CHAPTER XII.. The Water of Soils.—_continued._

THE WATER OF SOILS.—Continued.

SURFACE, HYDROSTATIC AND GROUND WATER; PERCOLATION.

Since all the water of soils and plants is directly or indirectly derived from the rainfall (including therein snow and hail), some general points regarding this factor require first consideration. While it is not the object of this work to discuss climatology in detail, yet the times of the year and the manner in which precipitation comes, acts upon and is disposed of in the soil under different climatic conditions, must of necessity form an essential part of its subject matter.

Amount of rainfall.—The rain falling in the course of a year is usually stated in the form of “inches” (or centimeters), implying the height of the water column that would be shown at the end of the year had it all been allowed to accumulate; or, the sum of all the successive rains (including snow) observed during the year. Since this amount ranges all the way from nothing, or a mere fraction of an inch (as in portions of the Andes, and of the great African and Asian deserts) to as much as 600 inches or fifty feet (Cherapundji in eastern India), the adaptation of agricultural practice to the maintenance of the proper moisture-supply to crops is largely a local question, oftentimes of not inconsiderable difficulty. This is especially the case where torrential rains, yielding several inches of rain in a few hours, alternate with light, soaking rainfall, as is very commonly the case in the interior of continents, and more especially in the United States east of the Rocky Mountains. Westward of the same the rainfall decreases so rapidly that at or about the one-hundredth meridian (the longitude of Bismark and Pierre, Dakota, and Dodge City, Kansas) we already reach the annual average of 20 inches, which is commonly assumed to be the limit below which crops cannot safely be grown without irrigation. The “cloudbursts” occasionally occurring within these limits are usually confined to mountainous regions, and the water they pour down on the dry soil is rarely of any direct benefit to agriculture; hence they cannot be properly counted in the general estimate of the effective rainfall. A region of high rainfall (up to 100 inches and over), however, extends along the Pacific coast from northern California through western Oregon and Washington across British Columbia to Alaska, to seaward of the Sierra Nevada, Cascade, and Alaskan coast ranges.

In the country east of the Mississippi river, the average annual rainfall ranges from 30 inches in the region of the Great Lakes, and 45 to 50 inches on the north Atlantic coast, to 60 inches in Louisiana and up to eighty in southern Florida. The average of the Mississippi Valley and Atlantic coast States is usually stated at about 45 inches, which is distributed more or less evenly throughout the year, excepting usually from six to eight weeks of more scanty precipitation in the latter part of August and in September—the “Indian summer” season; so that the winter is the season of greatest total rainfall.

Natural disposition of the Rain Water.—The rainfall is naturally first disposed of in two ways, viz., a portion which is absorbed by the soil, and another which is at once shed from the surface and constitutes the “surface runoff.” The portion absorbed into the soil is subsequently disposed of either by soakage downward into the subdrainage and through springs and seepage into the streams and rivers; or by evaporation. The latter again occurs in two different ways, viz., from the soil-surface itself, or through the roots and leaves of plants. The importance of each of these modes is sufficiently great to entitle each to detailed consideration.

The Surface Runoff.—This portion of the disposal of rain may range all the way from nothing to almost totality, according to the nature of the soil and the condition of its surface. Sandy soils, especially when coarse, may absorb instantly even a very heavy rainfall. Heavy clay soils when dry will at first also absorb quickly quite a heavy precipitation; but as the beating of the raindrops compacts the surface, the absorption quickly slows down, so that heavy downpours of brief duration, while wetting thoroughly into a plastic mass the first two or three inches of a clay soil, may leave all beneath dry, to be very gradually moistened by the slow downward percolation against the resistance of the air in the soil; while the greater part of the later portion of the shower will drain off the surface in muddy runlets. Certain soils classed as loams, having the property of crusting readily by rain followed by sunshine (see chapter 7, p. 111), in heavy showers behave hardly better than strong clay soils; shedding the water until the soaked crust gives way, and is carried off in muddy streamlets. Then begins the cutting-away of the soil that, in portions of the Cotton States, as well as north of the Ohio river, has been the cause of extensive devastation of once fruitful culture lands, the site of which is now marked by “red washes” and gullies but too familiar to the eye in many regions, especially of the southern United States.

