WATER-CONTENT
40. Value of different instruments. The paramount importance of water-content as a direct factor in the modification of plant form and distribution gives a fundamental value to the methods used for its determination. Automatic instruments for ascertaining the water in the soil are costly, in addition to being complicated, and often inaccurate. For these reasons, much attention has been given to developing the simpler but more reliable methods in which a soil borer or geotome is used. The latter is simple, inexpensive, and accurate. It can be carried easily upon daily trips or upon longer reconnaissances, and is always ready for instant use. In the determination of physiological water-content, it is practically indispensable. Indeed, the readiness with which geotome determinations of water-content can be made should hasten the universal recognition of the fact that it is the available, and not the total amount of water in the soil, which determines the effect upon the plant.
Geotome Methods
Fig. 1. Geotomes and soil can.
41. The geotome. In its simplest form, the geotome is merely a stout iron tube with a sharp cutting edge at one end and a firmly attached handle at the other. The length is variable and is primarily determined by the location of the active root surface of the plant. In xerophytic habitats, generally a longer tube is necessary than in mesophytic ones. The bore is largely determined by the character of the soil; for example, a larger one is necessary for gravel than for loam. Tubes of small bore also tend to pack the soil below them, and to give a correspondingly incomplete core. The best results have been obtained with geotomes of ½–1 inch tube. Each geotome has a removable rod, flattened into a disk at one end, and bent at the other, for forcing out the core after it has been cut from the soil. Sets of geotomes have been made in lengths of 5, 10, 12, 15, 20, and 25 inches. The 12– and 15–inch forms have been commonly used for herbaceous formations and layers. They are marked in inches so that a sample of any lesser depth may be readily taken. Such a device is very necessary for gravel soils and in mountain regions, where the subsoil of rock lies close to the surface.
Fig. 2. Fraenkel soil borer.
Fig. 3. American soil borer.
42. Soil borers. There is a large variety of soil borers to choose from, but none have been found as simple and satisfactory for relatively shallow readings as the geotome just described. For deep-rooted plants, many xerophytes, shrubs, and trees, borers of the auger type are necessary. These are large and heavy, and of necessity slow in operation. They can not well be carried in an ordinary outfit of instruments, and the size of the soil sample itself precludes the use of such instruments far from the base station, except on trips made expressly for obtaining samples from deep-seated layers. For depths from two to eight feet, the Fraenkel borer is perhaps the most satisfactory, except for the coarser gravels: it costs $14 or $20 according to the length. For greater depths, or when a larger core is desirable, the Bausch & Lomb borer, number 16536, which costs $5.25, should be made use of. This is a ponderous affair and can be employed only on special occasions. On account of the size of samples obtained by these borers, it is usually most satisfactory to take a small sample from the core at different depths. Frequently, indeed, a hand trowel may be readily used to obtain a good sample at a particular depth.
43. Taking samples of soil. In obtaining soil samples, the usual practice is to remove the air-dried surface, noting its depth, and to sink the geotome with a slow, gentle, boring movement, in order to avoid packing the soil. This difficulty is further obviated by deep notches with sharp, beveled edges which are cut at the lower end. In obtaining a fifteen-inch core, there is also less compression if it be cut five inches at a time. Repeated tests have shown, however, that the single compressed sample is practically as trustworthy as the one made in sections. The water-content of the former constantly fell within .5 per cent of that of the latter, and both varied less than 1 per cent from the dug sample used as a check. As soon as dug, the core is pressed out of the geotome by the plunger directly into an air-tight soil can. Bottles may be used as containers, but tin cans are lighter and more durable. Aluminum cans have been devised for this purpose, but on account of the expense, “Antikamnia” cans have been used instead. These are tested, and those that are not water-tight are rejected, although it has been found that, even in these, ordinary soils do not lose an appreciable amount of water in twenty-four hours. The lid should be screwed on as quickly as possible, and, as an added precaution, the cans are kept in a close case until they have been weighed. The cans are numbered consecutively on both lid and side in such a way that the number may be read at a glance. The numbers are painted, as a label wears off too rapidly, and scratched numbers are not quickly discerned.
Fig. 4. Field balance.
