The inorganic constituents are the various minerals and elements found alone, or in combination, in the earth, such as silica, aluminum, calcium, iron, carbon, sodium, chlorine, potassium, etc.
The characteristics of the soil depend upon its constituents, and upon the predominance of one or the other of its composing elements. The nature of the soil also depends upon its physical properties. When the disintegrated rock consists of quite large particles, the soil is called a gravel soil. A sandy soil is one in which the particles are very small. Sandstone is consolidated sand. Clay is soil consisting principally of aluminum silicate; in chalk, soft calcium carbonate predominates.
The organic constituents of the soil are the result of vegetable and animal growth and decomposition in the soil.
Ground Water.—Ground water is that continuous body or sheet of water formed by the complete filling and saturation of the soil to a certain level by rain water; it is that stratum of subterranean lakes and rivers, filled up with alluvium, which we reach at a higher or lower level when we dig wells.
The level of the ground water depends upon the underlying strata, and also upon the movements of the subterranean water bed. The relative position of the impermeable underlying strata varies in its distance from the surface soil. In marshy land the ground water is at the surface; in other places it can be reached only by deep borings. The source of the ground water is the rainfall, part of which drains into the porous soil until it reaches an impermeable stratum, where it collects.
The movements of the ground water are in two directions—horizontal and vertical. The horizontal or lateral movement is toward the seas and adjacent water courses, and is determined by hydrostatic laws and topographical relations. The vertical motion of the ground water is to and from the surface, and is due to the amount of rainfall, the pressure of tides, and water courses into which the ground water drains. The vertical variations of the ground water determine the distance of its surface level from the soil surface, and are divided into a persistently low-water level, about fifteen feet from the surface; a persistently high-water level, about five feet from the surface, and a fluctuating level, sometimes high, sometimes low.
Ground Air.—Except in the hardest granite rocks and in soil completely filled with water the interstices of the soil are filled with a continuation of atmospheric air, the amount depending on the degree of porosity of the soil. The nature of the ground air differs from that of the atmosphere only as it is influenced by its location. The principal constituents of the air—nitrogen, oxygen, and carbonic acid—are also found in the ground air, but in the latter the relative quantities of O and CO2 are different.
AVERAGE COMPOSITION OF ATMOSPHERIC AIR IN 100 VOLUMES
| Nitrogen | 79.00 | per cent. |
| Oxygen | 20.96 | " |
| Carbonic acid | 0.04 | " |
AVERAGE COMPOSITION OF GROUND AIR