The quiet seepage from the banks and beds of streams plays a much more important part in the increase of volume of flow than is commonly supposed, because unperceived save by measurement of the tributaries and comparison with the main streams. This is especially true of the drainage in the arid region, where the deep and pervious soils favor diffuse seepage as against definite spring flow.

Toumey (Yearbook U. S. Dep’t Agr. 1903) states that in the San Bernardino mountains in southern California, the first rainfall (in December) was absorbed to the extent of 95% in forested areas, against only 60% in the non-forested; but that later, after the soil had been partially saturated, 60% only was absorbed in the forested land, against 5% in the non-forested. While it is generally admitted that forests diminish the runoff, Rafter (Relation of Rainfall to Runoff, U. S. Geol. Survey Paper, No. 80, p. 53) contends that in New York State the reverse is true.

Washing-away and Gullying in the Cotton States.—Nowhere perhaps have these effects been so severely felt as in portions of northwestern and central Mississippi, and this case is so instructive as to deserve a more detailed description. In the regions in question the soil stratum consists of a yellow or brownish loam from three to seven feet in original thickness, constituting a very desirable class of gently rolling uplands, which at one time claimed to be the best cotton-growing portion of the State. It was originally covered with an open forest of oaks, with an abundant growth of grasses that afforded excellent pasture to deer and cattle; a natural park gay with flowers during most of the season.

When these lands were taken into cultivation little or no attention was paid to the direction of the furrows and rows of corn and cotton; most commonly the plowing was done “up-hill and down,” so that the “dead-furrow” afforded a ready opportunity for the formation of washes cutting into the subsoil, during the torrential rains sometimes falling during the summers. Even when filled with soil by plowing, these washes would frequently re-open during rains, shedding the soil in a muddy flood upon the lower lands. The washing-away of the surface soil, thus brought about, of course diminished the production of the higher lands, which were then commonly “turned out” and left without cultivation or care of any kind. The crusted surface shed the rain water into the old furrows, and the latter were quickly deepened and widened into gullies—“red washes”—whose presence rendered any resumption of cultivation difficult. In the course of a few years the soil-stratum of brown loam was penetrated into the loose or loosely cemented sand which underlies it almost everywhere, and is very readily washed away. Soon the water, gaining yearly in volume, undercut the loam stratum so as to cause it to “cave” into gullies in huge masses, which with the sand were carried into the valleys adjacent, filling the beds of the streams so as to cause their flow to disappear under the flood of sand. As the evil progressed, large areas of uplands were denuded completely of their loam or culture stratum, leaving nothing but bare, arid sand, wholly useless for cultivation; while the valleys were little better, the native vegetation having been destroyed and only hardy weeds finding nourishment on the sandy surface.

In this manner whole sections, and in some portions of the State whole townships of the best class of uplands have been transformed into sandy wastes, hardly reclaimable by any ordinary means, and wholly changing the industrial conditions of entire counties; whose county seats even in some instances had to be changed, the old town and site having, by the same destructive agencies, literally “gone down hill.” This destruction of lands was greatly aggravated by the civil war, during which, and for some time after, large areas of lands once under cultivation were left to the mercy of the elements.

Injury in the arid regions.—In the arid regions, where the rainfall frequently comes in heavy downpours or “cloudbursts,” immense damage to pasture lands has been brought about by overstocking, in Arizona and New Mexico; involving the destruction of the natural cover of vegetation and the loosening of the surface especially by sheep; after which a heavy rainfall will carry off the surface soil, the muddy water being gathered largely in the trails made by cattle going to water. Thus gradually gullies are formed, which enlarging more and more become ravines and cut up the pasture slopes into “bad lands,” useless equally for pasture and for agriculture. California, eastern Oregon and Washington, and Montana, offer striking and lamentable examples of the same destructive agencies.

Open Range and Irrigation Farming. R. H Forbes, in Forester, Nos. 7, 9, 1902.

