General Composition

Mineral matter, organic matter, soil water, and soil air are the four major components of a soil.

The proportions of these components may vary between horizons in a soil or between similar horizons in different soils. The ratio of soil water to soil air depends upon whether the soil is wet or dry. The mineral matter, composed of particles ranging in size from the submicroscopic to gravel or even rocks in some cases, accounts for the bulk of the dry weight of the soil and occupies some 40 to 60% of the soil volume. Organic matter, derived from the waste products and remains of plants and animals, occurs in largest amounts in the surface soil, but even here seldom accounts for more than 10% of the dry weight of the soil.

Soils are very porous bodies. Some 40 to 60% of the volume is interparticle space, or pore space. The pores, highly irregular in shape and size but almost all interconnected by passages, contain soil water, soil air, or both of these. The soil water reacts chemically with the soil solids and usually contains dissolved substances and perhaps suspended particles. The soil air approaches equilibrium with atmospheric air through movement of individual gases.

Bedrock is the ultimate source of the inorganic component in soils. When rock is exposed at the surface of the earth's crust, it is broken down into smaller and smaller fragments by physical forces. The fragments may be altered or decomposed by chemical reaction of mineral matter with water and air. Hundreds, thousands, or even millions of years may be required for the weathering or physical and chemical alteration of rock to produce the ultimate end products in soils. Once particles reach a sufficiently small size they can be moved by wind, water or ice when exposed at the surface. It is common, therefore, for small particles to be moved from one location to another. A single particle might occur in several different soils over a period of 100,000 years. Eventually, these particles or their decomposition products reach the ocean where they are redeposited as marine sediments.

The silicate group of minerals is dominant in soil systems. The terms, clay mineral and layer silicate, are used almost interchangeably. The dominant chemical elements in silicate clays are oxygen, silicon, aluminum and iron. Important constituents in relatively small amounts are potassium, calcium, magnesium and sodium. Other elements occur in very small amounts in silicates. Carbonates, oxides, phosphates and sulfates are other mineral groups that occur commonly in parent materials.

Lesson 3. Soil Physical Properties


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