CHAPTER XI.. The Water of Soils.

THE WATER OF SOILS.

HYGROSCOPIC AND CAPILLARY MOISTURE.

When it is remembered that from 65 to over 90% of the fresh substance of plants consists of water, the importance of an adequate and regular supply of the same to growing plants is readily understood. But it seems desirable, before discussing the relations of water to the soil and to plant life, to consider first the physical peculiarities which distinguish it from nearly all other substances known. That it is colorless, tasteless, inodorous, and also chemically neutral, alone constitutes a group of properties scarcely found in any other fluid. But its special adaptation to its functions in relation to vegetable and animal life are much more fundamental, as is shown in the table of its physical constants as compared with other well-known substances, given below.

PHYSICAL FACTORS OF WATER COMPARED WITH OTHER SUBSTANCES (PER UNIT WEIGHT). ================================+=================================== Capillary ascent in glass tubes | Specific Heats. of one mm. diameter. | | Water 14 mm.| Water 1.000 Alcohol 6 mm.| Ice .502 Olive oil 1 mm.| Steam .475 | Clay, Glass .180-.200 | Charcoal .241 HEAT RELATIONS. | Wood .032 Density. | Gold, Lead .032-.031 | Zinc .096 Water at 0° C. (freezing | Steel .119 pt.) .99988| Water at 4° (Maximum | Heat of fusion. density) 1.00000| Water (Ice) 80 Cal. Water at 15° C. (ordinary | Metals 5-28 “ temperature) .990 | Salts, (incl. silicates) 40-63 “ Ice at 0° (freezing pt.) .92800| --------------------------------+-----------------------------------

+=================================== | Heat of Evaporation. | | Water at 20°C. 613 Cal. | “ “ 100°C. 637 “ | Alcohol 209 “ | Spirits of Turpentine 67 “ +-----------------------------------

Summarizing the meaning of the data given in the above table with respect to organic life, we see, first, that water rises higher both in the soil and in the tissues of the plant than any other liquid. Second, that as its density decreases in cooling after a certain point is reached, it freezes at the surface instead of at the bottom, as other liquids do; and as solid water (ice) is lighter than fluid water, ice stays at the surface and is readily melted when spring comes. Third, since its temperature changes more slowly than that of any other liquid, it serves to prevent injuriously rapid changes of temperature in plants and animals as well as in soils. Its high “heat of fusion” also serves to prevent quick freezing of plant and animal tissues, so that the brief prevalence of a low temperature may be more readily borne. Finally, the large amount of heat absorbed in evaporation of water serves to keep both plants and animals cool under excessive external temperatures which would otherwise quickly destroy life.

Capillarity or Surface Tension.—In this table it will be noted, first, that water rises higher in fine (“capillary”) or hair tubes than the other fluids mentioned, which fairly represent all others. No other fluid approaches water in the height to which it will rise in either soils or plant tissues. Were its capillary factor no higher than, e. g., that of oil or alcohol, trees could not grow as tall as we find them, and the water supply from the substrata, and all the movements of water in the soil, and hence plant growth, would be similarly retarded. It is easy to verify these differences by immersing a cylinder of clay soil (or a cotton wick) in water on the one hand, and in oil or alcohol on the other. Notwithstanding the greater fluidity of alcohol as compared with water, the latter will be found to fill the porous mass much more quickly.

Excepting only the water-solutions of certain salts, among which common salt, kainit and nitrate of soda are of agricultural interest. Common salt may increase the capillary rise to the extent of more than five per cent.

The smaller the diameter of the tube, the higher will the water rise in it, and the greater will be the curvature of its upper surface, to which the rise is sensibly proportional. But in the case of liquids which do not “wet” the walls of the tube (as in that of mercury and glass), the curve (meniscus) is convex, instead of concave, and the liquid is depressed instead of rising.

It is in its relations to heat, however, that water is specially distinguished from other substances; and these differences are most vital not only to living organisms, but to the entire economy of Nature.

Density.—As regards the density or specific gravity of water (which is by common consent assumed as the unit of comparison), it will be seen from the “Density” table that whereas all other bodies contract and become more dense as they grow colder, water has its point of (fluid) “maximum density” at 4°C. (49°.2 Fahr.), and expands as it grows colder, until at 0°C. (32° Fahr.) it solidifies into ice. In so doing it departs still farther from the rule obtaining with all other bodies (excepting certain mixtures, such as type metal) and again expands so as to decrease the density from .99988 to .92800; thus causing ice to float on water at the freezing point. Hence water, unlike all other fluids, solidifies first on the surface; and but for this, the thawing of the winter’s ice, which would be formed at the bottom of rivers and lakes, would be deferred until late in summer. The expansion of water in freezing is forcibly illustrated in the bursting of water pipes and pitchers in winter; in the soil, the ice forming in the interstices serves to loosen the compacted land and give it better tilth for the ensuing season.

Specific Heat.—Considering next, the column showing the “specific heat” of water as compared with other substances, we see that it exceeds all other known bodies in the amount of heat required to change its temperature; hence again, its heat capacity is taken as the unit to which all others are compared. The figures given in the table show that even ice and steam require for equal weights only about half as much heat (or burning of fuel) to change their temperature (e. g., 1 degree) as would liquid water. But earthy matters, such as clay or soil and glass, require only one-fifth as much heat for a similar change; charcoal only about one-fourth as much. But vegetable matter as represented by wood on the one hand, and gold and lead on the other, require only about one-thirtieth as much heat as an equal weight of water; zinc about one-tenth as much, steel somewhat more.

It is thus plain that masses of water act powerfully, more than any other substance, as moderators of changes of temperature by their mere presence. The body of an animal or plant is protected against violent changes by the presence of from 60% to 90% of liquid water, the temperature of which can only be raised or lowered slowly; and the presence of the sea tempers the climates of coasts and islands as compared with the heat or cold occurring in the interior of the continents.

Ice.—Again, it is shown in the table that the heat required to melt ice is greater than in the case of any other substance, especially the metals; which when once heated to the fusing point, require only a very little more heat to become liquid. The fusion of salts (including silicate rocks) requires more heat than does that of the pure metals.

