CHAPTER II. THE HABITAT

27. Definition of the habitat.The habitat is the sum of all the forces or factors present in a given area. It is the exact equivalent of the term environment, though the latter is commonly used in a more general sense. As an ecological concept, the habitat refers to an area much more definite in character, and more sharply limited in extent than the habitat of species as indicated in the manuals. Since the careful study of habitats has scarcely begun, it is impossible to recognize and delimit them in an absolute sense. Visible topographic boundaries often exist, but in many cases, the limit, though actual, is not readily perceived. Contiguous habitats may be sharply limited, or they may pass into each other so gradually that no real line of demarcation can be drawn. Whatever variations they may show, however, all habitats agree in the possession of certain essential factors, which are universally present. On the other hand, a few factors are merely incidental and may be present or absent. The relative value and amount of these is probably similar for no two habitats, though the latter readily fall into groups with reference to the amount of some particular factor.

28. Factors.The factors of a habitat are water-content, humidity, light, temperature, soil, wind, precipitation, pressure, altitude, exposure, slope, surface (cover), and animals. To these should be added gravity and polarity, which are practically uniform for all habitats, and may, in consequence, be ignored in this treatise. Length of season, while it plays an important part in vegetation, is clearly a complex and is to be treated under its constituents. Of the factors given, all are regularly found in each habitat, though some are not constantly present. The first five, water-content, humidity, light, temperature, and soil are the most important, and any one may well serve as a basis for grouping habitats into particular classes with reference to quantity. As will be pointed out later, however, water-content and light furnish the most striking differences between habitats, and offer the best means of classification. As habitats are inseparable from the formations which they bear, the discussion of the kinds of habitats is reserved for chapter IV.

Classification of Factors

29. The nature of factors.The factors of a habitat are arranged in two groups according to their nature: (1) physical, (2) biotic. In the strict sense, the physical factors constitute the habitat proper, and are the real causative forces. No habitat escapes the influence of biotic factors, however, as the formation always reacts upon it, and the influence of animals is usually felt in some measure. Physical factors are further grouped into (1) climatic and (2) edaphic, with respect to source, or, better, the medium in which they are found. Climatic, or atmospheric factors are humidity, light, temperature, wind, pressure, and precipitation. Axiomatically, the stimuli which they produce are especially related to the leaf. Edaphic or soil factors are confined to the soil, as the term denotes, and are immediately concerned with the functions of the root. Water-content is by far the most important of these; the others are soil composition (nutrient-content), soil temperature, altitude, slope, exposure, and surface. The last four are of a more general character than the others, and are usually referred to as physiographic factors. Cover, when dead, might well be placed among these also, but as it is little different from the living cover in effect, it seems most logical to refer it to biotic factors.

30. The influence of factors.While the above classification is both obvious and convenient, a more logical and intimate grouping may be made upon the influence which the factor exerts. On this basis, factors are divided into (1) direct, (2) indirect, and (3) remote. Direct factors are those which act directly upon an important function of the plant and produce a formative effect: for example, an increase in humidity produces an immediate decrease in transpiration. They are water-content, humidity, and light. Other factors have a direct action: thus temperature has an immediate influence upon respiration and probably assimilation also, but it is not structurally formative. Wind has a direct mechanical effect upon woody plants, but it does not fall within our definition. Indirect factors are those that affect a formative function of the plant through another factor; thus a change in temperature causes a change in humidity and this in turn calls forth a change in transpiration; or, a change in soil texture increases the water-content, and this affects the imbibition of the root-hairs. Indirect factors, then, are temperature, wind, pressure, precipitation, and soil composition. Remote factors are, for the most part, physiographic and biotic: they require at least two other factors to act as middlemen. Altitude affects plants through pressure, which modifies humidity, and hence transpiration. Slope determines in large degree the run-off during a rainstorm, thusaffecting water-content and the amount of water absorbed. Earthworms and plant parts change the texture of the soil, and thereby the water-content. Indirect factors often exert a remote influence also, as may be seen in the effect which temperature and wind have in increasing evaporation from the soil, and thus reducing the water-content. This distinction between factors may seem insufficiently grounded. In this event, it should be noted that it centers the effects of all factors upon the three direct ones, water-content, humidity, and light. If it further be recalled that these are the only factors which produce qualitative structural changes, and that the classification of ecads and formations is based upon them, the validity of the distinction is clear.