44. Weighing. Although soil samples have been kept in tight cans outside of cases for several days without losing a milligram of moisture, the safest plan is to make it a rule to weigh cans as quickly as possible after bringing them in from the field. Moreover, when delicate balances are available, it is a good practice to weigh to the milligram. At remote bases, however, and particularly in the field, and on reconnaissance, where delicate, expensive instruments are out of place, coarser balances, which weigh accurately to one centigram, give satisfactory results. The study of efficient water-content values has already gone far enough to indicate that differences less than 1 per cent are negligible. Indeed, the soil variation in a single square meter is often as great as this. The greatest difference possible in the third place, i. e., that of 9 milligrams, does not produce a difference of .1 of 1 per cent in the water-content value. In consequence, such strong portable balances as Bausch & Lomb 12308 ($2), which can be carried anywhere, give entirely reliable results. The best procedure is to weigh the soil with the can. Turning the soil out upon the pan or upon paper obviates one weighing, but there is always some slight loss, and the chances of serious mishap are many. After weighing, the sample is dried as rapidly as possible in a water bath or oven. At a temperature of 100° C. this is accomplished ordinarily in twenty-four hours; the most tenacious clays require a longer time, or a higher temperature. High temperatures should be avoided, however, for soils that contain much leaf mould or other organic matter, in order that this may not be destroyed. When it is necessary on trips, soil samples can be dried in the sun or even in the air. This usually takes several days, however, and a test weighing is generally required before one can be certain that the moisture is entirely gone. The weighing of the dried soil is made as before, and the can is carefully brushed out and weighed. The weight of aluminum cans may be determined once for all, but with painted cans it has been the practice to weigh them each time.
45. Computation. The most direct method of expressing the water-content is by per cents figured upon the moist soil as a basis. The ideal way would be to determine the actual amount of water per unit volume, but as this would necessitate weighing one unit volume at least in every habitat studied, as a preliminary step, it is not practicable. The actual process of computation is extremely simple. The weight of the dried sample, w1, is subtracted from the weight of the original sample, w, and the weight of the can, w2, is likewise subtracted from w. The first result is then divided by the second, giving the per cent of water, or the physical water-content. The formula is: (w − w1)/(w − w2) = W. The result is expressed preferably in grams per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per cent of water-content may readily be figured on the basis of dry or moist soil.
46. Time and location of readings. Owing to the daily change in the amount of soil water due to evaporation, gravity, and rainfall, an isolated determination of water-content has very little value. It is a primary requisite that a basis for comparison be established by making (1) a series of readings in the same place, (2) a series at practically the same time in a number of different places or habitats, or (3) by combining the two methods, and following the daily changes of a series of stations throughout an entire season, or at least for a period sufficient to determine the approximate maximum and minimum. The last procedure can hardly be carried out except at a base station, but here it is practically indispensable. It has been followed both at Lincoln and at Minnehaha, resulting in a basal series for each place that is of the greatest importance. When such a base already exists, or, better, while it is being established, scattered readings may be used somewhat profitably. As a practical working rule, however, it is most convenient and satisfactory to make all determinations consecutively, i. e., in a series of stations or of successive days. Under ordinary conditions, the time of day at which a particular sample is taken is of little importance, as the variation during a day is usually slight. This does not hold for exposed wet soils, and especially for soils which have just been wetted by rains. In all comparative series, however, the samples should be taken at the same hour whenever possible. This is particularly necessary when it is desired to ascertain the daily decrease of water-content in the same spot. In the case of a series of stations, these should be read always in the same order, at the same time of day, and as rapidly as possible. When a daily station series is being run, i. e., a series by days and stations both, the daily reading for each place should fall at the same time. While there are certain advantages in making readings either early or late in the day, they may be made at any time if the above precautions are followed.
