CHAPTER XVII.. Relations of Soils and Plant Growth to Heat.

RELATIONS OF SOILS AND PLANT GROWTH TO HEAT.

The Temperature of Soils.—The rapid germination of seeds, as well as the development of plants to maturity, is essentially dependent upon the maintenance of the appropriate temperature. The temperature most favorable to germination or growth, as well as the degree of tolerance of high and low temperatures, varies greatly with different plants, governing mainly what is known as their climatic adaptation. A knowledge of these points with reference to the several crops is therefore of no mean importance to the farmer; for, to a certain extent, he can control the temperature in the soil itself, and he can mostly choose for sowing and planting, the time when the soil shall have the proper temperature for rapid germination or maturity. As a rule, it is not desirable to have either seeds or seedling plants in the ground for any length of time when the temperature is too low for active vegetation; for while they rest, other, lower organisms (fungi and bacteria), adapted to low temperatures, may continue in full activity at the expense of the vitality of the crop plant.

Water exerts controlling Influence.—Since the capacity of water for heat is approximately five times greater than that of the average soil, equal weights being considered, it follows that the temperature of soil-water must exert a controlling influence over that of the soil. Taking the case of a cubic foot of loamy soil, fully saturated with water, in which one-third of the volume may be assumed to be water: the weight of the dry soil being about eighty pounds per cubic foot, calculation shows that the amount of heat required to raise the temperature of the water contained, one degree, will be fully twice as great as for that required for the soil itself. It is thus obvious that the control of soil-temperature is largely dependent upon the control of the water-content of the same, which has been discussed in a former chapter. Even in the condition of moisture known to be most favorable to plants, viz., one-half of the maximum water capacity, the influence of the water-content upon the temperature will still be as great as that of the entire soil mass. This consideration emphasizes the importance of such control.

Cold and Warm Rains.—It is not surprising then that the occurrence of cold or warm rains or the use of cold or warm irrigation water at critical periods, may largely determine the success or failure of the crop. It is well known that the occurrence of a cold rain after vegetation has started actively in early spring, may not only destroy the season’s fruit crop by preventing the setting, or thereafter causing the dropping, of the fruit, but may even, if the suppression of vegetative action be continued for some length of time, result in serious injury to, or death of trees. Widely extended disastrous experience of the kind was had in California in February and March, 1887, resulting in the death of tens of thousands of fruit trees and vines during that and the following season. It is obvious that in such a case as this the rapid draining-off of the cold water through underdrains would have materially mitigated, if not wholly prevented, such injury.

Solar Radiation.—Aside, however, from such overwhelming influences as the above, the soil temperatures are measurably controlled by the extent to which they receive and absorb the sun’s heat rays, whether directly or through the mediation of the air. The direct effect of the sun’s rays upon the surface is, upon the whole, the most generally potent, although warm winds may occasionally exert a very strong influence. The varying influence of the sun’s rays depends primarily upon the change of seasons, which themselves result from the varying angles at which the sun’s rays strike the surface; as well as upon the duration of the day. The greater or less cloudiness or fogginess of the sky, of course, exerts a decided effect in this connection.

The Penetration of the Sun’s Heat into the Soil.—In the temperate regions of the earth the daily variations of temperature cease to be felt at depths ranging from two to three feet, according to the nature of the soil material and its more or less compacted condition. The monthly variations, of course, reach to greater depths; while the annual variations do not disappear in the temperate zone, e. g., at Paris, Zürich and Brussels, at a less depth than seventy-five feet. At these depths of constant temperature we find approximately the same temperature as that which we can deduce from the thermometric observations as the annual mean. From similar causes the mean annual temperature of any place may be approximately deduced from the observation of the water of wells and springs derived from moderate depths. For below the level of constant annual temperature the latter begins to ascend steadily as we progress downward, owing to the interior heat of the earth.

Change of Temperature with Depth.—The following table of observations made at Brussels illustrates the decrease of annual range of temperature with depth:

Average Annual At feet: temperature. range. (°C.) (°C.) 3.25 7.2 10.5 15.6 13.5 4.5 30.8 16.4 1.3 75.0 17.0 0.0

It is interesting to compare with this record that of a well sunk by Ermann at Yakutzk, Siberia, where the mean annual temperature is—9.7 C. (14.6 F.). This temperature was found a few feet below the surface. At 50 feet the temperature was—7.2 C. (19° F.); at 145 feet—5° Cent. (23° F.) at 350 feet—.9° C. (30.8° F.) showing that the ground was below the temperature of freezing water for some distance farther down; so that the search for liquid water was abandoned.

