An understanding of fundamental principles of soil science, or pedology, is indispensable to a geographer. Soils constitute a major environmental factor, influencing by their fertility and special qualities, not only whether a population can be fed, clothed, and housed but also the particular types of food and fiber or lumber products that can be obtained from a region.
Many persons think of the soil as a lifeless, residual layer, which has somehow accumulated over a long period of time and which merely holds a supply of things necessary for plant growth. As soil science has developed, however, it has become known that the soil is a dynamic layer in which many complex chemical, physical, and biological activities are going on constantly. Far from being a static, lifeless zone, it is a changing and developing body. We know now that soils become adjusted to conditions of climate, landform, and vegetation and will change internally when those controlling conditions change.
The soil scientist restricts the word soil, or solum, to the surface material, which, over a long period of years, has come to have distinctive layers, or horizons. A soil has certain distinctive physical, chemical, and biological qualities, which permit it to support plant growth and which set it off from the infertile substratum, which may consist of overburden or solid bedrock lying beneath. The true soil is composed both of mineral and organic particles.
Soil is made up of substances existing in three states: solid, liquid, and gaseous. For plant growth a proper balance of all three states of matter is necessary.
The solid portion of soil is both inorganic and organic. Weathering of rock produces the inorganic particles that give a soil and main part of its weight and volume. These fragments range from gravel and sand down to tiny colloidal particles too small to be seen by an optical microscope. The organic solids consist of both living and decayed plant and animal materials, most being plant roots, fungi, bacteria, worms, insects, and rodents. Collodial particles of organic matter share with inorganic collodial particles an important function in soil chemistry.
The liquid portion of soil, the soil solution, is a complex chemical solution necessary for many important activities that go on in the soil. Soil without water cannot have these chemical reactions, or can it support life. Oases in the open pore spaces of the soil form the third essential component. They are principally the gases of the atmosphere, together with gases liberated by biological and chemical activity in the soil.
For an understanding of soils, information is needed about
(1) the physical-chemical properties and materials of soils, and
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