Soil Action.— The soil acts directly upon the behavior and form of plants by reason of its influence upon water-content and temperature. In plant geography, all inorganic strata upon which plants grow are termed soils. The ex treme types are rock and water, between which are found all manner of gradations from gravel to mud. The weathering of rocks produces two kinds of soils quite different in their behavior with respect to water. The one is loose and composed of large particles — for example, sands and gravels; the other is composed of fine, compact particles, clays and loams. Sands and gravels absorb nearly all of the rain fall ing upon them but much of the water taken up passes through and is carried away along some impervious stratum. What remains as water-content is readily absorbed by the root hairs or is lost by evaporation from .the soil. On account of the large air-spaces between the grains, the water in the lower layers is raised with difficulty by capillary action. Plants withdraw from loose soils nearly all of their water-content. Compact soils, especially clay do not absorb rain rapidly, and much of the latter is carried away by surface drainage. Water once absorbed is held tenaciously, and the loss by underground drainage is slight. The pores of the soil are fine, and capillarity plays an important part in raising water from the lower levels. The particles attract the water films strongly, and in consequence clay yields its water to plant or air very slowly. Ordi nary plants will wilt in clay soils which still contain as high as 12 to 30 per cent of water. Rock is all but absolutely impervious to water. Mosses and lichens alone can grow upon it, in consequence of their power to decompose the surface and their ability to withstand drying out. The amount of soluble material in the soil has a direct effect upon the growth of plants. Nearly all ordinary soils contain an adequate supply of soluble minerals; a few, however, are deficient in these, and are unable to support more than a scanty vegetation. Other soils contain in excess soluble salts and acids which are harmful. The sparse, desert-like vegetation of salt basins and alkali wastes is due to the large quantities of sodium chloride, sodium carbonate, and• other salts present. In swamps and bogs the decay of plant and animal remains uses up the available oxygen and thus hinders the absorptive powers of the roots. The kind of material in the soil and the fineness and compactness of the grains deter mine its behavior with reference to the absorp tion and radiation of heat. The color of the soil, the amount of water present, and other conditions have also to do with this matter. Rock is warmed most readily in the sunshine, and at night most readily parts with its heat. Water is at the other extreme; it warms up slowly and, conversely, yields its heat reluctantly.
Utilization of Moisture.— It is a well-estab lished fact that plants in general are unable to absorb water-vapor from the air. The moisture must be condensed into water, and then must regularly find its way into the soil before it can be used by the plant. A comparatively small number of plants, lichens, mosses, and tree dwelling orchids absorb rain or dew directly through their leaf or thallus surfaces, but with the great majority of plants the water can only be taken in from the soil or substratum. The moisture of the air is condensed or precipitated in various forms — rain, dew, snow, sleet, hail, frost and fog. With the exception of the last, all of these contribute sooner or later to the water-content of the soil, the important differ ence being that the solid forms usually melt grad ually and are in consequence absorbed more completely. The water which falls upon the surface of the soil is partly absorbed, and partly carried away by drainage. The latter is known as urun-off its amount will depend upon the compactness of the soil and the steepness of the slope. The absorbed water passes into the lower
layers in part, where some of it is drained off as gravitation water, and some is retained as capillary water to be raised by pillary action into the upper layers of the soil. That which remains in the soil about the roots forms thin films about the soil-grains, and is known as the water-content. This alone can be used by the root-hairs. Part of it, however, is lost by direct evaporation from the soil. Rain has lit tle or no mechanical action upon plants, except perhaps in the tropics, where it falls in torrents. In the form of ccrun-off,)) however, it acts powerfully upon the surface of hills and mountains, and plays in consequence an ex tremely important part in the development of vegetation. Sleet and hail are very destructive in the breaking of twigs and branches and the cutting and tearing of leaves but are of little importance because of their relative infrequence. Snow, on the other hand, has had a great deal to do with the forms of trees, particularly the pines, spruces, and firs, in northern and moun tain regions. It is a poor conductor, and for this reason affords much protection to plant parts covered by it. Unlike rain, it is often unequally distributed by the wind, and there fore produces important local differences in the water-content of the soil.
Physiography.— The surface features of a region — its physiography — affect directly sev oral of the physical factors of habitats. Alti tude not only influences the rainfall, but it also increases water-loss by reason of the reduced air-pressure and the decreased humidity. The sunlight may be stronger, as the rays pass through fewer air-layers, and are absorbed in a less degree. The degree of slope is especially important, as it determines very largely the ra tio between "run-off° and absorbed water. In mountain regions particularly, it modifies the angle at which the sun's rays strike the sur face, and increases the amount of heat and light received. The exposure of the surface, that is, the direction in which it lies, affects the amount of heat and light, the intensity of the wind, and the snowfall. Furthermore, the character of the surface itself, whether level or uneven, will influence all of these factors in a less degree. Physiographic changes, such as elevation and subsidence, the erosion of river valleys, and the upbuilding of swamps, deltas, etc., have a profound effect upon the distribu tion of plants, and the development of vegeta tion.
Vegetable, Animal and Human Factors.— Dead vegetation increases the water-content of a habitat by checking the movement of the “run-off,o and thus increasing the absorption, and by protecting the surface of the soil from excessive evanoration. It equalizes the soil temperature by hindering the warming action of the sun's rays, and the cooling effect of radia tion. It also diminishes the force of the wind, and, finally, by decay, returns to the soil much of the nutrient material taken from it. Living vegetation has the same effect, but is different in that it constantly draws water and nourish ment from the soil, and often reduces the amount of light present. The activities of animals and man are extremely diverse. Earth worms and burrowing mammals enrich the soil by working it over repeatedly. Grazing animals have more or less effect upon grass lands. Insects are fundamentally important in fertilization, and doubtless often act decisively in the struggle for existence by destroying some plants and not others. Man is a biological factor of the first importance, even if we leave out of consideration all the changes that he has brought about in plants and vegetation in conse quence of cultivation. He changes habitats fundamentally by the removal of forests. by fires, by the construction of railroads and canals, by drainage, by irrigation, etc.