A CONTRIBUTION TO THE SUBJECT O F THE HYGROSCOPIC MOISTURF, OF SOILS BY CHAS. B. LIPMAN AND LESLIE
T. SHARP
In a recent publication of the Bureau of Soils, Patten and Gallagher’ give experimental data in support of the principle, enunciated by Knop and Schubler and adhered to by other investigators, that the power of soils to absorb hygroscopic moisture from a saturated atmosphere decreases with a rise in temperaturi and increases with a decline in temperature. The above-named investigators preface their work with a review of the more important investigations on the subject of hygroscopic moisture in soils and show that all but one investigator, Hilgard, have supported the results of Knop, as above stated. Hilgard, however, as they have pointed out, found in his investigations that not only does the power of soils t o absorb hygroscopic moisture not decrease with a rise in temperature, but that it actually increases provided the atmosphere is saturated. It appeared to us, therefore, that it was highly desirable to prosecute further investigations not only relative to the temperature question, above referred to, but also to the methods involved in the study of the powers of soils to absorb hygroscopic moisture, as well as other phases of that subject. In order that the data on the effects of temperature might be established on a well justified method, the first experiments were carried out on the effects of the depth of the soil layer on the absorption of hygroscopic moisture from a saturated atmosphere. Air-dried samples of a Berkeley clay adobe soil, from the campus of the University of California, were sifted through the 0.5 mm. sieve and distributed in duplicate in I-gram, 3gram and 6-gram portions in weighing bottles which were in. in diameter and I 16,”* in. in height. They were about I arranged on a glass plate which was placed on a porcelain acid dish partly filled with distilled water and set in turn on a Bureau of Soils, Bulletin No. 51, U.
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ground-glass plate, over \he whole of which was fitted a bell jar of the short, squat fo.-m, to the inside upper portion of which was pasted a con::iderable quantity of thoroughly moistened filter paper. Th: ground rim of the bell jar was well covered with vaseline so as to permit of no escape of the moisture from the bell jar and thus insure a thoroughly saturated atmosphere. The aqparatus was then placed on a shelf in a small room as far a3 possible from currents of air and where the temperature could be raised a t will by lighting a Bunsen burner. A centigrade thermometer was placed next to the bell jar so that the temperature could be taken accurately just previous to removihg the bottles from the jar for weighing. From the amounts of\\soil placed in the weighing bottles we had a depth in the c a s e y t h e I-gram samples of I .5 mm, in that of the 3-gram samples of 4 mm and in that of the 6-gram samples of 8.5 mm. The covers of the weighing bottles were protected from moisture and dust under a separate bell jar and were so arranged on a glass plate as bo allow of their being placed in their respective bottles in two or three seconds, from the time when the jar in which the samples were exposed was raised from the ground glass plate, thus' preventing any loss of moisture. Table I gives the results of the weighings made through a period of 3343/4 hours. The differences due to changes of temperature may be ignored here, since that matter is taken up in another series of experiments, the results of which are given below, but to which they lend further support. No further explanations of the technique and other details of the experiment need be given here since they are fully considered in the table. The results in Table I bring out quite clearly the fact that anything but a very thin layer of soil cannot be used for the determination of the hygroscopic coefficients of soils, since even with as thin a layer of soil as 3 mm it was impossible t o saturate it in less than three days and in the case of the 6 mm layer it was found impossible to saturate the soil until nearly the end of the experiment, or considerably over 400 hours. It must be said here also that the data given above as well as
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those given in the other tables represent averages of duplicate samples which throughout all the work agreed remarkably well, thus strengthening the accuracy of the work. In view of the fact that most of the work thus far done on the hygroscopic coefficients of soils was done with thick and in many cases uneven layers of soil lends to the data above presented great significance and renders questionable some of the conclusions adduced from the other determinations. Indeed the depth of the soil layer employed seems to have been considered in the past by most investigators an insignificant factor despite the evidence which Hilgard, in work above referred to, has brought to light in that connection. Schiibler and Knop have evidently not given the matter any consideration, and even in the very recent work of the Bureau of Soils above cited, Io-gram and 20-gram samples of soil were employed in weighing bottles, thus making a much thicker layer than the thickest employed by us in the experiment just described. In a short time, therefore, it