Deforestation.—The deforestation of hill and mountain lands has, the world over, led to similar results; causing not only the destruction of pasture and agricultural lands, but also the conversion of streams, flowing from springs and seepage all the year, into periodic torrents, flooding the lowlands during rains by the rapid running-off of the water from the bare and hard-baked mountain slopes, and then running dry within a short time, so as not even to afford drinking water to pasturing cattle in summer. Thus for half a century the unsolved problem of the “correction of the waters of the Jura mountains” was before the Swiss and French governments; and the great and costly public work involving re-forestation, deflection of torrents and filling-in of deep ravines and gullies, is not even yet nearly completed. In Spain, which in the time of the Roman occupation was largely a forest country with abundant rainfall, the same results are seen, notably in the South, in the wide, and mostly dry, sandy beds of streams once running deep and clear; and in the scarred hill-and mountain-sides, and scant vegetation of low shrubs (“chaparral”) that replace the once abundant tree growth, e. g., in Old and New Castile. Unfortunately the lessons taught by the bitter experience of the old world seem to require actual repetition in the new, before means of prevention are even thought of.

Prevention of Injury to Cultivated Lands from excessive Runoff.—The fundamental remedy for the injurious effects of excessive runoff from the land surface is, of course, to facilitate its absorption into the soil to the utmost extent possible, by deep tillage; or in cases where this is undesirable (as when in rainy climates excessive leaching of the land is feared), to so direct and control the surface drainage that its flow shall nowhere be so rapid as to carry with it any large amounts of earth, or to wash out the furrows. To this end its fall must be diminished by “circling,” i. e., plowing nearly at right angles to the slope instead of up-and-down, and on steep slopes especially also by maintaining open furrows or ditches having a gentle fall only, into which the water can shed and flow off quietly in case the furrows, left in plowing, prove insufficient to retain and shed gradually the water they cannot hold permanently. The early adoption of this simple expedient would have wholly prevented the enormous waste of fine agricultural lands referred to above.

The underdraining of lands liable to washing is a costly but highly effective means of preventing denudation; and the laying of underdrains in gullies already formed, to prevent farther deepening, is among the most obvious means of arresting farther damage. The beneficial effects of underdrainage in conserving moisture will be discussed farther on.

ABSORPTION AND MOVEMENTS OF WATER IN SOILS.

The phenomena and laws of capillary ascent of water in soils, as discussed in the preceding chapter, serve best to demonstrate the general behavior of liquid water within different soils and their several grain-sizes; because measurably independent of the physical changes that almost unavoidably accompany the percolation of water from above downward; whether such water comes in the form of rain, or irrigation, or even when applied with the utmost precautions in the laboratory. The “beating” of rains quickly compacts the surface to a certain extent, varying with the nature of the soil, its condition of more or less perfect tilth, and the degree of violence with which the rain strikes the surface. When the latter has been compacted by a previous rain and then dried, “baking” or incrusting the surface, the latter may almost wholly shed a rain of brief duration, which, had the surface been loose, would have been wholly absorbed, materially benefiting the crop. Such surface-crusting is, therefore, injurious in preventing the absorption of water from above; and in addition, it serves to waste, by evaporation, the moisture contained in the underlying soil and subsoil. For the crust being of a finer (single-grain) texture than the tilled portion beneath, it will forcibly abstract from the latter, by absorption, its capillary moisture, and evaporating it at the upper surface, continue to deplete the land, to the great injury of crop growth, until destroyed by cultivation.

This effect is well illustrated by the behavior of a dry brick laid upon a wet sponge. It will quickly absorb all the liquid moisture contained in the latter, while the sponge will be wholly unable to take any moisture from a fully-soaked brick.

The flow of irrigation water produces the same compacting effect, but to a less extent; the more as, unlike rain water, irrigation water usually contains a certain amount of alkaline and earth salts, which tend to prevent the diffusion of clay and of fine sediments, and therefore the disintegration of the soil-floccules into single grains. Nevertheless, it is in some soils as necessary to cultivate after surface-irrigation as after rains, in order to prevent great waste of moisture by evaporation.