Vaporization.—In the amount of heat required for its vaporization water is also especially pre-eminent, and potent in its influence upon organic life. The table shows that the evaporation of water requires six hundred heat units as compared with alcohol, requiring only two hundred; while spirits of turpentine, the representative of a large proportion of vegetable fluids, needs but sixty-seven.

A heat unit, or “calorie,” is the amount of heat required to raise the temperature of a unit-weight (pound, kilogram, or gram) of water one thermometric degree. According to the unit-weight and thermometric scale used, the figures will vary, but in this text the basis is understood to be kilograms and the centigrade scale.

The practical result is that evaporation of water from the surface of animals and the leaves of plants, is exceedingly effective in preventing excessive rise of temperature, the heat of the sun and air being spent in evaporating the perspiration of animals and plants before an injurious rise of temperature, such as would cause sunstroke in animals, and wilting or withering in plants, can occur. But since evaporation is most rapid in dry air, it follows that the cooling effect will be the greater in the arid regions than in the humid. In the latter, therefore, sunstroke is much more frequent than in the fervid regions of the arid west, even though the temperature in the latter may be higher by twenty or twenty-five degrees Fahrenheit. White men who would soon succumb if they attempted to work in the sun in Mississippi or Louisiana when the thermometer stands at 95°F. will experience no inconvenience under the same conditions in the dry atmosphere of the Great Valley of California.

Solvent Power.—To the exceptional properties of water discussed above, should be added another hardly less important one, viz., that of being an almost universal solvent especially of mineral matters, including even those which, like quartz, appear to be most insoluble and refractory (see chapt. 3). The water of the soil is thus enabled to convey to the roots of plants, in solution, all kinds of plant food contained in the soil. It should be noted that distilled (hence also rain-) water is a more powerful solvent, e. g., of glass, than ordinary waters containing mineral matter, and even free acids.

Practically, plants take up all their water supply from the soil in the liquid form; and hence the soil-conditions with respect to this supply are of the most vital importance to plant growth. The most abundant supply of mineral plant food may be wholly useless, unless the physical conditions of adequate soil-moisture, access of air, and warmth, are fulfilled at the same time. On the other hand, comparatively few plants are adapted to healthy growth in soils saturated with water, or in water itself; and but few among these are of special interest from the agricultural standpoint.

Water-requirements of Growing Plants.—The amount of water contained in any plant at one time, however large, is but a small proportion of what is necessary to carry it through its full development. When we measure the amount of water actually evaporated through the plant in the course of its normal growth, we find it to be several hundred times the quantity of dry vegetable substance produced; varying according to the extent and structure of the leaf-surface, the number and size of the breathing pores (stomata) of the leaves, and the climatic conditions (including specially the duration of active vegetation, and temperature during the same), from 225 to as much as 912 times the weight of the mature, dry plant.

The following are extreme figures for water consumption of different plants as reported by different observers, viz., Lawes and Gilbert in England, Hellriegel in northern Germany, Wollny in Southern Germany (Munich), and King in Wisconsin: Wheat, 225 to 359; barley, 262 to 774; oats, 402 to 665; red clover, 249 to 453; peas, 235 to 447; mustard and rape, 845 to 912 respectively; the latter figure being the maximum thus far reported. The highest figures given are throughout very nearly those of Wollny, working in the very rainy climate of Munich.

Evaporation from Plants in Different Climates.—It might be expected that in countries where the air is usually moist, the evaporation will, other things being equal, be less than where it is commonly far below the point of saturation. But the “guardian cells” (stomata) of the leaf pores possess the power of regulating, to a certain extent, the evaporation from the leaf-surface in accordance with temporarily prevailing conditions, so as to allow free evaporation in moist air, but to prevent the wilting and drying-up of the leaf in hot and dry air, save in extreme cases. Moreover, plants adapted to arid conditions are usually provided with additional safeguards in the form of thick, non-conducting layers of surface cells, or long channels connecting the interior tissue with the breathing-pores on the surface. Often hairy, scaly or viscous coverings serve the same end. On the other hand, when the air is very moist, so as to check evaporation, water is sometimes found secreted in minute droplets around the breathing-pores of the leaves, since its ascent is a necessary condition of nutrition and development.

Relation between Evaporation and Plant-growth.—There is not in all cases any direct relation between the amount of evaporation and plant growth; but experience, as well as numerous rigorous experiments have shown that under ordinary conditions of culture, and within limits varying for different soils and crops, production is almost directly proportional to the water supply during the period of active vegetation.

On the basis of Hellriegel’s results, showing that wheat uses (in Germany) about 435 tons, or nearly four acre-inches of water in the production of one ton of dry matter, and assuming the ratio of grain to straw to be 1:1.5, King calculates the following table of probable production under different moisture conditions (Physics of Agriculture, page 140):

YIELD PER ACRE. =========+===========+===========+=========+============= Number of| Weight of | Weight of | Total | Water used. Bushels. | Grain. | Straw. | Weight. | Acre-inches. | Tons. | Tons. | Tons. | ---------+-----------+-----------+---------+------------- 15 | .45 | .675 | 1.125 | 4.498 20 | .60 | .90 | 1.500 | 5.998 25 | .75 | 1.125 | 1.875 | 7.497 30 | .90 | 1.350 | 2.250 | 8.997 35 | 1.05 | 1.575 | 2.625 | 10.495 40 | 1.20 | 1.800 | 3.000 | 12.000 ---------+-----------+-----------+---------+-------------

S. Fortier has made several series of tests to determine the actual yield of grain crops under field conditions when supplied with different amounts of water. Two of these were made at the Montana experiment station in 1902 and 1903, (see reports of these years), in large tanks placed in a field, level with the ground. The results of the last year’s experiments are shown graphically in the figure below, from which it will be seen that the yield increased quite regularly with the amount of water supplied, up to the depth of 36 inches of water.

It should be noted that in this case (and as usual) not only the quantity but the quality of the grain was greatly improved as the water-supply increased, it becoming larger and more uniform in size.

Of similar experiments made in the San Joaquin Valley, California, in 1904, Fortier says:

“In experimenting with barley last winter the natural rainfall, which amounted to 4½ inches during the period of growth, produced at the rate of nine bushels per acre, while the application of sixteen inches of water increased the yield to twenty-two bushels per acre. In the same case; of wheat, the rainfall, alone, produced straw, but no grain; four inches of additional irrigation water produced a yield at the rate of ten bushels, and sixteen inches of water increased the yield to thirty-eight bushels per acre.”

