110. Records. The following form is used as a combined record for precipitation and wind:
| Day | Time | Formation | Station | Altitude | Exposure | Community | RAINFALL | Base | WIND | Base | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Inches | Duration | Velocity | Heig’t | Direction | |||||||||
| 29/8/04 | 6:30 P.M. | Half gravel | Hiawatha | 2550 m. | N.E. 17° | Asterare | 1 | 8 hours | 5 | 3 ft. | N. W. | ||
| 31/8/04 | 5:45 P.M. | „ | „ | „ | „ | „ | Trace | 10 min. | 12 | „ | „ | ||
| 2/9/04 | 4:00 P.M. | „ | „ | „ | „ | „ | .2 | 2 hours | 7 | „ | W. | ||
| 3/9/04 | 10:00 P.M. | „ | „ | „ | „ | „ | Trace | 18 | „ | „ | |||
SOIL
111. Soil as a factor. In determining the value of the soil as a factor in a particular habitat, it must be clearly recognized that its importance lies solely in the control which it exerts upon water-content and nutrient-content. The former is directly connected with the texture or fineness of the soil, the latter with its chemical nature. Accordingly, the structure of the soil and its chemical composition are the fundamental points of attack. These are not at all of equal value, however. Water is both a food, and a solvent for the nutrient salts of the soil. Furthermore, the per cent of soluble salts, as determined in mechanical analyses, is practically the same for all ordinary soils. Indeed, the variations for the same soil types are as great as for entirely different types. For these reasons, soluble salt-content may be ignored except where it is readily seen to be excessive, as in alkaline soils; and determinations of chemical composition are necessary only in those soils which contain salts or acids to an injurious degree, e. g., alkaline soils, peat bogs, humus swamps, etc. The structure of the soil, on the other hand, in the usual absence of excessive amounts of solutes, absolutely controls the fate of the water that enters the ground, in addition to its influence upon the run-off. It determines the amount of gravitation water lost by percolation, as well as the water that can be raised by capillarity. The resultant of these, the total soil water or holard, is hence an effect of structure, while the size and compactness of the particles are conclusive factors in controlling the chresard. It must be recognized, however, that these are all factors which enable us to interpret the amount of holard or chresard found in a particular soil. They have no direct important effect upon the plant, but influence it only in so far as they affect the water present.
112. The value of soil surveys. The full appreciation of the preeminent value of water-content, particularly of the chresard, greatly simplifies the ecological study of soils. The ecologist is primarily concerned with soil water only in its relation to the plant, and while a fair knowledge of soil structure is essential to a proper understanding of this, he has little concern with the detailed study of the problems of soil physics. For the sake of a proper balance of values, he must avoid the tendency noted elsewhere of ignoring the claims of the plant, and of studying the soil simply as the seat of certain physical phenomena. Accordingly, it is felt that mechanical and chemical analyses, determinations of soluble salt-content, etc., have much less value than has been commonly supposed. The usual methods of soil survey, which pay little or no attention to water-content, and none at all to available water, are practically valueless for ecological research. This statement does not indicate a failure to appreciate the importance of the usual soil methods for many agricultural problems, such as the use of fertilizers, conservation of moisture, etc., though even here to focus the work upon water-content would give much more fundamental and serviceable results. For these reasons, slight attention will be paid to methods of mechanical and chemical analysis. In their stead is given a brief statement of the origin, structure, and character of soils with especial reference to water-content.
113. The origin of soils. Rocks form soils in consequence of weathering, under the influence of physical and biotic factors. Weathering consists of two processes, disintegration, by which the rock is broken into component particles of various sizes, and decomposition, in which the rock or its fragments are resolved into minute particles in consequence of the chemical disaggregation of its minerals, or of some other chemical change. These processes are usually concomitant, although, as a rule, one is more evident than the other. The relation between them is dependent upon the character of the rock and the forces which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a rule disintegrate more rapidly than they decompose; sedimentary rocks, on the other hand, tend to decompose more rapidly than they disintegrate. In many cases the two processes go hand in hand. This difference is the basis for the distinction, first proposed by Thurmann, between those rocks which weather with difficulty and those which weather readily. The former were called dysgeogenous, the latter eugeogenous. Thurmann restricted the application of the first term to those rocks which produce little soil, but it seems more logical to apply dysgeogenous to those in which disintegration is markedly in excess of decomposition, and eugeogenous to those rocks that break down rather readily into fine soils. With respect to the general character of the soil formed, rocks are pelogenous, clay-producing, psammogenous, sand-forming, or pelopsammogenous, producing mixed clay and sand. The first two are divided into perpelic, hemipelic, oligopelic, perpsammic, etc., with reference to the readiness with which they are weathered, but this distinction is not a very practicable one. The grouping of soils into silicious, calcareous, argillaceous, etc., with reference to the chemical nature of the original rock, is of no value to the ecologist, apart from the general clue to the physical properties which it furnishes.
114. The structure of soils. The water capacity of a soil is a direct result of the fineness of the particles. Since the water is held as a thin surface film by each particle or group of them, it follows that the amount of water increases with the water-holding surface. The latter increases as the particles become finer and more numerous, and thus produce a greater aggregate surface. The upward and downward movements of water in the soil are likewise in immediate connection with the size of particles. The upward or capillary movement increases as the particles become finer, thus making the irregular capillary spaces between them smaller, and magnifying the pull exerted. On the contrary, the downward movement of gravitation water, i. e., percolation, is retarded by a decrease in the size of the soil grains and hastened by an increase. Hence, the two properties, capillarity and porosity, are direct expressions of the structure of the soil, i. e., of its texture or fineness. Capillarity, however, increases the water-content of the upper layers permeated by the roots of the plant, while porosity decreases it. On the basis of these properties alone, soils would fall into two groups, capillary soils and porous soils, the former fine-grained and of high water-content, the latter coarse-grained and with relatively little water. A third factor, however, of great importance must be taken into account. This is the pull exerted upon each water film by the soil particle itself. This pull apparently increases in strength as the film grows thinner, and explains why it finally becomes impossible for the root-hairs to draw moisture from the soil. This property, like capillarity, is most pronounced in fine-grained soils, such as clays, and is least evident in the coarser sands and gravels. It seems to furnish the direct explanation of non-available water, and, in consequence, to indicate that the chresard is an immediate result of soil texture.
Fig. 25. Sieves for soil analysis.
115. Mechanical analysis. From the above it is evident that, with the same rainfall, coarse soils will be relatively dry, and fine soils correspondingly moist. However, this difference in holard is somewhat counterbalanced by the fact that the chresard is much greater in the former than in the latter. The basis of these relations can be obtained only from a study of the texture of the soil. The usual method of doing this is by mechanical analysis. This is far from satisfactory, since the use of the sieves often brings about the disaggregation of groups of particles which act as units in the soil. Furthermore, the analysis affords no exact evidence of the compactness of the soil in nature, and tests of capillarity and porosity made with soil samples out of position are open to serious error. Nevertheless, mechanical analyses furnish results of some value by making it possible to compare soils upon the basis of texture. For ecological purposes, minute analyses are undesirable; their value in any work is doubtful. A separation of soil into gravel, sand, and silt-clay is sufficient, since the relative proportion of these will explain the holard and chresard of the soil concerned. The latter are also affected in rich soils, especially of forests, by the organic matter present. If this is in a finely divided condition, the amount is determined by calcining. When a definite layer of leafmold is present, as in forests and thickets, its water-value is found separately, since its power of retaining water is altogether out of proportion to its weight.