Determination of rate of percolation.—When water is allowed to soak into an air-dry soil column without sensible shock or motion, from a constant level, we obtain the nearest approach to a definite determination of the relative permeability of soils to water under the conditions usual in the arid region. A number of determinations thus made is tabulated in the diagram given below, which embodies the observations made by Mr. A. V. Stubenrauch in connection with a more extended investigation.

Rep’t Calif. Exp’t Station for 1898 to 1901, p. 165.

As these experiments were made with soils not in their field condition, but gently broken up with a rubber pestle, a standard of compactness was established by weighing the quantity which could conveniently be settled into a tube space of 100 centimeters capacity by tapping the sides and bottom of the tube, without touching the soil itself. In this way the following standards were established: For the University Adobe soil, 140 grams; for the Yuba loam soil, 110 grams; for the Stanislaus sandy soil, 170 grams. Tubes 1½ inches wide were used, and the soils were introduced in bulk, inside of a cylinder of stiff paper upon which previously to rolling it up the soils had been thoroughly mixed. After introducing the soil-filled paper roll it was gently withdrawn, leaving the soil column in the tube as uniform as before; a condition almost impossible of fulfilment when the soil is introduced piece-meal. The tubes were, of course, left open at the lower end, using a wire netting to keep the soil column in, so that the air could escape freely before the descending water column.

The results thus obtained do not, of course, apply directly to the same soils undisturbed in place in the field; where, moreover, the air is confined by the wetting of the surface and thus directly opposes penetration of the water. Still, they doubtless give a correct idea of their relative permeability for water when in the tilled condition. The water level was automatically maintained at the depth of half an inch above the surface of the soil columns. Pore-spaces given are calculated from volume-weight and specific gravity.

This diagram shows plainly that there is no direct relation between the total pore-space in a soil and the facility of water-penetration. The highest pore-space, in the fine-grained alluvial loam, allows more rapid percolation than the heavy clay or adobe soil, but is greatly exceeded by the coarser sandy soil. In all it is very apparent that the downward movement slows down as the water descends, doubtless because the great friction in a longer column gradually diminishes the effect of hydrostatic pressure. It may be presumed that at a certain distance from the surface the downward movement becomes practically uniform, and independent of the pressure from above.

Summary.—Two salient points are revealed by even a cursory inspection of the preceding diagram, viz.:

1. The downward percolation is most rapid in the same soils in which the capillary ascent is quickest, that is, in the coarse, sandy soil.

2. The rapidity of percolation decreases materially as the wetted soil column increases in length.

The first point is readily foreseen and needs no comment. As regards the second, it results from the fact that as the wetted column lengthens the frictional resistance increasingly counteracts the effects of the hydrostatic pressure from above, until the water’s descent becomes but little more rapid than would be its lateral diffusion, or its ascent at the end of a similar column supplied by capillary rise from below. In both cases the frictional resistance has so far counteracted the effect of gravity that the capillary coefficients of the soil-material become the controlling factors of the water movement.

Influence of Variety of Grain-sizes.—King (Physics of Agriculture, pp. 159, 160), compared the rapidity of the percolation of water through definitely graded pure sands on the one hand, and a sandy loam and a clay soil on the other. The materials were arranged in 8-foot columns fully saturated with water at the outset, and then allowed to drain freely. The following abridged table shows the tenor of his results:

TABLE SHOWING RELATIVE RAPIDITY OF PERCOLATION IN PURE SANDS AND SOILS, IN INCHES OF WATER DRAINED OFF. ===================+==================+==============+============ Diameter of Uniform| First | Second | Total Sand Grains. | 30 minutes. | 30 minutes. |in one hour. -------------------+------------------+--------------+------------ .475 mm. | 10.25 | 4.68 | 14.93 .155 “ | 5.67 | 4.52 | 10.19 .083 “ | 1.21 | .85 | 2.06 ===================+===========+======+===+==========+============ |First 21-23| First 10 |Second 10 | Total in Soils. | hours. | days | days | about | |following.|following.| 505 hours. -------------------+-----------+----------+----------+------------ Sandy loam | 2.64 | 5.07 | .91 | 8.62 Clay loam | 1.96 | 2.11 | .49 | 4.56 -------------------+-----------+----------+----------+------------