31. The need of exact measurement.Any serious endeavor to find in the habitat those causes which are producing modification in the plant and in vegetation can not stop with the factors merely. The next step is to determine the quantity of each. It is not sufficient to hazard a guess at this, or to make a rough estimate of it. Habitats differ in all degrees, and it is impossible to institute comparisons between them without an exact measure of each factor. Similarly, one can not trace the adaptations of species to their proper causes unless the quantity of each factor is known. It is of little value to know the general effect of a factor, unless it is known to what degree this effect is exerted. For this purpose it becomes necessary to appeal to instruments, in order to determine the exact amount of each factor that is present in a particular habitat, and hence to determine the ratio between the stimulus and the amount of structural adjustment which results. The employment of instruments of precision is clearly indispensable for the task which we have set for ecology, and every student that intends to strike at the root of the subject, and to make lasting contributions to it, must familiarize himself with instrumental methods. One great benefit will accrue to ecology as soon as this fact is generally recognized. The use of instruments and the application of results obtained from them demand much patience and seriousness of purpose upon the part of the student. As a consequence, there will be a general exodus from ecology of those that have been attracted to it as the latest botanical fad, and have done so much to bring it into disrepute.

32. The value of meteorological methods.At the outset there must be a very clear understanding that weather records and readings have only a very general value. This is in spite of the fact that the instruments employed are of standard precision. An important reason for this lack ofvalue is that readings are not made in a particular habitat; as a rule, indeed, they are made in towns and cities, and hence are far removed from masses of vegetation. They are usually taken at considerable heights, and give but a general indication of the conditions at the level of vegetation. The chief difficulty, however, is that the factors observed at weather stations—temperature, pressure, wind, and precipitation—are those which have the least value for the ecologist. It is true that a knowledge of the temperature and rainfall of a great region will afford some idea of the general character of its vegetation. A proper understanding of such a vegetation is, however, to be gained only through the exact study of its component formations. Ecology has already incurred sufficient censure as a subject composed of very general ideas, and the use of meteorological data, which can never be connected definitely with anything in the plant or the formation, should be discontinued. This must not be understood to mean that meteorological instruments can not be used in the proper place and manner, i. e., in the habitat.

33. Habitat determination.It is self-evident that determinations of factors by instruments can only be of value in the habitat where they are made. In other words, a habitat is a unit for purposes of measuring its factors, and measures of one habitat have no exact value in another. This fact can not be overstated. Thus, while it is perfectly legitimate, and indeed highly desirable, to locate thermographs in different mountain zones for ascertaining the rate at which temperature decreases with altitude, the data obtained in this way are not directly applicable in explanation of plant or formation changes, except when the same species occurs at different altitudes. Special methods are valuable and often absolutely necessary, but in view of the fact that the plant as well as the formation is the definite product of a definite habitat, the fundamental rule in instrumentation is that complete readings must be made within a habitat for that habitat alone. This necessarily presupposes a certain preliminary acquaintance with the habitat to be investigated, as it is imperative that the station for making readings be located well within the formation, in order to avoid transition conditions. In vegetation, there are as many habitats as formations, and in addition a large number of new and denuded habitats upon which successions have not yet started; a knowledge of each formation or succession must rest ultimately upon the factors of its particular habitat.

34. Determinable and efficient differences.The instruments employed in studying habitats can not be too exact, as there is no adequate knowledge as yet concerning the real differences which exist between related or contiguous formations. This is particularly true of differences which areefficient in producing a recognizable structural change in plant or formation. Investigations made by the writer have shewn that standard instruments will measure differences of quantity quite too small to produce a visible reaction. Efficient differences are not the same for different factors, and perhaps also for the same factor when found in various combinations. They vary widely for different species, being in direct relation to the plasticity of the latter. The point necessary to bear in mind in formulating methods for habitat investigation and in making use of instruments is that standard instruments should be used for the very reason that we do not yet know the relation between determinable and efficient differences. On the other hand, it is unnecessary to insist upon absolute exactness as soon as it is found that the determinable difference lies well within the efficient one. This by no means indicates that instruments are not to be carefully standardized and frequently checked, or that accurate readings should not be made. It means that a slight margin of error may be permitted in a machine which registers well within the efficient difference for that factor, and that instruments that read to the last degree of nicety are not absolutely necessary. In the fundamental work of determining efficient differences, however, instruments can not have too great precision. Moreover, these differences must be based upon the most plastic species of a formation, and the readings must be made under normal conditions.

35. Methods.In the field use of instruments two methods have been developed. The first in point of time was the method of simple instruments, devised especially for class work, and capable of being used only where a number of trained students are available. The method of automatic instruments was an immediate outgrowth of this, due to the necessity which confronts the solitary investigator of being in different habitats at the same time. In the gradual evolution of this subject, it has become possible to combine the two methods in such a way as to retain all the advantages of the automatic method, and most of those of the method of simple instruments.