47. Location of readings. Samples should invariably be taken in spots which are both typical and normal, especially when they are to be used as representative of a particular area or habitat. A slight change in slope, soil composition, in the amount of dead or living cover, etc., will produce considerable change in the amount of water present. Where habitat and formation are uniform, fewer precautions are necessary. This is a rare circumstance, and as a rule determinations must be made wherever appreciable differences are in evidence. The problem is simpler when readings are taken with reference to the structure or modifications of a particular species, but even here, check readings in several places are of great value. The variation of water in a spot apparently uniform has been found to be slight in the prairies and the mountains. In taking three samples in spots a few inches to several feet apart, the difference in the amount of water has rarely exceeded 1 per cent, which is practically negligible. Gardner[[2]] found that 16 samples taken to a depth of 3 inches, in as many different portions of a carefully prepared, denuded soil plot, showed a variation of 7½ per cent. This is partially explained by the shallowness of the samples, but even then the results of the two investigations are in serious conflict and indicate that the question needs especial study. It should be further pointed out that all readings should be made well within a particular area, and not near its edge, and that, in the case of large diversified habitats, it is the consocies and the society which indicate the obvious variations in the structure of the habitat.
48. Depth of samples. The general rule is that the depth of soil samples is determined by the layer to which the roots penetrate. The practice is to remove the air-dried surface in which no roots are found, and to take a sample to the proper depth. This method is open to some objection, as the actively absorbing root surfaces are often localized. There is no practical way of taking account of this as yet, except in the case of deep-rooted xerophytes and woody plants. It is practicable to determine the location of the active root area of a particular plant and hence the water-content of the soil layer, but in most formations, roots penetrate to such different depths that a sample which includes the greater part of the distance concerned is satisfactory. Some knowledge of the soil of a formation is also necessary, since shallow soils do not require as deep samples as others. The same is true of shaded soils without reference to their depth, and, in large measure, of soils supplied with telluric water. In all cases, it is highly desirable to have numerous control-samples at different depths. The normal cores are 12 or 15 inches; control-samples are taken every 5 inches to the depth desired, and in some cases 3–inch sections are made. It has been found a great saving of time to combine these methods. A 5–inch sample is taken and placed in one can, then a second one, and a third in like manner. In this way the water-content of each 5–inch layer is determined, and from the combined weight the total content is readily ascertained.
49. Check and control instruments. A number of instruments throw much light upon the general relations of soil water. The rain-gauge, or ombrometer, measures the periodical replenishment of the water supply, and has a direct bearing upon seasonal variation. The atmometer affords a clue to the daily decrease of water by evaporation, and thus supplements the rain-gauge. The run-off gauge enables one to establish a direct connection between water-content and the slope and character of the surface. The amount and rapidity of absorption are determined by means of a simple instrument termed a rhoptometer. The gravitation water of a soil is ascertained by a hizometer, and some clue to the hygroscopic and capillary water may be obtained by an artificial osmotic cell. All of these are of importance because they serve to explain the water-content of a particular soil with especial reference to the other factors of the habitat. It is evident that none of them can actually be used in exact determinations of the amount of water, and they will be considered under the factors with which, they are more immediately concerned.
Physical and Physiological Water
50. The availability of soil water. The amount of water present in a soil is no real index to the influence of water-content as a factor of the habitat. All soils contain more water than can be absorbed by the plants which grow in them. This residual water, which is not available for use, varies for different soils. It is greatest in the compact soils, such as clay and loam, and least in the loose ones, as sand and gravel. It differs, but to a much less degree, from one species to another. A plant of xerophytic tendency is naturally able to remove more water from the same soil than one of mesophytic or hydrophytic character. As the species of a particular formation owe their association chiefly to their common relation to the water-content of the habitat, this difference is of little importance in the field. In comparing the structure of formations, and especially that of the plants which are found in them, the need to distinguish the available water from the total amount is imperative. Thus, water-contents of 15 per cent in gravel and in clay are in no wise comparable. A coarse gravel containing 15 per cent of water is practically saturated. The plants which grow upon it are mesophytes of a strong hydrophytic tendency, and they are able to use 14½ or all but .5 out of the 15 per cent of water. In a compact clay, only 3½ of the 15 per cent are available, and the plants growing in it are marked xerophytes. It is evident that a knowledge merely of the physical water-content is actually misleading in such cases, and this holds true of comparisons of any soils which differ considerably in texture. After one has determined the physiological water for the great groups of soils, it is more or less possible to estimate the amounts in the various types of each. As an analysis is necessary to show how close soils are in texture, this is little better than a guess, and for accurate work it is indispensable that the available water be determined for each habitat. Within the same formation, however, after this has once been carefully ascertained, it is perfectly satisfactory to convert physical water-content into available by subtracting the non-available water, which under normal conditions in the field remains practically the same.