This table is very instructive in showing the great difference in the rapidity of percolation in materials of uniform, even-sized grains, as compared with such as contain particles of many different sizes, in which the interspaces of the larger ones are filled more or less closely by the smaller sizes of particles (see chapter 7, p. 109). While it is true that we have no definite physical analysis of the soils here used, the differences are so great as to be sufficiently striking. Compare the percolation through the sand of .155 mm. uniform grain-size (a fine sand), during the first half hour, with that through the sandy loam during the first 21 hours. Twice as much water has passed from the sand as from the soil in one forty-second part of the time. Comparing similarly the finest sand, .083 mm. in diameter, with the clay loam, we find the difference to be as one to seventy-three. It is thus evident that but for the variously assorted sizes of the soil-particles, water would not be held long enough to supply plant growth.

Percolation in Natural Soils.—In artificial percolation experiments, as well as during a fall of rain, the gradual settling of the fully wetted soil-column produces a compacting of that portion of the mass, that increasingly impedes the downward penetration. The effect of this under natural conditions is readily seen in the fact that after the first, rapid absorption of falling rain by the soil when in good tilth, there is a gradual slackening of the process even when the rain is fine and slow, causing a perceptible increase of the runoff until, should the rain continue for some time, the absorption becomes so slow as to cause all, or nearly all the water to drain off the surface. The soil is then called “saturated,” having really arrived at that point right at the surface, and to a depth varying according to the duration and amount of rain, and the natural perviousness of the land.

When the rain ceases, the visible saturation of the surface usually soon disappears in cultivated soils, and the zone of saturation begins to descend. The progress of this descent may be very strikingly observed in a series of holes (post-holes) dug or bored across a ridge; as indicated in the subjoined schematic diagram, in which the successive dotted lines represent the levels of the descending “bottom water” at successive intervals, as derived from the observation of the water levels in the several holes.

The exact record of these observations was unfortunately destroyed by fire; the soil was a heavy clay, and it took ten days before the water disappeared from the lowest hole.

It will be seen that while at first the upper surface of the zone of saturation coincides with the surface of the ground, in falling it descends most rapidly on the highest ground, while at the lower levels the holes may remain full or overflowing; the drainage taking place sideways as well as vertically. The curved surface connecting the levels in the several holes gradually flattens, rapidly at first, then progressively more slowly; the water disappearing entirely, first from the holes lying highest, then successively from those at lower levels; those located in valleys or drainage channels remaining full until surface-water ceases to run in such channels. But even after liquid water has ceased to be visible in the holes, the descent of the water continues within that portion of the soil, tending (unless more rain should come before that time), to establish the condition of equilibrium as existing in the soil columns shown in the diagram on p. 205, chapt. 11; such as results from the capillary ascent of water from below, but having above it a column of soil of minimum water-content, of greater or less height according to the length of time allowed for the water to descend. This is a very common state of things during the long summer droughts in the arid region, when neither rain nor irrigation has added to the water supply in the soil for many months, and yet ordinary deciduous fruit trees mature their normal crops. Frequently, however, before this state of equilibrium is reached, evaporation from the surface so draws upon the water supply within the first few feet, as to reduce the soil to undersaturation at the lowest point of the descending column, so stopping farther descent and soon reversing the direction of the movement. The latter is the usual condition of scantily irrigated ground.

Ground or Bottom Water, Water Table.—During and after long-continued and abundant rains, the zone of supersaturation continues to descend until it finally reaches a more or less permanent level, varying somewhat from season to season, but on the whole usually definable for each region and locality; being the depth to which wells must be sunk in order to secure a fairly permanent water supply. This is called the water table, ground water, bottom water, or “first water.” The proportion of the rainfall that reaches the permanent water level varies enormously, of course, in different soils and at different times. With brief and moderate rains, in soils of high water-holding power and slow percolation, it may never reach the bottom-water level; this is very commonly the case in the arid regions. Where, as in the humid regions, rains are frequent or much prolonged, one half and even more may finally reach the permanent level; runoff and evaporation disposing of the balance.

