116. Kinds of soils. It is very doubtful whether it is worth while to attempt to distinguish soils upon the basis of mechanical analysis. Unquestionably, the most satisfactory method is to distinguish them with respect to holard and chresard, and to regard texture as of secondary importance. A series of soil classes which comprise various soil types has been proposed by the U. S. Bureau[[7]] of Soils as follows: (1) stony loam, (2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine sand, (7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) silt loam, (12) clay loam, (13) clay, (14) adobe. These are based entirely upon mechanical analyses, and in some cases are too closely related to be useful. The line between them can nowhere be sharply drawn. Indeed, the variation within one class is so great that soils have frequently been referred to the wrong group. Thus, Cassadaga sand (gravel 22 per cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely related to Oxnard sandy loam (26–37–18–12) and to Afton fine sandy loam (28–43–18–8) than to Coral sand (61–29–3–4), Galveston sand (6–91–1–1), or Salt Lake sand (84–15–1–0). Elsinore sandy loam (8–38–35–10) is much nearer to Hanford fine sandy loam (9–36–33–14) than to Billings sandy loam (1–60–22–11) or to Utuado sandy loam (48–23–19–8). The soil types are much more confused, and for ecological purposes at least are entirely valueless. Lake Charles fine sandy loam has the composition, 1–34–52–9; Vernon fine sandy loam, 1–37–54–7, while many other so-called types show nearly the same degree of identity.
117. The chemical nature of soils. The effect of alkaline and acid substances in the soil upon water-content and the activities of the plant is far from being well understood. It is generally recognized that salts and acids tend to inhibit the absorptive power of the root-hairs. In the case of saline soils, this inhibitive effect seems to be established, but the action of acids in bogs and swamps is still an open question. It is probable that the influence of organic acid has been overestimated, and that the curious anomaly of a structural xerophyte in a swamp is to be explained by the stability of the ancestral type and by the law of extremes. Apart from the effect which excessive amounts of acids and salts may have in reducing the chresard, the chemical character of the soil is powerless to produce structural modification in the plant. Since Thurmann’s researches there has been no real support of the contention that the chemical properties of the soil, not its physical nature, are the decisive factors in the distribution and adaptation of plants. It is not sufficient that the vegetation of a silicious soil differs from that of a calcareous one. A soil can modify the plants upon it only though its water-content, or the solutes it contains. Hence, the chemical composition of the original rock is immaterial, except in so far as it modifies these two factors. Humus, moreover, while an important factor in growth, has no formative influence beyond that which it exerts through water-content.
PHYSIOGRAPHY
118. Factors. The physiographic factors of a definite habitat are altitude, exposure, slope, and surface. In addition, topography is a general though less tangible factor of regions, while the dynamic forces of weathering, erosion and sedimentation play a fundamental role in the change of habitats. It is evident, however, that these, except where they affect the destruction of vegetation directly, can operate upon the plant only through more direct factors, such as water, light, and temperature. While they are themselves not susceptible of measurement, they can often be expressed in terms of determinable factors, i. e., slope, exposure, and surface. Fundamentally, they constitute the forces which change one habitat into another, and, in consequence, are really to be considered as the factors which produce succession. The static features of physiography, altitude, etc., lend themselves readily to determination by means of precise instruments. These factors, though by no means negligible, are remote, and consequently their mere measurement is insufficient to indicate the nature or extent of their influence upon the plant. It is necessary to determine also the manner and degree in which they affect other factors, a task yet to be done. Readings of altitude, slope, and exposure are so easily made that the student must carefully avoid the tendency to let them stand at their own value, which is slight. Instead, they should be made the starting point for ascertaining the differences which they produce in water-content, humidity wind, and temperature.
Altitude
119. Analysis into factors. Of all physiographic features, altitude is the most difficult to resolve into simple factors. Because of general geographic relations, it has a certain connection with rainfall, but this is vague and inconstant. Obviously, in its influence upon the plant, altitude is really pressure, and in consequence its effect is exerted upon the climatic and not the edaphic factors of the habitat. Theoretically, the decrease of air pressure in the increased altitude directly affects humidity, light, and temperature. Actually, while there is unquestionably a decrease in the absorption of the light and heat rays owing to the fact that they traverse less atmosphere, which is at the same time less dense, this seems to be negligible. Photometric readings at elevations of 6,000 and 14,000 feet have so far failed to show more than slight differences, which are altogether too small to be efficient. The effect upon humidity is greater, but the degree is uncertain. Continuous psychrographic records at different elevations for a full season, at least, will be necessary to determine this, since the psychrometric readings so far made, while referred to a base psychrograph, are too scattered to be conclusive. Finally, the length of the season, itself a composite, is directly dependent upon the altitude. This relation, though obscure, rests chiefly upon the rarefaction of the air which prevents the accumulation of heat in both the soil and the air.
Fig. 26. Aneroid barometer.
120. The barometer. To secure convenience and accuracy in the determination of altitude, it is necessary to use both a mercurial and an aneroid barometer. The latter is by far the most serviceable for field work, but it requires frequent standardizing by means of the former. The mercurial form is much more accurate and should be read daily in the base station. It is practically impossible to carry it in the field, except in the so-called mountain form, which is of great service in establishing the altitudes of a series of stations. In use the aneroid barometer may be checked daily by the mercurial standard, or it may be set at the altitude of the base station, thus giving a direct reading. After the normal pressure at the base has once been ascertained, however, the most satisfactory method is to set the aneroid each day by the standard, at the same time noting the pressure deviation in feet of elevation (see p. [46]). The absolute elevation of the various stations of a series may be determined either by adding or subtracting this deviation from the actual reading at the station, or by noting the change from the base station, and then adding or subtracting this from the normal of the latter. When it is impossible to check the aneroid by means of a mercurial barometer, the average of a series of readings made at different days at one station, especially if taken during settled weather, will practically eliminate the daily fluctuations, and yield a result essentially accurate. Even in this event, the accuracy of the aneroid should be checked as often as possible, since the mechanism may go wrong at any time. The barograph, while a valuable instrument for base stations, is not at all necessary. These instruments can be obtained from all makers of meteorological apparatus, such as H. J. Green, and J. P. Friez. Aneroid barometers reading to 16,000 feet cost about $20; the price of the Richards aneroid barograph is $45. Ordinary observatory barometers cost $30–$40; the standard instrument sells at $75–$100. The mountain barometer, which is altogether the most serviceable for the ecologist, ranges from $30–$55, depending upon accessories, etc.