Typical garden soil is composed of 50% solid material by volume, 25% water, and 25% air. The solid material portion includes about 45% minerals and 5% organic matter.
The mineral portion of the soil is derived from the bedrock from which it was formed. Some of the mineral particles, such as sand, still consist of rock. They are chemically and structurally the same as the parent rock, but have been ground by weather, water, glaciers, and other natural forces into small pieces. Other particles—including most of the microscopic clay particles—have been dissolved and precipitated again, perhaps many times, to reach a form that is quite different from the native rock.
The sizes of individual particles of the mineral component are responsible for the texture of the soil. Soil textures are classified as sand (the coarsest texture), silt, and clay (the finest texture, with particles 1,000 times smaller than large sand particles). Loam is a mixture of sand, silt, and clay. For more information, see Soil Texture.
Living and dead plant and animal matter in various stages of growth and decay constitute the organic part of the soil. Most native, or unamended, soils contain from less than 1% to 5% organic matter, whereas a well-amended garden soil may contain 30% or more. Adding organic matter is one of the best things you can do to your soil. In addition to being a reservoir of nutrients, organic matter improves the soil structure (see Soil Structure).
Live organic matter includes earthworms, insects, microorganisms, and plant roots. Earthworms and plant roots perform a valuable service by creating tunnels for air and water to flow through the soil. An invisible world of soil bacteria, fungi, and algae is even more crucial. These microorganisms decompose organic matter and contribute to the chemical reactions that allow plants to absorb nutrients.
Soil water enables plants to absorb minerals by first dissolving them. Water is also needed for the physiological and chemical processes of plant growth.
Water is so strongly attracted to small spaces, or pores, in the soil that it moves from large spaces to smaller ones, even if the movement is upward or sideways. That is why a soil with mostly small pores, such as clay, holds water so well. An ideal soil has a mix of large and small spaces, so that it holds both water and air. (See Soil Texture.)
Soil with a loose surface and large pores permits air to diffuse easily into it. Entry is limited if the soil is crusted over or compacted. Soil air is more humid than the air that humans breathe, and it has a higher carbon dioxide content. The oxygen it contains is vital to the root growth of plants. In fact, roots grow only where oxygen is present in the soil.
Roots and decaying organic matter give off carbon dioxide, which diffuses to the surface and dissipates in the air as oxygen diffuses to the depths of the soil.