“Water and Forest,” January, 1905. “The Use of Water,” by S. Fortier.

It is thus obvious that, other things being equal and with conditions sufficiently favorable for the growth of crops, the rule as formulated above is verified in practice.

Whitney (Bulletin 22, Bureau of Soils, U.S. Dept. Agr.), has carried this rule so far as to claim that in all soils, the moisture supply is the only important factor, and that so long as this is provided for, soil fertility continues indefinitely without replacement of ingredients withdrawn. The latter conclusion is so thoroughly disproved by experience as well as experiment that it hardly requires discussion here.

Whether plants, especially cultivated ones, are capable of adapting themselves to arid conditions so as to be capable of producing satisfactory crops with less water than is actually consumed in the humid region, has not been directly determined. Such is, however, the impression produced by farming experience; and the fact that among the common weeds of arid California are mustard and rape, cited by Wollny as requiring over three times as much water as does maize for the production of one part of dry matter, lends color to the supposition that in some manner these, and probably other plants, use more water in humid than in dry climates (see this chapt. p. 212).

See Wollny’s experiments, Forsch. Agr. Phys. Vol. 20, p. 58.

It is therefore impossible to assign a definite figure for the amount of water required by vegetation at large; and even for one and the same plant, only approximations conditioned upon climatic factors can be given. We can in many cases, however, assign for one plant, or for certain groups of plants, the amounts of water producing the best results (“optimum”) and the least amount (“minimum”) compatible with a paying crop, that must be furnished during the growing season, to produce certain results. For when instead of fruiting, it is desired that the crop should produce the largest possible amount of vegetable substance, as in the case of forage crops, a larger amount of water will usually be serviceable.

Different conditions of Soil-Water.—Water may be contained in the soil in three different conditions, viz.:

1. From absorption of water vapor; Hygroscopic water.

2. Liquid water held suspended between the soil particles so as to exert no hydrostatic pressure; capillary water, or water of imbibition.

3. Liquid water seeking its level; bottom, ground or hydrostatic water.

HYGROSCOPIC WATER.

Soils artificially dried so as to deprive them of all their moisture, when exposed to moist air absorb water vapor with great energy at first; both the rapidity of absorption and the amounts absorbed, when full time is given, varying greatly with their nature. Sandy soils, broadly speaking, absorb the smallest amounts; while clayey soils, and those containing much humus, or finely divided ferric hydrate, take up the largest proportion.

The figure expressing the amount of aqueous vapor absorbed at the standard temperature of 15° Cent., is called the coefficient of moisture absorption. For one and the same substance, this coefficient rises as the grain becomes finer, the surface being correspondingly increased (see chapt. 6).

The table below indicates the effect of the three substances mentioned in increasing moisture absorption as compared with a very sandy soil from the pine woods of Mississippi, and a gray silt or “dust” soil from Washington, very fine-grained but poor both in humus and ferric hydrate. (For details of the physical composition of the Mississippi soils see table in chapt. 6, p. 93). A highly ferruginous soil from Oahu shows plainly the effect of that substance.

TABLE SHOWING INFLUENCE OF SILT, SAND, CLAY, FERRIC HYDRATE, AND HUMUS ON MOISTURE ABSORPTION. ================================+======+======+=====+========= | 248 | 79 | 238 | 230 | | | | |Miss. |Wash’n|Miss.| Miss. |Pine | Dust |White|Flatwoods |Hills | Soil.|Pipe | Clay |Sandy | |Clay.| Soil. |Loam. | | | | | | | | % | % | % | % --------------------------------+------+------+-----+--------- Hygr. Moisture | 2.48| 4.92| 9.09| 9.33 Clay | 2.94| 1.27|74.65| 25.48 Ferric Hydrate | 1.64| | .15| Humus | .55| .44| 0.00| .50 Finest Silts (.01-.0250 mm.) | 60.10| 45.04|23.15| 68.60 Sands, f. and c. (.0250-.50 mm.)| 31.20| 42.40| .20| 4.70 --------------------------------+------+------+-----+--------- ================================+===========+===========+=====+===== | 246 | | 220 | 215 | | | | | Misc. | Oahu |Miss.|Miss. |Ferruginous|Ferruginous|Marsh|Marsh | Clay | Laterite. |Muck.|Soil. | Soil. | | | | | | | | | | | | % | % | % | % --------------------------------+-----------+-----------+-----+----- Hygr. Moisture | 18.60 | 19.66 |21.00|15.40 Clay | 28.15 | ? | Tr. | Tr. Ferric Hydrate | 12.10 | 41.00 | | Humus | little | 3.33 |66.10|19.83 Finest Silts (.01-.0250 mm.) | 40.33 | } 45.66 |33.94| 8.70 Sands, f. and c. (.0250-.50 mm.)| 15.61 | } | |70.18 --------------------------------+-----------+-----------+-----+-----

It will be noted that the greater fineness of grain in the Washington dust soil induces a higher absorption of moisture than occurs in the sandy soil from Mississippi, although the latter contains more clay. Comparison of the figure for the Mississippi pipe-clay and clay soil with the ferruginous soils, from the same state and from Oahu, indicate plainly the influence of the ferric hydrate in increasing absorption; although in the latter case the clay determination was not made, because of the excess of ferric hydrate. The influence of humus is plainly shown in the case of the marsh muck and soil, neither of which contain any appreciable amount of either clay, or ferric hydrate in the finely diffused condition. The relatively slight difference in the absorptions of muck and soil is due to the only partial humification of the organic matter in the former, while in the soil the humification is sensibly complete, and the sand forming the body of the material serves to render it more loose.

These data, referring to natural materials, while not as complete as could be desired, are sufficient to prove the facts, and seem preferable to any artificially devised imitation of their kind.

