To most people, soil isn't very exciting stuff. However, look at it closely and it's a complex, changing, active substrate essential to life. A soil suitable for most plant growth consists of 45 percent mineral particles, 25 percent water, 25 percent air and 5 percent organic matter. The organic component is vital because as it decomposes, elements get recycled and made available to plants and other organisms. How quickly soil organic matter decomposes depends on a number of factors.
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Type of Organic Matter
The decay rate of soil organic matter depends in large part on what kind of organic matter it is. Three categories exist, based on the time it takes them to decompose. First is active soil organic matter, which breaks down in a short time -- a few weeks to a few years. It contains fresh material such as insect bodies and newly dead plant remains. Next is the slow soil organic material category, which consists of gradually decomposing detritus that's already partially decomposed. This can take years to decades to complete decay. At the bottom of the scale is passive soil organic material, called humus, which isn't actively decomposing and may take hundreds or thousands of years to fully decay.
The agents of decay are living organisms that break down organic matter and are also part of the organic matter. Larger organisms such as earthworms, insects, slugs and small mammals eat soil organic matter, further breaking it down in their digestive systems. Soil microfauna such as nematodes and protists live in water films within soil. Soil bacteria exist in large numbers, but they have a small biomass. Fungi secrete enzymes that break down organic tissues. Active soil organic material has a high population of all kinds of living organisms. Factors that promote the growth of soil organisms speed the rate of decomposition.
Temperature and moisture affect the rate of soil organic matter decomposition. Cool weather and dry conditions slow down decomposition, and warm temperatures and humid conditions speed it up. Tropical areas, even though they have a high rate of organic material deposited on forest floors, don't have much soil organic matter in the topsoil because it breaks down so quickly. As you move away from the equator into cooler, drier areas, soil organic matter content increases because the decomposition rate is lower. Hot, humid conditions favor increased microbial growth in soils. Decay rates increase when soils are alternately dry and wet, with decay decreased in soils that are constantly wet or dry.
The kind of particles that make up the soil's mineral content affects how much soil organic matter occurs. Sandy soils have large particle sizes, resulting in good aeration and low soil moisture, conditions that don't favor buildup of organic matter because good aeration speeds decomposition. Clay soil, with a fine particle size and high moisture content, allow buildup of organic material because it doesn't decay as quickly. Tilling suddenly increases the supply of oxygen in the soil, and bacterial growth increases. The bacteria decompose the soil more quickly as a result, leaving the soil depleted of soil organic matter.
- National Oceanic and Atmospheric Administration: National Estuarine Research Reserve System: Soil Composition and Formation
- Virtual Soil Science Learning Resources: It's Alive!: Decomposition
- Soil Ecology and Management; Joann K. Whalen and Luis Sampedro
- The Ohio State University Extension: Understanding Soil Microbes and Nutrient Recycling
- University of Florida IFAS Extension: Sabine Grunwald: Soil Organic Matter (SOM)
- U.S. Department of Agriculture Natural Resources Conservation Service: Soil Organic Matter