Planetary life is sustained by topsoil because that is where food grows. Topsoil health depends on its organic matter content and the practices that build organic matter into the soil. Soil with little or no organic matter content erodes easily and loses the ability to grow things well. It becomes grey and lifeless. Organic matter in soil provides the nutrients that create healthy food.
The University of Wisconsin Extension defines soil organic matter as a "fraction of the soil composed of anything that once lived." This includes plants and animal remains in various stages of decomposition, root and microbial exudates and humus. Humus is defined as the biological residue that is left over when decomposition is complete. "Compost" is the common term for soil organic matter.
The Midwest Organic and Sustainable Education Service describes soil organic matter as "the most fundamental source of fertility in organic agriculture." Humus, or compost, is described "a bank which holds nutrients and can release them in response to plant or microorganism needs." Chemical fertilizers force-feed nutrients to plants rather than responding to a plant's nutritional needs.
The function of soil organic matter is to stabilize and hold soil particles together, to supply and store nutrients and to provide carbon/energy for microbes. Organic matter in soil also acts as a filter system for pollutants. The Environmental Protection Agency has determined that compost (organic soil matter) helps decontaminate soil by transmuting chemical residues. Chemical fertilizers are manufactured in the same process as ammunition and leave they heavy metal residue in the ground. The compost process also transmutes wood preservatives and pesticides residues.
Biologically active (labile) and stable (recalcitrant) are two types or stages of soil organic matter. Soil is active in the process of decomposition, such as a compost pile that has not yet turned to a rich, dark crumbly substance. Stable organic matter is that which has completed a decomposing process and becomes finished compost. It contributes to the structure and color of the soil and interacts with organic matter.
Organic matter in garden soil feeds growing plants and creates higher nutrient levels in food. Building soil organic matter is done by adding compost, through crop rotation and return of crop residues, and by the addition of organic soil amendments. The "no-till method" also helps maintain the soil's organic matter content.
Creating soil organic matter in the home garden benefits the eco-system that sustains the garden's life. Organic matter in the form of compost is the foundation for the health of growing plants. Beneficial insects, birds and butterflies are attracted to a garden without pesticides and chemical fertilizers. On a farm, soil organic matter reduces disease and insect problems, helps soil erosion problems and optimizes crop yields.