Influence of Temperature, and Degree of Air-Saturation.—The amount of moisture absorbed varies materially both with the temperature, and with the degree of saturation of the air to which the soil is exposed. Schübler, Knop and other earlier observers, operating with earth exposed to air only partly saturated, and with soil layers of considerable thickness (in watch glasses), found that the absorption decreased as the temperature increased, according to a law formulated by Knop. The writer found that under the conditions established in the experiments of Knop and others, the air was not nearly saturated, so that these determinations are marred by ineliminable faults, the more as the soils used are only designated in general terms, as “garden soil,” “loam,” “peaty land,” etc., without any definite indication of their actual physical or chemical constitution. The writer therefore undertook to correlate these coefficients, determined with respect to completely saturated air, with the physical composition of certain soils, as determined by means of the methods heretofore described.

It should be understood that it is by no means easy to insure full saturation in any considerable volume of air.

It has generally been considered sufficient to cover with water the bottom of the space in which absorption was to occur. The writer found that in order to insure uniform results, it was necessary to cover the entire inner surface of the vessel with wet blotting paper, and even then to exclude carefully all circulation of air by padding the joints with such paper. When only the bottom of the box was covered, samples placed at different levels above the water surface gave discordant results. It was also observed that whenever the thickness of the soil layer exceeded about one millimeter, a long time was required for full saturation; during which inevitable changes of temperature would bring about a deposition of dew on the soil, greatly exaggerating the absorptive coefficient.

In the chamber used at the California station for soil saturation, dimensions 12 × 18 × 19 inches high, the same soil was exposed on a shelf close to the surface of the water, another midway up, a third near the lower surface of the cover; liquid water being in the bottom of the chamber, and the rest covered with wet blotters. It was found that despite these precautions, the lowest soil layer absorbed in the same time as much as ¾% more than the uppermost one.

Some of the data so obtained are given in the table of physical soil composition on page 93, chapt. 6. They have since been extensively supplemented by additional determinations, but without materially changing the coefficients approximately corresponding to the several designations accepted in farm practice. Experiments conducted by the writer have conclusively shown that Knop’s law of decrease of absorption with rise of temperature not only is not true for fully saturated air, but must be reversed; the fact being that the amount of water absorbed by the soil increases in a fully saturated atmosphere (i.e., in presence of excess of water) as the temperature rises, at least between 15 and 35 degrees Cent. Thus, fine sandy soil which at 15° absorbed 2% of moisture, took up 4% at 34°; while loam soil absorbing 7% at 15°, showed nearly 9% at 35°; an increase of 2% in each case. But in partially saturated air it was found that, as stated by Knop, the amounts absorbed steadily decrease, though not according to the law announced by him. Taking as a unit the moisture absorbed at 15°, it was found that in air three-fourths saturated, ¾ of the unit was taken up by the soil; at half saturation, nearly the proportional amount; but at one-fourth saturation the earths absorb materially more than a similar proportion, being then capable of withdrawing moisture from greatly undersaturated air. Since air thus undersaturated occurs not uncommonly in the arid regions of the world, the fact that the soil cannot be farther dried by such air of the same temperature, is of some practical significance.

The partial saturation to a definite extent was effected by means of solutions of calcium chlorid of different degrees of concentration, according to the determinations of Wüllner (Pogg. Ann.). These solutions were placed in a wide, flat dish, over which a layer of soil 1 mm. in thickness was exposed, all being covered with a bell glass lined inside with the same solution, so as to insure equal saturation.

In view of the highly variable composition of soils and of the doubtless varying hygroscopic properties of their several physical constituents, it is not to be expected that any one numerical law will hold good exactly for all kinds of lands. Mineral powders, colloidal clay, ferric hydrate, aluminic hydrate, the zeolites, humus, and other hydrates known to occur, doubtless each follow a different law in the absorption of moisture and gases; so as to modify the hygroscopic properties of the soil in accordance with their relative predominance in each case. (See table of absorption of gases, chapter 14).

Utility of Hygroscopic Moisture to Plant-growth.—The early experimenters considered the hygroscopic moisture of the soil to be of very great importance to the welfare of crops. Within the last twenty-five years much doubt has been cast upon this claim, even to the extent of stating that “the hygroscopic efficacy of soils must be definitely eliminated from among the useful properties” (Mayer’s Agriculturchemie, vol. 2, p. 131). Yet Mayer himself concedes the cogency of the experiments made by Sachs, which proved that dry soil immersed in a (probably not even fully) saturated atmosphere is capable of supplying the requirements of normal vegetation; thus explaining the obvious beneficial effects on vegetation of the summer fogs prevailing in portions of the arid region, e. g.; on the coasts of California and Chile.

E. A. Mitscherlich (Bodenkunde für Land-und Forstwirthe, p. 156 et al.) claims that all determinations of soil hygroscopicity thus far made are grossly incorrect on account of the dew liable to be condensed on the soil layer from fully saturated air, as the result of slight changes of temperature. He therefore would have all such determination made either in an air-vacuum, or over a 10% solution of sulfuric acid.

Such dew-formation, however, cannot happen to any appreciable extent under the conditions maintained in the writer’s work, viz, absorption within a thick-walled (two-inch) wooden box of the dimensions given above, and sunk in the ground in a cellar in which the temperature varies only a few tenths of a degree during 24 hours. The soil layer of one millimeter thickness being put down in the morning, the 7 hour absorption period falls at the time of slightly rising temperature, as an additional precaution against dew-deposition. Mitscherlich fails, moreover, to show that this source of error produces any wide or serious discrepancies except under such long absorption periods as he finds it necessary to use because of the great thickness of his soil layers. It is doubtful whether the limits of errors in soil sampling do not greatly exceed any of those involved in the writer’s method, and whether such accuracy as is attempted by Mitscherlich is of any practical significance.

Mayer’s experiments relied upon to prove the uselessness of hygroscopic moisture to plant growth, were carried out in flower-pots, in which it was plainly shown that the plants wilted before even the visible liquid (capillary) moisture of the earth was entirely exhausted. But this simply proves that under such artificial conditions, plants cannot withdraw moisture from the soil rapidly enough for their needs. In nature, and notably in the arid regions, the chief supply of water is received through the deep-going main roots, while the bulk of the active feeding roots of the plant may be surrounded by almost air-dry soil; under which conditions, as Henrici (Henneberg’s Journ., 1863, p. 280) has shown, slow growth and nutrition occurs even in such plants as the raspberry, a native of humid climates. But in the arid region this is the normal condition of the native vegetation through most of the rainless summer. That a higher moisture-coefficient does not necessarily imply that a larger amount of moisture can be withdrawn from the soil by the plants, is undoubtedly true in some, but not in all cases; for in soils rich in humus, the moisture is more freely shared with the roots than in non-humous, clay lands.