is impossible to saturate a Io-gram sample of soil in a weighing bottle placed in a saturated atmosphere. I t would appear from our experiments that even with a thin layer of soil of 1.5 mm a t least 24 hours is necessary to saturate it in a weighing bottle. It is very probable though that in our regular method of determining the hygroscopic coefficients of soils the point of saturation can be reached in 8-10 hours for ordinary arable soils. This method is one devised by Hilgard and consists in sifting a quantity of the air-dried soil to be tested in a layer I mm thick on a piece of glazed paper which in turn is laid on a small wooden table with metal legs which stands in a box 1 2 x 18 x 19 inches high and made of boards I ~ / , inches in thickness. The box and air-tight cover are lined with blotting paper which is thoroughly moistened and a layer of water a t the bottom of the box about I inch in thickness is constantly maintained. The small tables upon which the soil samples are spread are so made that the tops are only about I inch above the surface of the water. This is absolutely necessary since it is difficult to obtain a saturated atmosphere
Hygroscopic Moisture of Soils
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at any distance from the layer of water as Hilgard has found in his experiments on this subject. The box just described is kept in a cellar in which the temperature remains practically constant throughout the day and night. After an exposure of 8 hours the samples are quickly transferred to weighing bottles, the latter are stoppered and weighed. The stoppers are then removed, the bottles placed in an oven a t 105-110' C and heated until the weight is constant. The percentage of hygroscopic moisture is thus calculated without any but small chances of loss. The earlier determinations carried out by Mitscherlich, Knop and others in watch glasses with uneven depths of soil cannot have given correct results, especially since not only the depth of soil interfered but also the lack of a saturated atmosphere. It should be noted here also that there is a constant rise in the amount of moisture absorbed by the thicker layer of soil regardless of the temperature conditions until the point of saturation is reached. Having thus again justified the method which could be relied on for the most accurate results, it was thought desirable to study the effects of temperature on the hygroscopic powers of soils in saturpf-d O+nlospheres,and since the apparatus used by Hi'_ ' - v e could not be conveniently placed undel lture conditions during the course of the experimel,, :was conducted in a bell jar as was the preceding series a1l;i the layers of soil used in the weighing bottles were made as nearly as possible I mm in depth. Three different soils, one of a low, another of a medium, and a third of a high, hygroscopic power were chosen for the work. The first was a light sandy soil from Anaheim, California, the second a clay adobe soil from the campus of the University of California, and the third was a tule soil with a high humus content from Klamath Lake, Oregon. All precautions employed with reference to the technique of the work in the preceding experiment were observed here. The results follow :
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The results in Table I1 would seem to give an unequivocal reply to the question of the effects of temperature on the hygroscopicity of soils. It would appear that as soon as the soil is saturated we have a rise of temperature accompanied by an increased absorption of hygroscopic moisture, and a fall in temperature accompanied by a loss of moisture from the soil. With the exception of the results of one or two weighings of the Anaheim soil towards the end of the experiment we find the results are so clearly in favor of the temperature effects just enunciated that no better evidence could be desired, to establish them. We have here, therefore, data which confirm the results of Hilgard' and so far as we are aware the only data which agree with those of that investigator. In addition to the results of Knop and Schiibler opposed t o those of Hilgard, we have the exhaustive investigations of Schlosing, and also the work of Ammon,3von Dobeneck4 and others who worked with the various soil constituents instead of with soil. The objection to their methods must be urged however that they, as well as Patten and Gallagher, whose recent work is above cited, worked either with thick or with uneven layers of soil, or with both uneven and thick layers of soil, even though it be admitted that in some cases a saturated atmosphere was insured in the experiments. We are emphasizing the importance of the saturated atmosphere advisedly for in our own experiments, it has been noted on several occasions that when there was a condensation of vapor on the walls of the glass bell jar which trickled down t o the ground glass and was allowed to remain there during one or two removals and replacements of the bell jar, that several places on the rim of the bell jar would be found free from vaseline and perfectly dry thus allowing an escape of vapor from the jar and undersaturation of the atmosphere within it. These occurrences I
Wollny's Forsch. a. d. Gebiete der Agrikulturphysik., 8, 93. Ibid., 7, 322. Ibid., 2, I . Ibid., 15, 163.