We thus see that in the Arctic regions, owing to the presence of water in the form of ice, the melting of which impedes the access of solar heat, the level of no variation is found at the distance of a few feet below the surface, despite the great variations in temperature between the short but hot summer and the extremely cold winter. In the tropics, also, the annual temperature-variation disappears at a less depth than 2 feet, in consequence of the very slight difference between the two seasonal extremes of temperature.

Surface—Conditions that influence Soil-Temperature. Among these, color has already been mentioned, and to a certain extent discussed. While it is true that, broadly speaking, dark-colored soils absorb more of the sun’s heat than light-colored ones, other things being equal, it must still be understood, that the nature of the color-giving substance exerts a very material influence upon the amount of heat absorbed. Thus charcoal is among all known substances the one absorbing and radiating the sun’s heat rays most powerfully, and all kinds alike; so much so, that its absorbent power is taken as 100. But other substances which to the eye appear equally black, have by no means the same absorbing power. The heat absorption by black humus is high, though not quite equal to that of charcoal; and many gray soils, though appearing to the eye of rather light tint, really absorb more heat than others which, to our perception, have the darker tint, but are colored by other substances. Gardeners and especially vine growers in the colder portions of Europe often take advantage of the powerful absorbing power of carbon by spreading charcoal or black slate powder over the surface of the soil where early maturity is specially desired; and slate powder is similarly used by the peasants at Chamouni to hasten the melting of the snow.

Heat of High and Low Intensity.—It must also be kept in view that the surfaces, and especially the colors that favor absorption of the intense rays of the sun, may comport themselves quite differently toward heat rays of low intensity, such as those thrown back from the soil at night when it cools. Were this otherwise, a soil that absorbs much heat in the daytime would lose it with corresponding rapidity at night. But this is true only of charcoal; in the case of most other substances, there is a material difference in favor of the retention of the heat, of low intensity, by slower radiation into a “heat-trapping” atmosphere.

Reflection vs. Dispersion of Heat.—Theoretically, a smooth surface reflects more heat than a rough one, and warms much more slowly by absorption; as is strikingly shown by the use of polished metal screens placed on walls to prevent their being overheated by a flue near by. In the case of soils, also, the condition of the surface affects materially the absorption of heat, but not in accordance with the above rule so far as the result is concerned. For it is found that, other things being equal, a loose or cloddy surface disperses in many directions the heat it receives, and does not permit it to penetrate by conduction to so great extent as would a more compact soil, whose smooth surface would waste less of the heat received by radiation.

King has called special attention to the difference of temperature existing between soils smoothed and compacted by a roller, and the unrolled soil having a loose surface. He found that the former at a depth of one and a half inches was as much as 5.5°C. (10°F.) warmer than the loose soil, and that even at a depth of three inches a difference as high as 3.5°C. (6.5°F.) existed between the two. He observed at the same time that the temperature of the air over the unrolled ground was considerably warmer than above the rolled, thus corroborating the differences observed in the soil itself. But at night the heat is given out more rapidly from the rolled than from the unrolled surface, the latter acting as a non-conductor and keeping the soil warmer than that of the more compact rolled land. King gives as the average difference observed between rolled and unrolled land on eight Wisconsin farms, 1.6°C. (3°F.) in favor of the rolled land between 1 and 4 p.m.

It will thus be seen that the loose tilled layer, while impeding the penetration of the sun’s heat into the deeper portions of the soil during the day, on the other hand serves to retain it at night better than a more compact soil. This obviously places it within the power of the farmer to exert considerable control over the soil-temperature at critical times; restraining or favoring the access of the sun’s heat in accordance with the requirements of the climate or season, as the case may be.

Influence of a Covering of Vegetation, and of Mulches.—A cover of either living or dead vegetation depresses the temperature of the soil as compared with the bare land, as elaborately shown by Wollny and Ebermeyer. In the monthly averages these differences rarely exceed .8° C. (1.5 degrees F.), and are mostly below .50° C. (1° F.), but during different parts of the day they may rise to 2.2 to 2.5° C. (4 to 4.5° F.), at 4 inches depth. In summer they are greater than at other seasons. Of course the density of the vegetation or the thickness of the mulch influences them materially. Forests exert the greatest cooling influence upon the soil, and next to these the dense herbaceous crops, such as clover, and the legumes generally.