The importance of knowing the available water is even greater in those habitats in which salts, acids, cold, or other factors than the molecular attraction of soil particles increase the amount of water which the plant can not absorb. Few careful investigations of such soils have yet been made, and the relation of available to non-available water in them is almost entirely unknown. It is probable that the phenomena in some of these will be found to be produced by other factors.
51. Terms. The terms, physiological water-content, and physical water-content, are awkward and not altogether clear in their application. It is here proposed to replace them by short words which will refer directly to the availability of the soil water for absorption by the plant. Accordingly, the total amount of water in the soil is divided into the available and the non-available water-content. The terms suggested for these are respectively, holard (ὅλos, whole, ἅpδov, water), chresard (χοῆςις, use), and echard (ἕχω, to withhold).
52. Chresard determinations under control. The determination of the chresard in the field is attended with peculiar difficulties. In consequence, the method of obtaining it under control will first be described. The inquiry may be made with reference to soils in general or to the soil of a particular formation. In the last case, if the plants used are from the same formation, the results will have almost the value of a field determination. When no definite habitat is the subject of investigation, an actual soil, and not an artificial mixture, should be used, and the plants employed should be mesophytes. The individual plants are grown from seeds in the proper soils, and are repotted sufficiently often to keep the roots away from the surface. The last transfer is made to a pot large enough to permit the plant to become full-grown without crowding the roots. The pot should be glazed inside and out in order to prevent the escape of moisture. This interferes slightly with the aeration of the soil, but it will not cause any real difficulty. The plant is watered in such a way as to make the growth as normal as possible. After it has become well established, three soil samples are taken in such a manner that they will give the variation in different parts of the pot. One is taken near the plant, the second midway between the plant and the edge of the pot, and the third near the edge. The depth is determined by the size of the pot and the position of the roots. The holard is determined for these in the usual way, but the result is expressed with reference to 100 grams of dry soil; the average is taken as representative. The soil is then allowed to dry out slowly, as sudden drouth will sometimes impair the power of absorption and a plant will wilt although considerable available water remains. Plants often wilt in the field daily for several successive hot dry days, and become completely turgid again during the night. If the drying out takes place slowly, the plant will not recover after it has once begun to wilt. The proper time to make the second reading is indicated by the pronounced wilting of the leaves and shoots. Complete wilting occurs, as a rule, only after the younger parts have drawn for some time upon the watery tissues of the stem and root, by which time evaporation has considerably deceased the water in the soil. It is a well-known fact that young leaves do not wilt easily, especially in watery or succulent plants. Three samples are again taken and the average water-content determined as above. This is the non-available water or the echard. The latter is then computed on the basis of 100 grams of dry soil, and this result is subtracted from the holard to give the chresard in grams for each 100 grams of dry weight. The chresard may also be expressed with respect to 100 grams of moist soil. As a final precaution in basal work, it is advisable to determine the chresard for six individuals of the same species under as nearly the same conditions as possible. When it is desired, however, to find the average chresard for a particular soil, it is necessary to employ various species representing diverse phyads and ecads. Such an investigation is necessarily very complicated, and must be made the subject of special inquiry.
53. Chresard readings in the field. The especial difficulties which must be overcome in the field are the exclusion of rain and dew and the cutting off of the capillary water. It is evident, of course, that experiments of this sort must also be entirely free from outside disturbance. The choice of an area depends upon the scope of the study. If the chresard is sought for a particular consocies, the block of soil to be studied should show several species which are fairly representative. In case the chresard of a certain species is to be obtained, this species alone need be present, but it should be represented by several individuals. Check plots are desirable in either event, and at least two or three which are as nearly uniform as possible should be chosen. The size and depth of the soil block depends upon the plants concerned. It must be large enough that the roots of the particular individuals under investigation are not disturbed. There is a limit to the size of the mass that can be handled readily, and in consequence the test plants must not be too large or too deeply rooted. The task of cutting out the soil block requires a spade with a long sharp blade. After ascertaining the spread and depth of the roots, the block is cut so that a margin of several inches free from the roots concerned is left on the sides and bottom. If the block is to be lifted out of place, so that the sides are exposed to evaporation, this allowance should be greater. In some cases, it may be found more convenient to dig the plant up, place it in a large pot, and put the latter back in the hole. As a general practice, however, this is much less satisfactory.