The higher moisture-absorption is however of the most unquestionable service in the case of the occurrence of the hot, dry winds that so frequently threaten the entire crops of some regions. In this case the soil containing the greater amount of moisture requires a much longer time to be dried, and heated up to the point of injury to the roots, than in the case of sandy soils of low absorptive power, whose store is exhausted in a few hours and then permits the surface to be heated up to the scalding point, searing the stems and root crowns. That such injury occurs much sooner in sandy lands than in well-cultivated clay soils, is a matter of common note in the arid region.

Summary.—The significance of hygroscopic moisture in connection with plant growth may then be thus summarized:

1. Soils of high hygroscopic power can withdraw from moist air enough moisture to be of material help in sustaining the life of vegetation in rainless summers, or in time of drought. It cannot, however, maintain normal growth, save in the case of some desert plants.

2. High moisture-absorption prevents the rapid and undue heating of the surface soil to the danger point, and thus often saves crops that are lost in soils of low hygroscopic power.

CAPILLARY WATER.

The liquid water held in the pores of the soil, in the form of surface films representing the curved surface seen in capillary tubes, and therefore tending to cause the water to move upwards, as well as in all other directions, until uniformity of tension is established, is of vastly higher importance to plant growth than hygroscopic moisture. It not only serves normally as the vehicle of all plant food absorbed during the growth of the usual crops, but also, as a rule, to sustain the enormous evaporation by which the plant maintains during the heat of the day, a temperature sufficiently low to permit of the proper operation of the processes of assimilation and building of cell tissue.

Comparatively few plants have roots adapted to healthy action while submerged in water, excluding them from free access of the oxygen of the air; and when such roots are formed by plants not naturally growing in water or swampy ground, they differ so far from earth-roots in their structure that when transferred to soil they usually die, normal earth-roots being gradually formed instead. Conversely, there is for all land plants a definite time-limit beyond which their roots cannot live, or at least remain healthy, in submersion. Thus grain fields will with difficulty recover from a week’s total submersion; while young rice fields will resist considerably longer. When in the resting (winter) condition vineyards will bear submergence for thirty-five and even forty days, deciduous orchards about three weeks; but when in the growing condition, injury is suffered much more quickly.

It follows that whenever the soil-pores remain completely filled with water for a length of time, there is danger to the welfare of nearly all plants commonly cultivated in the temperate zones. It is therefore important to know how much water will bring about this undesirable condition in the different kinds of soil.

To determine this point we may either employ the determination of pore space by a comparison of the density of the soil constituents (see chap. 7, p. 107) with the volume weight of the soil; or we may measure directly the amount of water required to fill the pore-space. For the latter purpose it is only necessary to measure the amount of water (conveniently flowing from a graduated pipette) which, rising slowly from below in a U-shaped tube so as to expel all the air before it, is required to fill a definite weight or volume of the soil entirely full, so as to rise to its surface. We thus ascertain the amount of empty space existing within the soil, which in the absence of water will ordinarily be filled by air.

Simple as this operation appears to be, it is found to be by no means easy to expel with certainty every small air bubble without resorting to means which would destroy the natural condition of the soil; such as boiling, or the use of the air-pump. These determinations cannot therefore lay claim to great accuracy.

In most cultivated soils, as already stated, the air-space constitutes about 25% to 50% of their volume; and this space when filled with water represents what is commonly termed their maximum water capacity or saturation point. It is of interest to know this, because it has been ascertained from experience that in order that plants may reach their best development, the capillary water present should not amount to more than 60%, or less than 40% of its maximum water-holding capacity; thus leaving about half the pore-space filled with air. This optimum, however, varies somewhat for different plants, some, like celery, being more tolerant of excess, and others being more tolerant of a deficiency of moisture, as is the e. g., egg-plant, originally a desert growth.

Capillary Ascent of Water in Soil Columns.—When a column of dry soil (e. g., contained in a glass tube closed with muslin at the lower end) is brought in contact with water, the latter is soon seen to ascend in the soil, wetting it and thus changing its color so as to permit of ready observation of its progress. At first the rise is comparatively rapid, in some cases as much as an inch in one minute; but it soon slows down and after a time ranging from a few days to many months, reaches a maximum height beyond which the liquid water will not rise. The ascent is most rapid, and stops soonest, in coarse sandy soils; it rises most slowly, but in the end considerably higher, in heavy clay soils. The most rapid continuous rise, and ultimately the highest, occurs in salty soils containing but a small proportion of clay. The maximum height of capillary rise thus far observed, viz. 10.17 feet, was noted in the case of quartz tailings from a stamp mill, ranging from .005 mm. to .016 mm. in diameter; but it took about 18 months’ time to reach this maximum. The excessively fine texture of clay opposes great frictional resistance to the movement of the water, and the same is true of the finest silts, which, like clay, remain almost indefinitely suspended in water. But it must be remembered that while pure grains of silt will in wetting remain unchanged in size, clay particles, and the clay incrusting silt grains, will on wetting swell greatly, and thus fill up the interstices, largely closing them up against the passage of water.

These facts are exemplified and graphically illustrated below.

The soils selected for this illustration, from California localities, are the following:

No. 233. Very sandy soil from near Morano, Stanislaus County. Typical of the noted wheat-growing region of the lower San Joaquin Valley, from northern Merced to Southern San Joaquin Counties; bench or plains lands. First foot.

No. 1197. Sandy alluvial soil from near the confluence of the Gila and Colorado rivers, near Yuma. Very deep, light and easily cultivated. First foot, but almost identical to 15 feet.

No. 168. Silty alluvial soil from the old alluvium of the Santa Clara River, near Santa Paula, Ventura County. Very deep, very easily tilled; a typical alluvial loam of the arid region.

No. 1697. Black adobe or clay soil, from the experiment station grounds, Berkeley. A heavy clay soil, originally a swamp deposit, becoming very tenacious when wet. An excellent wheat soil.

The physical analyses of these soils are given below.