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were invariably accompanied by losses of moisture from the soils tested regardless of the temperature variations which had occurred during the period of exposure. Moreover, Hilgard has found in some more direct experiments, which remain as yet unpublished, but which were communicated verbally to one of us, that when the determinations were carried out in a tall bell jar with no saturated blotting paper in the upper portion of it, the soil samples placed a t different heights from the water a t the bottom of the jar showed different powers of absorption and namely those nearest to the water absorbed the most and those furthest from the water the least hygroscopic moisture. It must also be stated here that our results do not show any increase in moisture absorption according to any geometrical or arithmetical ratio. Hilgard’ has claimed that so far from following Knop’s law of a steady decrease of moisture absorption with a rise in temperature according to a geometrical ratio, his experiments show an increase of moisture absorption, provided there is a saturated atmosphere, with a rise in temperature according to an arithmetical ratio. It is interesting to note here also that Hilgard adds, that in an undersaturated atmosphere, he was able to duplicate Knop’s results and obtained a decrease in moisture absorption with a rise in temperature, according to Knop’s geometrical ratio. The next series of experiments was carried out to note the changes, if any, taking place in the hygroscopic coefficients of the same soils’used in the preceding series when the bell jar was placed in the incubator instead of in a room, under changes of temperature, produced by adjustments of the thermostat. The technique employed was the same as that described above. Table I11 shows the results of the experiment. “ Soils,” Macmillan Co., 1903,p. 198. Owing t o the breaking of one of the bottles containing the Anaheim soil, the latter is left out of consideration here.
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In the table above given we find further confirmation, under incubator conditions, of the effects of temperature on the hygroscopicity of soils already noted in the preceding experiments. During a period of 9 days in the incubator, we find the soils to absorb more hygroscopic moisture with a rise in temperature and less with a fall in temperature. Here again by using the incubator, as did Patten and Gallagher, we obtain results that are the very reverse of their results. An interesting difference between the moisture absorptions taking place when the bell jar is in the incubator and those taking place when it is in the room, may be observed from a comparison of Tables I1 and 111. We are here confronted with a case of the absorptions taking place according to the same law in both experiments, namely, rising and falling with the temperature and yet the total capacity for holding hygroscopic moisture by the same soil in the incubator is considerably lower than i t is in the room, the atmosphere being saturated in both cases. This is a striking phenomenon which we are at a loss t o explain and for which indeed we have been unable to obtain an explanation from physicists and physical chemists t o whom we have appealed. We trust that this problem will, however, find solution in the near future. While thus the results obtained by the writers constitute a confirmation of the data obtained by Hilgard and while in our opinion there cannot be any other effects of temperature on the hygroscopicity of soils except as above enunciated, we cannot confirm any statement of a law for the increase in moisture absorption by a soil according to any arithmetical or geometrical ratio. At least the data adduced in our experiments do not justify the formulation of any such law. It will be found on an examination of the data above presented that in some cases the same amounts of moisture are not held by a given soil at the same temperature on two different days, especially when the temperature had been raised to 34" or 3.5' C from room temperature and then again cooled down to something near the room temperature. This can
720
Charles B . Lipman and Leslie T . Sharp
undoubtedly be explained by the deposition of dew, owing to the sudden lowering of the temperature in the latter case and the subsequent lack of time, before weighing, for the saturated atmosphere of the soil and air to come to equilibrium as a result of which the soil holds more water after sudden cooling from a high temperature to the room temperature than it did a t the latter before the temperature was raised. '
General Remarks and Practical Considerations It seems quite incomprehensible to us that data such as
those obtained by other investigators with the exception of Hilgard could have been found in saturated atmospheres, for in our experiments it was only under conditions where the atmosphere was manifestly undersaturated that soils displayed any tendency toward a loss of hygroscopic moisture with a rise in temperature. There must be mentioned here in further support of Hilgard's results and ouz own the work of Van Bemmelen' which, while carried out along different lines, brought to light the fact that the absorption of moisture by gelatinous hydrates increases with the vapor pressure. That being the case it appears clear that in a saturated atmosphere, a rise in temperature would be accompanied by a larger amount of vapor and hence of a greater vapor pressure and a greater moisture absorption. We feel constrained to emphasize again the importance of the eRect of the depth of the soil layer on the accuracy of the determination of the hygroscopic coefficients of soils. It is manifest from our experiments that only a very thin layer of soil can be saturated in a short time, and the latter consideration is important because it excludes the inevitable errors which must enter into such determinations through the deposition of dew during long exposure of the soil. We, therefore, must strongly urge the adoption of the Hilgard method for the determination of the hygroscopic powers of soils above described. We are now contemplating further experiments with the Hilgard method, looking toward the Zeit. anorg. Chernie, 5, 467, and literature there cited.