After the block has been cut, it may be moved if the soil is sufficiently compact, and then allowed to dry out in its own formation or elsewhere. The results are most valuable in the first case, though it is often an advantage to remove blocks cut from shade or wet formations to dry, sunny stations where they will dry more rapidly. The most satisfactory and natural method, however, is to leave the block in place, and to prevent the reestablishment of capillary action by enclosing it within plates. This is accomplished by slipping thin sheet-iron plates into position along the cut surfaces. The plate for the bottom should be somewhat wider than the block, and is slipped into place by raising the block if the soil is not too loose; in the latter event, it is carefully driven in. The side plates are then pushed down to meet the former. The size of the plates depends upon the block; in general, plates of 1, 2, and 3 feet square, with the bottom plates a trifle larger, are the most serviceable. Access of rain and dew is prevented by an awning of heavy canvas which projects far enough beyond each side of the block to prevent wetting. The height will depend of course upon the size of the plants. The awning must be used only when rain or heavy dew is threatened, as the shade which it produces changes the power of the plant to draw water from the soil.
The time necessary to cause wilting varies with the habitat and the weather. When the block is large and in position, two or three weeks are required. This period of drying incidentally furnishes an excellent opportunity for determining the rate at which the particular soil loses water. The holard sample is taken daily for several days before the block is cut out, in order to obtain an average, care being taken of course to avoid a period of extreme weather. The echard samples are taken as soon as the wilting is sufficient to indicate that the limit of available water is reached. The air-dry soil above the roots is first removed. The treatment of the samples and the computation of the chresard are as previously indicated.
54. Chresard values of different soils. The following table gives the water-content values of six representative soils. The per cents of holard (at saturation) and of echard are those determined by Hedgcock[[3]] with six mesophytes as test plants for each soil. The chresard has been computed directly from these.
| HOLARD | ECHARD | CHRESARD | ||||
|---|---|---|---|---|---|---|
| Sand | 14.3 | 12.6 | .3 | .25 | 14 | 12.3 |
| Clay | 47.4 | 32.5 | 9.3 | 6.3 | 38.1 | 26.2 |
| Loess | 59.3 | 37.1 | 10.1 | 6.4 | 49.2 | 30.7 |
| Loam | 64.1 | 39.1 | 10.9 | 6.6 | 53.2 | 32.5 |
| Humus | 65.3 | 39.6 | 11.9 | 7.2 | 53.4 | 32.4 |
| Saline | 68.5 | 40.8 | 16.2 | 9.6 | 52.3 | 31.2 |
The first column indicates the per cent based upon the dry weight, the second upon the weight of the moist soil.
While these can not be considered absolute for a particular soil other than the ones investigated, they are found to correspond somewhat closely to the results obtained for other soils of the respective groups. For accurate research, the chresard must of course be ascertained for each formation with respect to its peculiar plants and soil. The influence of the ecad in more or less determining the echard is also shown by Hedgcock, who found that floating plants wilt at 25 per cent, amphibious ones at 15–20 per cent, mesophytes at 6–12 per cent, and mesophytic xerophytes at 3–6 per cent. The echard is also somewhat higher for shade plants than for heliophytes.