PHYSICAL ANALYSES OF TYPICAL SOILS. ==============================+=====+=========================+====== | | Silt. |Sand, | +------------+------------+2.0 to |Clay.| Fine, | Coarse, |64 mm. | | <.25 to |.5 to 2. mm.|h. v. | |.5 mm. h. v.| h. v. | ------------------------------+-----+------------+------------+------ No. 233. Morano sandy soil | 2.82| 3.03 | 3.49 |89.25 No. 1197. Gila bottom soil | 3.21| 5.53 | 15.42 |72.05 No. 198. Ventura silty soil |15.02| 15.24 | 25.84 |45.41 No. 1697. Berkeley adobe soil |44.27| 25.35 | 13.47 |13.37 ------------------------------+-----+------------+------------+------

The most striking feature in this diagram is the very rapid and high ascent in the combination of sediments represented by the Gila bottom soil. It outstrips at once both the sandy soil from Stanislaus, which contains a trifle less of clay, and the silt soil from Ventura, from which at first sight it does not seem to differ widely, but which contains considerably more clay. It is doubtless the latter which so greatly retards the motion of the water, as is still farther seen in the case of the clay or adobe soil. It will be noted that on the second and third days, the Gila soil had raised the water nearly twice as high as the adobe, and that it took only 18 hours to raise it nearly the same height as that attained by the Ventura silt in so many days. But it ceased to rise after the 125th day, while the Ventura soil, continuing for 195 days, finally rose 3 inches higher. The adobe also continued its rise, but did not reach the same height as the Gila soil by nearly two inches. There can be no doubt that the energetic and high rise of the latter proves an important factor in the culture of these lands.

The ascent is of course most rapid, in the large tubes almost instantaneous, when the capillary space is entirely clear; but in the complex system of connected air spaces in soils, the curved paths and the friction obstruct the movement.

The coarse sandy soil reached its highest limit, 16½ inches, within six days, when the silty Gila soil stood at about double that height.

Ascent of Water in uniform Sediments.—Loughridge has ascertained the rate of ascent of uniform sediments of different grain-diameters, with the results shown in the diagram subjoined, together with the maximum height reached by each. The diagram is very eloquently illustrative of the great differences in the capillary properties of granular sediments of the various grades; and it would seem that it ought to be possible to deduce from it by a somewhat complex formula the rate and height of ascent of water in any soil of known physical composition. In nature, however, the presence of clay and the greater or less degree of flocculation of mixed sediments will always vitiate to a very great extent the results deducible from such calculations; hence the data conveyed by the observations of Loughridge must be considered applicable only to granular sediments free from clay and entirely deflocculated.

I. e., uniform between the narrow limits given.

It is curious that in this case the “clay” showed a rise markedly below that of the finest granular sediment, despite the extreme fineness of its particles. This proves plainly that the physical nature of colloid clay is unlike that of the granular sediments; as has been repeatedly mentioned above.

Maximum and Minimum of Water-holding Power.—It is clear that at the base of the columns of soils just considered, the maximum of water-absorption of which the soil is capable will have been brought about; while at the top of the same column, the minimum of possible liquid absorption (continuous films of water) will exist. The same minimum moisture-condition will be produced when a limited quantity of water is placed with a large mass of soil; the moisture will spread to certain limits, until the surface films of water have all acquired uniform tension; and will then cease to extend, except by evaporation and hygroscopic absorption. It is clear that the same condition will be brought about in the course of time at the top of a soil column in which water has percolated from above; and hence the minimum mentioned, aside from evaporation, represents approximately the usual condition of the soil near the surface within a variable time after a rain, or irrigation, when the descending water column has attained a length corresponding to the height to which the water would have risen from below in a tube arranged as shown on p. 205. It is therefore a condition of very frequent occurrence in the arid region.

Ad. Mayer (Agriculturchemie 2, p. 141) designates this minimum content of liquid water as the “absolute” water capacity of the same; but it is not obvious wherein this factor is better entitled to this name than would be the maximum (see Wollny’s Forsch., 1892, p. 1.). M. Whitney (Rep. Proceedings Ass’n Agr. Coll. & Exp’t St’ns, Nov. 1904) gives as a new observation the fact that in soils approaching the drought condition water “does not obey the ordinary physical laws as we recognize them in capillarity.” This evidently refers simply to the well-known phenomenon mentioned above.

Capillary Water held at Different Heights in a Soil Column.—To determine the amounts of water held in the different portions in columns of soils in which water ascends by capillary rise, the following plan was adopted by the writer in collaboration with Loughridge (Rep. Calif. Sta. 1892-4, p. 99).

Instead of glass tubes the soils to be tested were placed in copper tubes one inch in diameter, divided into segments six inches long, and flattened on one side. In the flattened side a slot half an inch wide was left, and glass plates, held in position by rubber elastics, were cemented on the slotted side by means of paraffin, to prevent a sifting-out of the soil. The short sections can be connected at the ends like joints of stove-pipe, and the earths can be easily introduced in proper, even condition. It was thus possible to gain access to any portion of the column at any time, for the taking of samples.

WATER CONTENTS OF SOIL COLUMNS AT VARIOUS HEIGHTS ABOVE WATER LEVEL. ==================+===========+===============+================ No. | 233 | 1197 | 1679 ------------------+-----------+---------------+---------------- Height above Water|Sandy Soil,|Sandy Alluvium,|Adobe, Berkeley. Level. | Morano. | Gila. | ------------------+-----------+---------------+---------------- 47 inches | | 4.33 | 42 inches | | 10.26 | 36 inches | | 11.99 | 30 inches | | 15.26 | 24 inches | | 21.39 | 10.26 18 inches | | 27.63 | 29.48 12 inches | 3.93 | 32.48 | 33.04 6 inches | 14.15 | 35.04 | 38.47 3 inches | | | 38.49 1 inch | 24.34 | 36.64 | 44.41 ------------------+-----------+---------------+----------------

This figure represents only a temporary condition; the full height of 46 inches was not reached until the 195th day.