In contradistinction to other levels or “streams” of water which may usually be found lower down, separated from the first water by some impervious stratum of clay, hardpan or rock, and very commonly under sufficient pressure to rise somewhat higher than the point at which it was struck, owing to connection with higher-lying sources of supply. When such pressure is sufficient to cause an overflow at the surface of the ground, we have “Artesian” water as commonly understood.

Lysimeters.—For the determination of the amount of water percolating to given depths, water-tight receptacles called lysimeters are usually employed. The best way to establish such receptacles is to isolate a unit-area (usually a square meter) by digging all around it to the depth desired, then surrounding it with a metal sheet soldered tightly at the cut edges, and finally driving in a sharp-edged, stiff metal sheet so as to form the bottom when soldered to the upright walls; leaving on one side an outlet for the percolating water, which is then received into a measuring receptacle somewhat like a rain gauge.

Hall (The Soil, p. 75) states that at Rothamstead, where an average rainfall of 31.3 inches is distributed rather uniformly through the season, and where the soil is a moderately clayey loam, a little less than half percolates through 20 inches of soil, and about 45% through 60 inches.

Surface of Ground Water; Variations.—The surface of the water table, however, is rarely level except in level and very uniform ground, or after long periods of drought. The undulations of its surface conform, in general, to that of the ground surface, but are less abrupt; so that the water lies nearer to the surface in low than in high ground, as is indicated in the diagram above.

King has shown, moreover, that the level of the ground water shows sensible variations due to increased or diminished barometric pressure, as well as to variations of temperature in the soil, which cause the air in the pores to expand or contract to a degree sufficient to bring about variations in the flow of springs and underdrains to the extent of 8 and 15% respectively, in conformity with the daily changes of temperature and pressure.

Physics of Agriculture, p. 270.

The Depth of the Ground Water most Favorable to Crops cannot be stated in a general manner, as it depends materially upon the nature of the crop, its root habit, and the nature of the soil. As has already been said, the amount of soil-moisture most favorable to plant growth is about half of the maximum it can hold; and this condition, as is shown in the table in chapter 11, p. 208, is reached about the middle of the maximum height to which the water can rise by capillarity from the water level. Below this point the access of air to the roots becomes too limited, and in case of continuous rains the root-ends would soon begin to suffer from want of aeration. On “sub-irrigated” land, therefore, which is generally considered desirable, crops must be carefully selected with respect to their root habits. Thus while alfalfa needs considerable moisture to do its best, its deep-rooting habit renders it undesirable when the ground water is at less than five feet depth; but red clover may be grown even with the water level at three feet.

In clayey soils root-penetration is always less than in sandy lands; and although in the former the capillary ascent of water goes higher than in the latter, yet its movement in clays is so much slower than in sandy materials that unless water is within comparatively easy reach, the plants may suffer from drought. Experience has long ago fixed the proper depth at which to lay underdrains limiting the rise of bottom water, at from three to four and a half or even five feet in clay soils; greater depths are only exceptionally used, partly because the laying of drains then becomes too expensive.

A mass of four feet of clay-loam soil is commonly, then, considered as sufficient to supply the needs of a crop; it being understood that in the humid region at least, such soils are usually the richest in plant food, so that a deeper range of the root system is not called for. It is quite otherwise in the sandy soils of the same region, which being usually poor in plant food, must afford a deeper penetration in order that an adequate amount of the same shall be within reach of the roots. Sandy lands, then, should be deep in order to repay cultivation; and fortunately this is usually the case. But when this is otherwise; when for instance a sandy soil four feet in depth is underlaid by impervious clay, underdrains may be quite as necessary as in the clay lands; since the depth of actually available soil mass would otherwise be reduced to two or two and a half feet only, by the water stagnating on the clay surface and rising from 16 to 24 inches in the sand. Soils thus shallowed can with difficulty be maintained in good productive condition even by the most energetic fertilization.

