-supply. Perhaps in imitation of these, an industrious farmer has practiced a similar system on the high benches of Kern River, and has successfully grown excellent fruit for years, on land that originally would grow nothing but cactus. Sub-irrigation from pipes has been applied in a similar manner. The principle flowing from the above is simply that the most economical mode of using irrigation water is to put it "where it will do the most good," close to the stem of the plant or trunk of the tree, and let it soak downward so as to form a moist path for the roots to follow to the greatest possible depth. It is this deep penetration to natural moisture, as a matter of fact, which enables the small quantities sup- plied to produce such marked effects. Basin irrigation. — It will be noticed that this principle is practically the same as that of the basin irrigation of orchards, which was originally largely practiced in California, but has now been mostly abandoned for furrow irrigation . The latter has been almost univer- sally adopted, partly because it requires a great deal less hand-labor, partly under the impression that the whole of the soil of the orchard is thus most thoroughly utilized; partly also because of the injurious effect upon trees produced at times by basin irrigation. The explanation of such injurious effects is, essentially, that cold irrigation water depresses too much the temperature of the earth immediately around the roots, and thus hinders active vegetation to an injurious extent, sometimes so as to bring about the dropping of the fruit. This, of course, is a very serious objection, to obviate which it might be necessary to reservoir the water so as to allow it to warm before being applied to the trees. In furrow irrigation the amount of soil soaked with the water is so great that the latter is soon effectually warmed up, besides not coming in contact too intimately with the main roots of the tree; along which the water soaks very readily when applied to the trunk, thus affecting their temperature much 9 more directly. It is for the fruit-grower to determine which consider- ation should prevail in a given case. If the water-supply be scant and warm, the most effectual use that can be made of it is to apply it immediately around the trunk of the tree, in a circular trench dug for the purpose. When, on the contrary, irrigation water is abundant and its temperature low, it will be preferable to practice furrow irri- gation, or possibly even flooding. As to the more complete use of the soil under the latter two methods, it must be remembered that while this is the case in a horizontal direction, yet unless irrigation is practiced rather sparingly under the furrow system, it may easily happen that the gain made horizontally is more than offset by a corresponding loss in the vertical penetration of the root- system. This is amply apparent in some of the irrigated orange groves of Southern California, where the fine roots of the trees fill the surface soil as do the roots of maize in a corn field of the Mississippi States; so that the plow can hardly be run without turning them up and under. In these same orchards it will be observed, in digging down, that at a depth of a few feet the soil is too water-soaked to permit of the proper exercise of the root functions, and that the roots existing there are either inactive or diseased. That in such cases abundant irrigation and abundant fertilization alone can maintain an orchard in bearing condition, is a matter of course; and there can be no question that a great deal of the constant cry for the fertilization of orchards in the irrigated sections is due quite as much to the shallowness of rooting induced by over- irrigation, as to any really necessary exhaustion of the land. When the roots are induced to come to and remain at the surface, within a surface layer of eighteen to twenty inches, it naturally becomes necessary to feed these roots abundantly, both with moisture and with plant food. This has as naturally led to an over-estimate of the requirements of the trees in both respects. Had deep rooting been encouraged at first, instead of over-stimulating the growth by surface fertilization and frequent irrigation, some delay in bearing would have been amply compensated for by less of current outlay for fertilizers, and less liability to injury from frequently unavoidable delay, or from inadequacy, of irrigation. Conservation of Soil Moisture. — Alongside of economy in the use of irrigation water, the conservation of the moisture imparted to the soil either by rains or irrigation is most important; Critically so where irrigation is unavailable. Utilization of winter rains, and winter irrigation. — However strong is the popular demand for storage of the winter rainfall and flood waters, too many do not appreciate the importance of the storage they can command without the use of reservoirs, within their own soil 10 mass. While there is a well-grounded objection to subjecting plowed land to the leaching action of the abundant rains in the humid region, no such objection holds in the case of lands lying within the limits of 20 to 25 inches of annual rainfall. Here the absorption of the winter rains should be favored to the utmost, for the run-off is mostly a dead loss. Fall plowing wherever the land is not naturally adaquately absorbent, and is not thereby rendered liable to washing away, is a very effectual mode of utilizing the winter's moisture to the utmost, so as to bring about the junction of the season's moisture with that of the previous season, which is generally considered as being a condition precedent for crop production in dry years. The same of course holds true of winter irrigation; the frequent omission of which in presence of a plentiful water supply at that season is a prolific cause of avoidable crop failures. Moistening the ground to a considerable depth by winter irrigation is a very effective mode of promoting deep rooting, and will thus stand in lieu of later irrigations, which, being more scant, tend to keep the roots near the surface. Knowledge of the subsoil. — It cannot be too strongly insisted upon that in our arid climate farmers should make themselves most thoroughly acquainted with their subsoil down the depth of at least four, but preferably six or eight feet. This knowledge, important enough in the East, is doubly so