Records and Results
55. The field record. It is superfluous to point out that a definite form for field records saves much time and prevents many mistakes. The exact form may be left to personal taste, but there are certain features which are essential. Many of these are evident, while others may seem unnecessary; all, however, have been proved by experience to have some value in saving time or in preventing confusion. The two fundamental maxims of field work are that nothing is too trivial to be of importance, and that no detail should be entrusted to the memory. The field record should contain in unmistakable terms all that the field has yielded. These statements apply with especial force to water-content, in many senses the most important of physical factors. The precise character of the record depends upon the way in which the readings are made, whether scattered or in series. As the day-station series is of the greatest importance, the record is adapted for it especially, but it will also serve for all readings. The record is chronological, since this is the only convenient method for the field. A proper form for a field record of water-content is the following:
| Can No. | Date | Formation | Station | Community | Soil | Sample | HOLARD | ECHARD | Chresard | NOTES | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Weighings | % | Weighings | % | Sky | Rainfall | ||||||||||||
| 1st | 2d | Can | 1st | 2d | Can | ||||||||||||
| 10 | 2/8/04 | Spruce forest | Jack Brook | Mertensiare | Loam | 10 | 58.7 | 50.1 | 25.52 | 2 | 58.7 | 53.41 | 25.52 | 10 | 16 | Cloudy | 0 |
| 17 | „ | Spruce forest | Milky Way | Gentianare | ¼ mold ¾ gravel | 10:2 | 64.25 | 57.5 | 21.35 | 16 | 64.25 | 59.6 | 21.35 | 5 | 11 | Cloudy | 0 |
| 40 | „ | Gravel slide. | Hiawatha | Asterare | Gravel | 2:10 | 78.55 | 74.3 | 22.85 | 8 | 78.55 | 74.85 | 22.85 | 1 | 7 | Cloudy | 0 |
A general designation of the soil composition is a material aid, especially where there is a difference in the core. For example, in a mountain forest or meadow, the upper layer will usually be mold, the lower sand or gravel. A careful estimate of the relation between the two throws much light upon the chresard. Under “sample” the number taken to reach the desired depth, if more than one, is indicated by placing the number before the depth, thus 2:10. When two or more full cores are included in the same sample for a check, the order is 10:2. It has already been shown, however, that these precautions are not necessary for ordinary purposes. In computing the holard and echard, there is no need to show the figuring, if the process is checked and then proved. Notes upon sky conditions aid in explaining the daily decrease in water-content. The amount of rain and the period during which it falls are of great importance in understanding the fluctuations of the holard. Under community it is highly desirable to have a list of all the species, but it is impossible to include this in the table, and a glance at the formation list will show them. The form indicated above serves for a day-station series, a daily series in one station for any number of check series in one spot, and for scattered readings. In many cases the echard will not be determined, but on account of its primary importance, there should be a space for it, especially since it may be desirable to determine it at some later time.
56. The permanent record. This should be kept by formation, or if the latter exhibits well-defined associations, the formational record may be divided accordingly. This may seem an unnecessary expenditure of time, but a slight experience in finding the water-content values of a particular habitat, when scattered through a chronological field record, will be convincing. The form of permanent record is the same as for the field, except that the column for the formation and that for the society are often unnecessary.
57. Sums and means. From the great difficulty of determining the absolute water-content, and of obtaining a standard of comparison between soils on account of the varying ratio between bulk and weight, water-content sums are impracticable. For the same reasons, means of actual water-content are practically impossible, and the mean water-content must be expressed in per cents. Daily readings are not essential to a satisfactory mean. In fact, a single reading at each extreme enables one to approximate the real mean very closely; thus, the average of 26 readings in the prairie formation is 18 per cent. The extremes are 5 per cent and 28 per cent, and their average 16.5 per cent. A few readings properly scattered through moist and dry periods will give a reliable mean, as will also a series of daily readings from one heavy rain through a long dry period. The one difficulty with the last method is that such periods can not well be determined beforehand. Means permit ready comparison between habitats, but in connecting the modifications of a species with water-content as a cause, the extremes are significant as indicating the range of conditions. Furthermore, the extremes, i. e., 5 per cent and 28 per cent, make it possible to approximate the mean, 18 per cent, while the latter gives little or no clue to the extremes. It is hardly necessary to state that means and extremes should be determined for a certain habitat, or particular area of it, and that the results may be expressed with reference to holard and chresard.
58. Curves. The value of graphic methods and the details of plotting curves are reserved for a particular section. It will suffice in this place to indicate the water-content curves that are of especial value. Simple curves are made with regard to time, place, or depth. The day curve shows the fluctuations of the water-content of one station from day to day or from time to time. The station curve indicates the variation in water from station to station, while the depth curve represents the different values at various depths in the same station. These may be combined on the same sheet in such a way that the station curves of each day may be compared directly. Similar combinations may be used for comparing the day curves, or the depth curves of different stations, but these are of less importance. A combination of curves which is of the greatest value is one which admits of direct comparison between the station curves of saturation, holard, chresard, and echard.