Influence of the Nature of the Soil-Material.—Aside from the surface condition, the nature of the material itself exerts a certain influence, not only upon the rate of introduction of heat, but also upon the amount taken up. Thus quartz sand having the highest density (greatest weight per cubic foot) and also the highest capacity for heat among the usual mineral soil-ingredients, will, mass for mass, experience a smaller rise of temperature than would clay or loam soil, of less density or volume-weight, and also of lower heat-capacity. While this holds good theoretically, differences corresponding to this consideration rarely occur in nature, for the reason that the much greater influence of the mechanical condition of the soil mostly overbalances these effects. Thus Wollny has shown that while quartz is a better heat-conductor than clay, quartz cobbles or gravel will materially increase the temperature of the soil in which they are imbedded. Yet compact clay is a better conductor of heat than loose sand; hence the latter, when exposed to the intense heat of the summer sun in the desert, becomes intensely hot on the surface, yet allows of the existence of abundant moisture at a depth of ten or twelve inches; while clay in the same region, being usually in a compacted condition, will show a lower surface-temperature and will be warmer and drier at a depth at which sand will still retain abundant moisture, and be comparatively cool (See chap. 13, p. 257.) So much indeed depends upon the state of mechanical division and flocculation in which the several soils may happen to be, that a hard-and-fast statement in regard to their relations to heat cannot and should not be given, as it would only lead to disappointments and practical mistakes; the more as in all cases the moisture-condition exerts an influence predominating by far over that of the dry material itself, and this moisture-condition is subject to rapid changes, owing to intrinsic differences in the several classes of soils. Wollny states as the result of his experiments, that in summer sandy soils are warmest; then humous, lime and loam soils; while in winter humous soils are warmest, then loams; and sand coldest.

Influence of Evaporation.—In treating of the Conservation of Soil Moisture (chapter 13), the effects, conditions and control of evaporation from the soil have already been discussed from several points of view; so that a summary review of the subject must suffice in this place.

It has been stated above that in the case of an average loam soil saturated with water, the heat required to raise the temperature of the water one degree would be about twice that needed to so change the dry soil material itself. But if it is required to evaporate the same amount of water from the soil, nearly ten (9.667) times that amount of heat will be required; or in the case assumed, twenty times as much as would suffice to raise the temperature of the dry soil through an equal interval of temperature. While in a few cases the cooling of the soil by evaporation is desirable, in the vast majority of cases it is injurious to the progress of vegetation, and should be restricted as much as possible by the means outlined in a former chapter.

Formation of Dew.—There is, however, another aspect of evaporation from the soil which has been long misunderstood, although the true state of the case was partially recognized long ago. Dew is in common parlance said to “fall,” it being supposed that, like rain, it is derived from the atmosphere. While this is partially true, inasmuch as from very moist, and notably from foggy air dew is frequently deposited on grass and foliage generally, as well as on wood and other strongly heat-radiating surfaces; yet as a matter of fact, in by far the majority of cases, as shown by H. E. Stockbridge and confirmed by everyday observation, dew is formed from the vapor rising from the warmer soil into a colder atmosphere, and condensed on the most strongly heat-radiating surfaces near the ground, such as grass, leaves both green and dry, wood, and other objects first encountering the rising vapor. In manifest proof of this it will be noted that very heavy dews may be seen on the ground, when the roofs of houses as well as the higher shrubs and trees remain perfectly dry. In winter this may be most strikingly seen in the deposition of hoar-frost in and immediately around the cracks of plank sidewalks, whose surface remains dry.

“Rocks and Soils,” pp. 175-189.

Dew rarely adds Moisture.—Candid observations will convince any one, therefore, that in most cases the supposed addition to the moisture of the soil from dews is an illusion. Whatever dewdrops fall on the ground are in general simply the return to the soil of a part of what came from it; while the dew that evaporates from the bedewed leaves or other objects represents simply a delayed outgo of moisture from the soil, which for a time retards evaporation direct from the soil, and thus effects a slight saving of moisture.

But while this is measurably true of inland and especially of continental areas like the great plains of North and South America, it is also true that in deep moist valleys, and on the rainy and foggy coast regions of continents, dews are found to both fall and rise, not uncommonly to such an extent as to be equivalent to a not inconsiderable aggregate precipitation. Thus in the moist coast belt of Oregon and Washington lying west of the Cascade range of mountains, the morning dews of summer are frequently so copious that the water falls in showers from the lower trees and shrubs, so as to necessitate the use of water-proof clothing when traversing the woods in the morning, quite as much as though rain was actually falling. In hilly and more especially in mountainous regions the cold air descending from above and flowing down in the ravines will often cause a heavy condensation of dew in these, while the bordering ridges, which rise above the cold currents, remain free from dew. These descending currents as a rule not only bring no surplus moisture with them, but in their downward course become warmer by contraction and therefore relatively drier. In these cases also, therefore, the dew is purely moisture derived from the ground, which in rising encounters the cold air and is thus condensed.