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determination of the number of hours necessary for the determinations of the hygroscopic coefficients of various types of soils. As t o the practical significance of the hygroscopic moisture of soils we need add but little to the very thorough investigations of Sachs and the criticisms thereof by Heinrich, Liebenberg and Mayer, based on elaborate experiments which are familiar to all engaged in investigations on soils. To these have also been added the observations of Hilgard' which point to the fact that hygroscopic moisture is neither sufficient for supplying plants with moisture as claimed by Sachs and others, nor is i t to be considered as of no significance a t all for plants as claimed by Mayer; but is to be looked upon as a protective agent for plants from the scorching effects of the hot, dry winds which, were it not for hygroscopic moisture and the cooling effects due to its evaporation by them, would cause the wilting and destruction of plants. From the data adduced by us it is plain that plants, especially those of the arid regions, would find great protection from the large amounts of hygroscopic moisture absorbed by soils with rising temperatures. We cannot agree further with Patten and Gallagher that equilibrium conditions are seldom found in the field, for the frequent and heavy fogs which occur so constantly in many climates are without doubt accompanied by equilibrium conditions and allow of the absorption of hygroscopic moisture as indicated by us. Moreover, field conditions cannot enter into a consideration of the proper method for determining hygroscopic moisture, nor into a study of the physical conditions which limit or enhance the powers of soils to absorb hygroscopic moisture from the atmosphere, because it is almost impossible in laboratory studies to simulate or even nearly to approximate a simulation of field conditions and therefore in our opinion the method which will give an accurate -
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Society for the Promotion of Agricultural Science, 1882,p. 118. See Hilgard, in publication just cited, for fuller discussion,
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Charles B . Lipman and Leslie T . Sharp
measure of soils with respect t o their hygroscopic powers is the only one that is tenable. That a t least will indicate the maximum hygroscopic powers of soils and give us relative hygroscopic coefficients of soils which will be of very considerable practical, as well as theoretical value. For a further interesting practical consideration of the value of hygroscopic moisture to plants and the importance of the proper method for its determination, we beg t o refer the reader to Hilgard's Soils," p. 214, in which is to be found a table which correlates the condition of piants in various soils with various amounts of water. The data there presented speak for themselves. I'
Conclusions ( I ) It is necessary to use thin layers of soils in determining their hygroscopic coefficients. A layer as nearly as possible I mm in depth has been found the best. Hilgard's method above described for the determinations is recommended to be used as a standard in all soil work on hygroscopicity. ( 2 ) A rise in temperature is accompanied by a greater absorption of hygroscopic moisture. A fall in temperature by a decreased absorption. These do not take place according to any definite law. (3) For the arid regions in particular, the hygroscopic moisture in soils has a certain definite practical importance. (4) Similar results on the effect of temperature on the absorption of hygroscopic moisture by soils are obtained in the incubator as in the room. The total absorptions in the incubator, however, as well as the variations with temperature are much smaller than in the room. We take occasion here to express our sincere thanks to Dr. E. W. Hilgard for kindly criticism and helpful suggestions in these investigations. Research Laboratory / o r Soils, Uiziversity of Califorlzia