Since gravity limits the capillary ascent in a progressive ratio, as shown in diagram 39, it is obvious that the true maximum saturation can exist only in a very short (strictly speaking, an infinitesimally short) vertical column. The least practicable height for experimental work being about 1 cm. (⅖ in.), the writer has adopted for the purpose of rapid determination of this factor, the use of a brass cylinder 1 cm. high and of such width as to contain, for the sake of convenience, 25 or 50 cm. of soil. This cylinder has a finely perforated bottom, which may be covered with filter paper; after being filled with soil which has been struck level, and weighing, it is immersed to 1 mm. depth in distilled water and allowed to rest for an hour; then quickly dried outside and beneath with filter paper, and again weighed. The amount of water found by difference should for all practical purposes be referred to the volume, not to the weight, of the soil, so as to eliminate the error arising from the varying specific gravity of the latter.

In most cases the surface of the soil in the sieve cylinder remains level after wetting; but sometimes it swells so as to rise above its dry level, even to the extent of nearly 30% (see chapter 7, p. 114). This happens especially in strongly ferruginous soils. In the case of “black alkali” soils, in wetting an enormous collapse sometimes takes place (see chapter 22).

If it be desired to determine also the minimum liquid absorption (see below), the surface of the wet soil is first covered with air-dry soil, to absorb the surplus moisture, and finally with soil previously saturated with hygroscopic moisture; the added soil being each time thrown off and finally the surface “struck” level with a tense silk thread before weighing. Corrections must be applied for the usual increase in weight, from the addition of soil, and for the hygroscopic moisture.

While the minimum of liquid absorption can thus be determined quickly, without awaiting the capillary ascent of a water column, and if sufficient time is given can also be determined in higher columns, as proposed by Mayer (Wollny’s Forsch. Vol. 3), the maximum cannot thus be determined without gross inaccuracy. In determinations made by the writer it was found that the figures for the minima of very different soils (clayey and sandy) of the arid region, differ proportionally much less than do the respective maxima. In few of these soils it was found to exceed about 10 per cent, and it scarcely fell below 4 per cent even in very sandy soils. A very deep, sandy soil, which had been irrigated in May, and upon which no rain had since fallen, showed in July in the second foot, upon which rested ten inches of fully air-dried soil free from vegetation, a water-percentage of eight per cent.

Hall (The Soil, p. 66) gives for the minima in the case of soils examined by him the following figures: coarse sandy soil, 22.2, light loam, 35.4, stiff clay, 45.6, sandy peat, 52.8. These figures are very much higher than for apparently similar materials used by the writer, and the differences exceed those between the maxima given for the same. This discrepancy I am unable to account for.

Capillary Action in Moist Soils.—In the preceding discussion the case of columns of air-dry soils, so common in the arid regions, has been considered. It is obvious that a soil column holding the minimum of capillary water may be of any height; so that when, as happens in the open field, the rain water soaks down beyond the range of capillary rise in a given soil, the upper portions of the latter, above that range, will remain at the minimum of moisture-content so long as it is not depleted by evaporation. King has made extended observations on soil columns ten feet high and moistened throughout the mass. Capillary movement takes place in moist soils much more rapidly than in dry ones, although when sufficient time is given the final adjustment will of course be the same. King’s experiments showed that evaporation at the surface of the tenfoot columns caused a sensible depletion of the water content originally existing at the depth of ten feet, in the course of 314 days. While so slow a movement might not be of any benefit during the growth-period of shallow-rooted annual crops, the fact shown is of importance to permanent plantings, as of trees and vines.

Another and not so readily intelligible effect observed by King is that when the surface-soil is wetted, moisture may be withdrawn toward the surface from the lower layers. In one experiment he found that when water was applied on the surface so as to add two pounds of water to each surface foot in several soils, at the end of 26 hours there had been an increase of three pounds in the same, and a loss of one and three quarter pounds from the second and third feet. The cause of this translocation is probably a “distillation” of the subsoil moisture toward the cooled soil; the fact that it occurs is of practical interest, since it seems to show that wetting the upper portion of the soil by cold rain or irrigation may tend to raise additional supplies from below. At the change of seasons we not uncommonly find, in digging tree holes or wells, a wet streak at from 9 to 18 inches below the surface, caused evidently by the condensation of subsoil moisture, at the limit of a cold zone resulting from the penetration of unseasonable temperature (“cold snap”) from above. Such movements of soil-moisture by means of evaporation and recondensation within the soil can of course take place even when the minimum of liquid absorption has been reached and direct capillary movement has ceased. It is, as it were, dew within the soil.

Proportion of Moisture Available to Growing Plants.—Not all the capillary moisture contained in soils is available to plants, as can readily be seen from the fact that many plants, especially when growing in pots, begin to wilt while the soil still appears visibly moist. The limit of wilting differs greatly in different plants, and in the open ground it is difficult to ascertain that limit, because the deeper roots continue to supply moisture from moister substrata. Hence potted plants wilt while the soil appears much moister than when the same grow in the field. King has determined the amounts of moisture down to 43 inches in a Wisconsin soil in which clover and corn were at the wilting point, as in the following condensed table:

===============================+=========+========+======== | Clover. | Maize. | Fallow | | | ground. -------------------------------+---------+--------+-------- First 12 inches, clay loam | 8.44 | 7.03 | 17.01 Second 12 inches, reddish clay | 12.84 | 11.79 | 19.86 24 to 30 inches, sandy clay | 13.52 | 10.84 | 18.56 40 to 43 inches, sand | 9.53 | 4.17 | 15.90 -------------------------------+---------+--------+--------

Physics of Agriculture, p. 135.

It is plainly shown here that the roots of clover and corn were unable to utilize the higher moisture-content of the subsoil-clay to the same extent as the smaller amounts present in the surface foot, and in the sandy substrata. Evidently the moisture in the clay soil was more tenaciously retained.

This is doubtless due, as King shows, to the equal thinness of the moisture film remaining on the soil grains in either case; the number of grains, and therefore the aggregate surface holding these films, being much greater in the clay than in sands; hence the higher water content.

It is interesting to compare these figures given by King for clover and maize at the wilting-point, and fallow ground adjacent, with those given by Eckart (Rep. Expt. Sta. Haw. Sugar Planters’ Ass’n., 1903) for those affording good growing conditions for sugar-cane on the (highly ferruginous) soils of that station. The plots were irrigated at the rate of one, two and three inches of water per week, allowance being made for the rainfall. Two inches proved, on the whole, to give the best average results for production. The moisture determination of the soil under the two-inch regime gave an average moisture content of 29.13% in the first foot of soil. It is not stated what was the hygroscopic coefficient of that soil, but it was probably very high; in the neighborhood of 21.5%, judging by the determinations made with six Hawaiian soils at the California Station. This would indicate about 7.63% of free moisture as the optimum for sugar-cane.