36. Method of simple instruments.By simple instruments are denoted those that do not record, but must be read by the observer at the time. They are standard instruments of precision, but possess the disadvantage of requiring an observer for each one. They are well illustrated by the thermometers and psychrometers used by the Weather Bureau. In the hands of trained observers the results obtained are unimpeachable; in fact, standard simple instruments must be constantly employed to check automaticones. As physical factors vary greatly through the day and through the year, it is all-important that the readings in habitats which are being compared should be made at the same instant. This requires a number of observers; as many as twelve stations have been read at one time, and there is of course no limit to the number. It is very important, also, that observers be carefully trained in the handling of instruments, and in reading them accurately and intelligently at the proper moment. In practice it has been found impossible to do such work in elementary classes, and, even in using small advanced classes, prolonged drill has been necessary before trustworthy results could be obtained. When a class has once been thoroughly trained in making accurate simultaneous readings, there is practically no limit, other than that set by time, to the valuable work that can be done, both in instruction and investigation.

37. Method of automatic instruments.The solitary investigator must replace trained helpers by automatic instruments or ecographs. These have the very great advantages of giving continuous simultaneous records for long periods, and of having no personal equation. They must be regulated and checked, to be sure, but as this is all done by the same person, the error is negligible. There is nothing more satisfactory in resident investigation than a series of accurate recording instruments in various habitats. Ecographs have two disadvantages. The chief perhaps is cost. The expense of a single “battery” which will record light, water-content, humidity, and temperature is about $250. Another difficulty is that they can be used only within a few miles of the base, since they require attention every week for regulation, change of record, etc. While this means that ecographs in their present form are not adapted to reconnaissance, this is not a real disadvantage, as the scattered observations possible on such a journey can best be made by simple instruments.

38. Combined methods.The best results by far are to be obtained by the combined use of simple and automatic instruments. This is particularly true in research, but it applies also to class instruction. The ecographs afford a continuous, accurate basal record, to which a single reading made at any time or place can be readily referred for comparison. On the other hand, it is an easy matter to carry a full complement of simple instruments on the daily field trips, and to make accurate readings in a score or more of formations in a single day. An isolated reading, especially of a climatic factor, has little or no value in itself, but when it can be compared with a reading made at the same time in the base station by an ecograph, it is the equivalent of an automatic reading. This method renders a set of simple instrumentsmore desirable for a long trip or reconnaissance than a battery of automatic ones. It is practically impossible to carry the latter into the field, and in any event a continuous record is out of the question. As there are other tasks at such times also, it becomes evident that the taking of single readings which can be compared with a continuous record offers the most satisfactory solution.

39. The selection of instruments.In selecting and devising instruments for the investigation of physical factors, emphasis has first been laid upon accuracy. This is the result of a feeling that it is better to have instruments that read too minutely than those which do not make distinctions that are sufficiently close, particularly until more has been learned about efficient differences. On the other hand, no hesitation has been felt in employing instruments which are not absolutely accurate, when it was clear that the error was less than the efficient difference. Similarly, the margin of error practically eliminates itself in the case of simultaneous comparative readings, when the instruments have been checked to the same standard. Simplicity of construction and operation are of great importance, especially in saving time where a large number of instruments are in operation. Expense is likewise to be carefully considered. It is impossible to have too many instruments, but cost practically determines the number that can be obtained. It is further necessary to secure or invent both simple and automatic instruments for all factors, except such invariable ones as altitude, slope, etc. Simple instruments must be of a kind that can be easily carried, and so constructed that they can be used at a minimum of risk. The sling psychrometer, for example, is very readily broken in field use, and it has been replaced by a protected modification, the rotating form.

In describing the construction and operation of the many factor instruments, there has been no attempt to make the treatment exhaustive. Those instruments which the author has found of greatest value in his own work are given precedence, and the manner of using them is described in detail. Other instruments of value are also considered, though with greater brevity. Some of the most complex and expensive ones have been ignored, as it is altogether improbable that they can come into general use in their present form. While the conviction is felt that the methods described below will enable the most advanced investigators to carry on thorough work, it is hoped that they will be seen to be so fundamental, and so attractive, that they will appeal to all who are planning serious ecological study.

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.

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 geotomehas 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 lessthan 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 entirelyreliable 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 fromw. 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 abase 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.

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 thehabitat. 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), andechard(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 subtractedfrom 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.

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 accurateresearch, 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.

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:

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