Moisture supplied by tap roots.—In most cases, sandy lands do not require underdraining; and in them, root-penetration may reach to extraordinary depths in the case of certain plants, especially when taprooted. Thus the roots of alfalfa (lucern) are very commonly found to reach depths of twenty to twenty-five feet, and even sixty feet has been credibly reported for the same plant in the arid region. It is obvious that for such plants, a high level of bottom water is wholly undesirable, since they are enabled to obtain their moisture supply from great depths, and can thus utilize for their nutrition much larger soil-masses than can shallow-rooted plants.

Reserve of Capillary Water.—It must be remembered that it is not only, nor usually, the bottom water that supplies moisture to plant growth; for all soils of proper texture for cultivation retain within them a certain amount of capillary moisture after the ground water has reached its permanent level (see this chap. p. 226), and when the tap or main roots are plentifully supplied with water, the upper and chief feeding roots draw but lightly upon the moisture within their immediate reach for the purpose of leaf evaporation. This fact can be plainly observed in the arid region, when on the advent of the summer drought, young plantlets whose tap roots have reached a certain depth continue to flourish and develop, while others practically of the same age, but slightly behind, quickly succumb, though the feeding roots of both may draw upon the same soil layer. It is especially in sandy soils that moisture is naturally thus conserved in the upper layers, because of the failure of the water to rise by capillary ascent so as to evaporate from the surface layer. It is often surprising to find a good amount of moisture in the sandy soils of desert regions at the depth of eight or ten inches, when the surface is so hot as to scorch the fingers; and this moisture continues very uniformly to great depths, probably to bottom water lying twenty or more feet below the surface, which in such materials may readily be reached by taprooted plants such as the “sagebrush” (Artemisia tridentata), the saltbushes (Atriplex) and others.

Injurious Rise of Bottom Water resulting from Irrigation.—In the deep, pervious sandy lands of the arid region, especially where the rainfall is very low and can wet the soil annually only to two or three feet depth, the substrata are sometimes found to be barely moist to depths of thirty and forty feet, and the short-lived spring vegetation carries off during its growth all the moisture supplied by the winter rains. When such lands are subjected to irrigation and the ditches carrying the water are simply dug into the natural sandy land, the thirsty soil absorbs the water greedily, so that even a considerable volume of water makes but slow progress toward the farther end of the canals. Gradually, as the rapidity of absorption decreases, the diminution of flow becomes less sensible, but still the loss thus experienced may be a very considerable percentage of the whole supply. Thus in the Great Valley of California, as well as in portions of Wyoming (Bull. 61, p. 32), the permanent loss from seepage is in the case of some extensive irrigation systems estimated at fully 50 per cent. When such lands have a considerable slope, the injury commonly ends with the loss of the water, which in many cases is again gathered and utilized at a lower level. But when the lands have but a slight slope, the drainage may become so slow as to permit of the gradual rise of the seepage water in the substrata, until finally it may come to within a few feet of, or actually to the surface.

Consequences of the Swamping of Irrigated Lands.—The injurious consequences of this swamping of the irrigated lands may readily be imagined. The first effect is usually noted in the sickening or dying-out of orchards and vineyards, consequent upon the submergence of the deeper roots, which in such lands frequently reach to from fifteen to twenty feet below the surface. But even where pre-existing plantations are not in question, the shallowing of the soil- and subsoil-strata from which the plants may draw their nourishment, constitutes a most serious injury to the cultural value of the land. It has become unsuited to deep-rooted crops; and where the natural soil, alone, would have perpetuated fertility for many years, fertilization becomes necessary within a short time. The injury becomes doubly great when, as is frequently the case, the rising bottom water brings up with it to the surface soil the alkali salts which previously were distributed throughout many feet of substrata, frequently rendering profitable cultivation impossible where formerly the most luxuriant crops were grown.