Moisture-requirements of Crops in the Arid Region.—Plants (particularly broad-leaved ones) which have made a brash growth during a period of abundant moisture, will wilt quickly when sunshine returns, and take some time to adapt themselves to the drier conditions. On the other hand, plants accustomed to dry air and scanty soil-moisture, will not wilt or suffer under what would elsewhere be considered very rigorous conditions. Loughridge has made numerous determinations of moisture in soils in which crops were beginning to suffer, and others on similar soils that were growing normally, and found that in general, not only were the differences in moisture content considerably less than in the case above quoted from King’s observations, but that the amounts of free moisture required by various crops in the arid climate of California were surprisingly small.

Rept. Cal. Expt. Sta. 1897-08, pp. 65-96.

The tables below show the results of observations made by Loughridge during several drought years in California; so arranged as to show the differences of moisture content for the same crop in different soils. It will be observed that in all cases where a crop growing on a clay soil could be compared with the same on a lighter soil, the moisture required to keep the crop in good condition was very much greater in the clay than in the loam or sandy soils. In the case of apples, e. g., 8.3% of water was abundant to keep the trees in excellent condition on a loam soil, while on a clay soil holding 12.3% the condition was very poor. That this difference is due in the main to the difference in the hygroscopic-moisture coefficient of the respective soils, is plainly apparent in several cases. It is therefore not the total moisture content, but the free moisture present in excess of what is held by hygroscopic absorption, that determines the welfare of the plant.

By determining, first, the total moisture in the soils, as taken in the field, then, after allowing them to become air-dry, determining the maximum of hygroscopic moisture they would absorb (see p. 198), Loughridge found by difference the amount of free moisture, or liquid water which must be present in the soil to prevent the crops from suffering. An exceptionally good opportunity for these observations was offered by the dry season of 1898, during which crops suffering and not suffering, on identical lands, could easily be found. The determinations were always made for each foot of the upper four feet of the land in the immediate neighborhood of the trees or among the field crops. The first table exemplifies the method of procedure; the second gives the summary of results for the several crops and trees, as calculated from observations made during the season.

TABLE SHOWING CONDITION OF CROPS ON VARIOUS SOILS UNDER DIFFERENT MOISTURE-CONDITIONS. ============+=============+=========+============================== | | |Per cent Moisture in four feet. Kind of | Kind of |Condition+------+------------+-----+---- Crop. | Soil. | of | | | |Tons | | Crop. |Total.|Hygroscopic.|Free.| per | | | | | |acre. ------------+-------------+---------+------+------------+-----+---- Wheat |Very sandy |Poor | 2.6 | 1.9 | .7 | 56 “ |Sandy loam |Good | 12.8 | 5.6 | 7.2 | 576 “ |Clay |Dead | 14.1 | 10.5 | 3.6 | 288 Maize |Clay adobe |Very good| 12.9 | 8.8 | 4.1 | 328 “ |Sandy loam |Fair | 6.1 | 2.3 | 3.8 | 304 Barley |Black adobe |Wilting | 10.7 | 8.8 | 1.9 | 152 Sugar Beets |Black loam |Good | 12.4 | 5.6 | 6.8 | 544 Vines |Loam |Good | 8.5 | 5.0 | 3.5 | 280 “ |Sandy loam |Poor | 1.9 | 1.5 | .4 | 32 Almonds |Loam |Good | 8.5 | 6.6 | 1.9 | 178 “ |Same field |Suffering| 7.9 | 6.9 | 1.0 | 80 Apples |Loam |Excellent| 8.3 | 5.5 | 2.8 | 224 “ |Clay |Poor | 12.3 | 10.8 | 1.5 | 120 Apricots |Loam |Excellent| 6.3 | 3.3 | 3.0 | 240 “ |Gravelly loam|Poor | 6.9 | 5.0 | 1.9 | 152 Figs |Red loam |Good | 5.2 | 3.8 | 1.4 | 112 “ |Heavy loam |Wilting | 8.6 | 8.6 | 0 | 0 Olives |Red loam |Good | 5.2 | 3.8 | 1.4 | 112 “ |Sandy loam |Suffering| 1.9 | 1.9 | 0 | 0 Peaches |Red loam |Good | 8.2 | 5.0 | 3.2 | 256 “ | “ |Poor | 6.8 | 5.0 | 1.8 | 144 Prunes |Gray loam |Excellent| 11.2 | 9.0 | 2.2 | 176 “ | “ |Poor | 6.4 | 5.4 | 1.0 | 80 Citrus fruits|Sandy loam |Good | 6.3 | 3.1 | 3.2 | 256 “ “ |Sandy soil |Leafless | 3.1 | 2.4 | .7 | 56 ------------+-------------+---------+------+------------+-----+----

TABLE SHOWING DROUGHT-ENDURANCE OF VARIOUS CROPS IN ARID REGION. ===================+=========================+==================== Free water in four | | feet of soil. | Crops that did well in |Crops that suffered ----------+--------+ lowest amount of |in highest amount of | Tons | moisture mentioned |moisture mentioned Per cent.| per | in first column. |in first column. | acre. | | ----------+--------+-------------------------+-------------------- 0 to 1.0 | 80 {|Apricots, Olives, Grapes,|Citrus, Pears, | {|Peaches, Soy-bean. | Plums, Acacia. | | | 1.0 to 1.5| 120 |Citrus, Figs. |Almonds, Apples. 1.5 to 2. | 160 |Almonds, Plums, Saltbush.|Barley. 2 to 2.5| { 176 |Prunes. | | { 200 |Walnuts, Eucalyptus. |Prunes. 2.5 to 3 | 224 |Apples. | 3 to 3.5| 288 |Pears. | 3 to 4 | 322 |Hairy Vetch. |Wheat. 4 to 5 | 400 |Wheat, Maize. | 5 to 6 | 480 |Sugar beets, Sorghum. |Sugar beets. ----------+--------+-------------------------+--------------------