here, since all root functions are and must be carried on at much greater depths. It is hardly excusable that a business man calling himself a farmer should omit the most elementary precaution of examining his subsoil before planting orchard or vine- yard, and should at the end of five years find his trees a dead loss in consequence of an unsuitable subsoil. Similarly, no irrigator should be ignorant of the time or amount of water it takes to wet his soil to a certain depth. We have lately seen a whole community suffering from the visible decline of the thrift of its fruit trees, which occurred despite what was considered abundant irrigation; i.e., allowing the water to run for a given length of time, deemed to be sufficient. Yet on being called in to investigate the causes of the trouble, the station staff found that the irrigation water had failed to penetrate during the allotted time to any beneficial extent, so that the trees were, in the main, suffering from lack of moisture — a fact that could have been verified by any one of the owners concerned, by simply boring or digging a hole or two. But no one had thought of doing so, and all kinds of mysterious causes were conjectured to be at work in the suffering orchards. A definite knowledge of the rapidity with which irrigation water penetrates downward and sideways in his soil should form a part of the mental equipment of every irrigator, particularly in arranging his head ditches. For in sandy lands it may easily happen 11 that when these are too far apart, the water near the head ditch is already wasting into the country drainage at the depth of ten or twelve feet, before any has reached the end of the furrows, or has wetted the lower half adequately. Many such cases come under our observation, and such ignorance of the conditions governing one of the most important factors of success is hardly excusable in any one. Nor is the quality of the water used indifferent in this connection; for waters containing alkali will fail to penetrate the soil as quickly as would ordinary stream waters. Preventing evaporation. — But supposing the moisture to have reached the depths of the soil, whether from rains or from irrigation, it is essential that proper means be employed for retaining it in the land, and especially to prevent evaporation. That this is best accom- plished by a mulch on the surface, and that the best mulch for the purpose, which need not be hauled on or off and is always ready, is a surface layer of loose, well- tilled soil, is now pretty well understood by all. But the extent to which the presence or absence of such a non- evaporating layer influences plant growth and fruit production in a critical time, is not so fully appreciated. Plates 3 and 4, at end of Bulletin, give an illustrative example of trees and fruit grown this season on adjacent fields, with only a lane between, the soil and all natural conditions being absolutely identical; the only difference being the presence and absence of cultivation. In the present case the cultiva- tion was omitted on principle by one owner, who considered cultivation superfluous on the loose, generous soil of Alameda creek; while his neighbor, across the way, held the opposite belief, and had this season cultivated to an extra depth to conserve moisture. The cultural results are sufficiently shown in the plates and need no comment, although it may be of interest to mention that the year's growth on the one hand was over three feet, on the other barely three inches. The effect on the fruit is shown in piate 4. The determination of the moisture held by the soil in July to the depth of six feet gave the following results : Depth in Soil. Cultivated. Uncultivated. Percent, j ^ Percent.] Tj-P- First Foot Second Foot ... . 6.4 128 5.8 116 4.3 86 4.4 8S Third Foot 6.4 128 3.9 78 Fourth Foot Fifth Foot Sixth Foot 6.5 130 5.1 102 6.7 134 3.4 68 6.0 120 4.5 90 Total for six feet... 6.3 756 4.2 512 12 The difference of 244 tons per acre of ground shown by the analyses is quite sufficient, according to the data given at the beginning of this bulletin, to account for the observed difference in the cultural result. The cause of this difference was that in the uncultivated field there was a compacted surface layer several inches in thickness, which forcibly abstracted the moisture from the substrata and evaporated it from its surface; while the loose surface soil on the cultivated ground was unable to take any moisture from the denser subsoil. This is well illustrated by the familiar fact that while a dry . brick will suck a wet sponge dry, a dry sponge (corresponding to the loose surface soil) is unable to take any water from a wet brick. Besides, the tilled surface soil forms a non-conducting layer protecting the subsoil from the sun's heat and the dryness of the air. In the East, where this principle is well understood, it is con- sidered that a surface layer three inches in thickness is sufficient to afford effective protection. But what is adequate in the region of summer rains is quite insufficient in California and in the arid region generally. It takes fully twice the thickness mentioned, and preferably more, to afford protection against the drought and heat lasting five or six months at a stretch. Here again we find an important point in which our practice must differ from that of the East and of the Old World. The beneficial effects of summer fallow in California are assuredly due quite as much to the conservation of moisture brought about by the tilled surface layer, as by the weathering of the soil to which the efficacy of the fallow is commonly ascribed. Witness the fact that weeds come up freely on summer-fallow as late as August, when unplowed land is as bare as a barn floor. Similarly on our mostly new and unexhausted lands, the bad effects of weed growth are doubtless due fully as much to the waste of moisture going on through their leaves as to the competition with the crop in plant food. Hence all good orchardists are very careful about keeping their ground clean in summer; but it must not be forgotten that by doing so they quickly deplete their lands of vegetable matter, which requires systematic replacement if production is to continue normally. Yet of the two evils, the loss of moisture is more to be dreaded, and very generally in practice the more difficult to remedy. , * «*- $6 *- *-' * 4S <"t h- ™*