Composting Basics

— Written By and last updated by Nancie Mandeville
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hands holding compost

Composting is the aerobic decomposition of organic materials by microorganisms under controlled conditions into a soil-like substance called compost. During composting, microorganisms such as bacteria and fungi break down complex organic compounds into simpler substances and produce carbon dioxide, water, minerals, and stabilized organic matter (compost). The process produces heat, which can destroy pathogens (disease-causing microorganisms) and weed seeds.

Raw materials are composted fastest when conditions that encourage the growth of the microorganisms are established and maintained. The most important conditions include the following:

  • Organic materials blended to provide the nutrients that support microbial activity and growth, including a balanced supply of carbon and nitrogen (C:N ratio)
  • Sufficient oxygen to support aerobic organisms
  • Moisture levels that uphold biological activity without hindering aeration
  • Temperatures needed by microorganisms that grow best in a warm environment

As soon as appropriate materials are mixed into a pile, the microorganisms set to work and the composting process begins. During this active stage, oxygen consumption and heat generation are at their highest levels. Then a curing period follows, in which materials compost much more slowly. The decomposition of organic material will continue until all of the nutrients are consumed by microorganisms and most of the carbon is converted to carbon dioxide. Before it reaches this point, however, compost is judged to be finished according to its intended use and factors such as C:N ratio, oxygen demand, temperature, and odor.

Factors affecting the composting process include nutrient ratio, moisture content, oxygen concentration, pH, surface area, temperature, and retention time.

Compost may be used as a soil incorporate, mulch, turf or other plant top dressing, an amendment to growing mixes, and a blend with topsoil. Currently, the number one use of compost is in planting beds. It is predicted that compost will be used widely for field nursery production.

Some of the benefits of mature compost to soil and plants include its ability to:

  • Improve soil structure
  • Reduce fertilizer requirements
  • Improve water infiltration and drought tolerance
  • Reduce soil compaction and crusting
  • Improve root growth and yields
  • Increase microbial and earthworm populations in soil
  • Protect plants from disease
  • Slowly release nutrients to plants
  • Improve nutrient-holding capacity
  • Increase ease of cultivation

Compost has also proven to be useful in pollution prevention and remediation. It is being used to prevent erosion of hillsides, embankments, and roadsides. Compost can also bind heavy metals in contaminated soils, degrade many pesticides, and absorb odors and degrade volatile organic compounds. In addition, compost is being used in wetland damage mitigation, stormwater filtration, and biofilters.