ED-STEEP: Education Solutions to Environmental
and Economic Problems
Organic Matter Fact Sheet
What is organic matter?
Soil organic matter consists of living organisms and all
the remains of dead organisms in the soil. The living
organisms are typically bacteria, fungi, plants, insects,
mites, nematodes, and other micro-invertebrates. Many of
these organisms use non-living organic matter for food. The
non-living organic matter occurs in different forms:
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organic matter suspended in water
particulate organic mater, which is the partially
decomposed, sand-sized fraction that serves as a
reservoir of soil nutrients that are released during the growing season
humus, which is usually the largest fraction of organic matter and is a mixture of organic
compounds that are relatively resistant to change (e.g., cellulose) and other organic
compounds that help “glue” soil particles together to form aggregates (e.g., fulvic acid,
humin).
inert organic matter, typically from the burning of plant material
The formation of organic matter
When plant and animal debris is added to soil, it is broken down by soil invertebrates, fungi, and
bacteria, initially into particulate organic matter and then into humus. Some materials, such as
lignins, are difficult to break down. During the process of decomposition, the number of living
organisms in the soil can increase greatly.
The fate of organic matter
Soil organic matter is lost in several ways. Soil bacteria and fungi use organic material to live,
and in the process, convert much of it into CO2 and water during respiration. On farmland, the
loss of organic matter is facilitated by soil erosion, plowing fields, and using fallow fields during
crop production. Fire can drastically reduce soil organic matter.
The function of organic matter
Organic matter provides numerous and essential ecosystem services:
 provides carbon, nitrogen, and energy for soil bacteria and fungi
 supplies nutrients for plants
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acts as a “glue” to bind soil particles together to stabilize soils
serves as a reservoir for plant nutrients
serves as a sink for CO2, thus reducing greenhouse gases
contributes to high soil biodiversity
binds pesticides and heavy metals, thus reducing water pollution
enhances water and nutrient holding capacity of soils, thus improving plant productivity
Factors that affect the amount of organic matter in the soil
Typically, less that 5% of soils is composed of organic matter. The organic matter content of
some agricultural soil is < 1%. Several factors affect the amount of organic matter, including:
 Climate – the rate of decomposition doubles for every 8-9ºC increase in mean annual
temperature
 Soil type – clay soils retain more organic matter than sandy soils
 Vegetation – the more vegetation and litter produced, the more organic matter in the soil;
also, high carbon:nitrogen ratios of vegetation slow down decomposition
 Topography – organic matter can accumulate in soils with poor drainage
 Tillage – tilling soil causes a decrease in organic matter by facilitating its decomposition
Management of Organic Matter
Considering how important organic matter is for sustaining life, it’s important that we develop
ways to preserve and enhance the organic matter in soils.
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Ways to prevent the loss of organic matter:
o control erosion
o use no-till planting methods
o use cover crops instead of fallow fields
o prevent catastrophic wild fires that reduce soil organic matter
o eliminate stubble burning
o manage grazing to maximize soil organic matter production
o use diverse, high biomass crops
o manage for maximum surface residues