The fact that dew is most commonly derived from the soil could have been foreseen from the other fact, long ascertained and known, that during the night the soil is as a rule warmer than the air above it; as has been shown by the earlier observers, as well as more specifically by Stockbridge.

Dew within the Soil.—It is obvious that whenever dew is formed above the surface of the soil, the air within the latter must be at or near its point of saturation with vapor, as in fact is usually the case a few inches below the surface. It follows that when a depression of temperature occurs within the soil, e. g., at night, dew must be deposited within the soil down to the depth to which the nightly variation reaches, increasing at that depth as the vapor from the warmer soil below rises, to be in its turn condensed. There is thus formed at that level a zone of greater moisture, which may sometimes be noted in digging pits, by a deeper tint, without any corresponding variation in the nature of the soil. The daily repetition of this process, at varying depths, and its greater or less recurrence at or near the limit-levels of monthly and even annual variations, must exert a not inconsiderable influence upon the vertical distribution of moisture in the soil; which instead of being usually found in horizontal bands or zones of varying moisture-contents, is usually remarkably uniform for considerable depths, despite the fitfully recurrent additions from rains. It is at least probable that this process of dew-formation within the soil materially assists capillarity in effecting a measurably uniform vertical distribution of moisture. (See also page 207, chapter 11).

Plant-development under different Temperature Conditions.—In the arctic regions the ground, frozen in winter to unknown depths, may thaw to only three to five feet during the summer, notwithstanding the great length and continuous sunshine of the arctic day. The shallow-rooted arctic flora develops very rapidly under the influence of the continuous daylight and heat, in the course of from five to eight weeks. The seeds of these plants must, of course, be capable of germinating at very low temperatures; and as a matter of fact, we find that both in the arctic regions and in the higher mountains, certain plants are found growing and blooming on slopes flecked with snow; each plant surrounded by a small circle of bare ground, where the snow has been melted under the influence of the dark-tinted earth and leaves. It is clear that here germination has occurred, the foliage has been formed, and the roots have been exercising their vegetable functions, in ground soaked with water practically ice-cold.

Germination of Seeds.—While wild plants of special adaptation may thrive in very low (or high) temperatures, it is also true that few of our cultivated plants will germinate, and still less grow thriftily, at such low temperatures. The limit below which most cultivated plants may be considered as remaining practically inactive lies between 4.4 and 7.2° C. (40 and 45° F.). Few tropical plants will germinate much below 23.8° (75° F.) and in some cases not below 35° Cent. (95° F.). Even maize and pumpkins, according to Haberlandt, germinate most rapidly between 35 and 38.3° C. (95 and 101° F.), while for wheat, rye, oats and flax the best temperature for germination lies between 21.1 to 26.1 (70 to 79°). Under the most favorable conditions of temperature and moisture, some small seeds which readily absorb moisture will germinate in from twenty-four to forty-eight hours, while at a lower temperature they may require from three days to two weeks. Thus Haberlandt found that while oats would germinate in two days at a temperature of 17.2 to 17.5° C. (63° to 63.5°), it took a full week for germination when the temperature was only 5° C. (41° F.). It is obvious that seeds remaining inert in the soil for such lengths of time will be subject to a variety of vicissitudes that may injure or destroy their vitality. There are many bacteria and fungous parasites which at low temperatures are perfectly capable of attacking and destroying the water-soaked seed. There is thus for each plant, from the lowest to the highest, a certain temperature most favorable to development; and both above and below this, the vegetative activity is seriously interfered with or wholly checked. A knowledge of these limits is manifestly of the utmost practical importance.

The influence of too high a temperature in preventing the germination of cinchona seed from India, was curiously exemplified when it was subjected to a supposedly favorable steady temperature of 23.8° C. (75° F.) under otherwise most favorable conditions. Not a single one came up in the course of six weeks, and the box in which it had been sown was put away outside of the hothouse as a failure. Within two weeks a full stand of seedlings was obtained, at temperatures ranging between 12.7 and 15.5° C. (55° and 60° F.). The fact that the cinchona is a tree of the lower slopes of the Andes (three to five thousand feet) although at home strictly within the tropics, explains the apparent anomaly.

PART THIRD.

CHEMISTRY OF SOILS.