Theoretically of course it is perfectly easy to avoid or remedy these troubles. It is only necessary to render the ditches water-tight by puddling with clay, cement, or otherwise. But the heavy cost of this improvement forms a serious obstacle to its adoption by the ditch companies who are not themselves owners of land. Thus, extensive areas of lands which when first irrigated were among the most productive, have in the course of eight or ten years become almost valueless to their owners, to whom legislation thus far affords but distant promise of relief; although the case seems in equity to fall clearly within the limits of the laws governing trespass.

Permanent Injury to Certain Lands.—In cases like those alluded to the remedy usually available for higher ground-water does not always afford relief, even when otherwise available. Long-continued submergence produces in many soils effects which cannot easily, if at all, be overcome by subsequent aeration. This is most emphatically true of soils containing a large proportion of ferric hydrate in the finely divided form in which it is usually present in “red” soils.

The first effect of the stagnation of water in such lands (as already explained in a former chapter 3, p. 45) is to set up a reductive (bacterial) fermentation of the organic matter of the soil, transforming the ferric into ferrous hydrate, which in the presence of the carbonic acid simultaneously formed, becomes ferrous carbonate, readily soluble in carbonated water. That this compound is poisonous to plant growth, has been stated (chap. 3, p. 46). The carbonates of lime and magnesia are simultaneously dissolved by the same, as is also calcic phosphate, the usual form in which phosphoric acid is present in the soil. Under the influence of partial aeration from the surface, the ferrous carbonate is slowly re-transformed into ferric hydrate, aggregated in the form of spots or concretions of “bog ore” (see chapter 5, p. 66). In this process the greater part of the phosphoric acid of the soil is also abstracted from its general mass and concentrated in the bog ore (chap. 5, p. 65), in which it is wholly unavailable to vegetation, and cannot be made available while in the ground, by any known process. The soil is therefore permanently impoverished in phosphoric acid; it is also deprived of its content of ferric hydrate, and is transferred from the class of “red” to that of “white” soils, well known everywhere to be unthrifty and to require early fertilization. Not only is this true, because of their almost invariable poverty in phosphoric acid, but also usually in lime, which like the iron, if not leached out, is aggregated into concretions in the subsoil, leaving the surface soil depleted of this important ingredient. The humus, also, is either destroyed or at least “soured” at the same time.

Reduction of Sulfates.—Should such a soil contain any considerable amount of sulfates, especially in the form of gypsum or calcic (or magnesic) sulfate, the reductive process results in the formation of iron pyrite (ferric sulfid, chap. 5, p. 75); while at the same time the soil is often sufficiently impregnated with sulfuretted hydrogen as to be readily perceived by the odor, or by the blackening of a silver coin. This is very commonly the case in sea-coast marshes, where a hole made with a stick thrust into the mud will be found to give forth both carburetted and sulfuretted hydrogen, while a careful washing of the soil will reveal the presence of minute crystals of iron pyrite. Hence the need of prolonged aeration of marsh soils, effecting the peroxidation of the ferrous compounds, and the conversion of the pyrite first into ferrous sulfate, and subsequently into innocuous, yellow, insoluble ferric oxy-sulfate.

Ferruginous Lands.—The injurious effect of the swamping of ferruginous lands has been especially conspicuous in some of the irrigated rolling lands of the Sierra Foothills of California, where orchards planted in relatively low ground and in full bearing have succumbed to the poisonous effects of the ferrous carbonate formed in the subsoil, long before the water had risen so high that, had the trees been grown afterwards, they would have adapted their root system to the existing conditions and fared moderately well at least. Underdrainage of the lower lands is, of course, the only possible remedy for this state of things, although even then the root-penetration is much more restricted, and therefore natural fertility of much shorter duration, than would have been the case without the rise of the irrigation water.

It is thus clear that in the practice of irrigation, the liability of injury to the lower ground by “swamping” through the rise of the ground water should always be kept in view; that, in fact, irrigation and provision for drainage should always go hand in hand. The legal provisions facilitating the rights-of-way for irrigation ditches should be made equally cogent with respect to drainage.