Organic
Organic Matter
and its
Role in Soils
Fred C. Schoenagel III
Resource Soil Scientist
USDA--NRCS
USDA
908-735908735-0737 x 106
[email protected]
What is Soil Organic Matter?
• is typically expressed as a percent by weight in soil survey information
• typically makes up less than 5 percent by weight off a mineral soil
• is composed primarily of carbon (approximately 58 percent), but also
contains hydrogen, oxygen, phosphorus, nitrogen, and sulfur
• in unfertilized soils, it is the primary source of nitrogen and sulfur for plants
Reference: Singer, M.J., and D.N. Munns (1987) “Soils, An Introduction”. Macmillan, New York.
Carbon Sources for Soil Organic Matter
• fresh and decomposing plant tissue such as roots and leaves
• living and dead soil microbes and exudates
• living and dead animal tissue and exudates
Reference: Singer, M.J., and D.N. Munns (1987) “Soils, An Introduction”. Macmillan, New York.
Forms of Soil Organic Matter
Humus
• colloidal organic soil material comprised of highly decomposed
organic matter and/or byproducts of microbial activity
• can produce brown and black colors within soils
• is very “reactive” (electrically charged), so it contributes to “cation
exchange capacity” in soils
• is not “static” within soils; it is always being created and
destroyed
Reference: Singer, M.J., and D.N. Munns (1987) “Soils, An Introduction”. Macmillan, New York.
Forms of Soil Organic Matter
Cellulose
• is comprised mainly of the simple sugar glucose
• is the primary component in plant tissue
• provides a “food” source for soil microbes
Reference: Singer, M.J., and D.N. Munns (1987) “Soils, An Introduction”. Macmillan, New York.
Forms of Soil Organic Matter
Lignin
• is the main constituent in woody plants and plant cells with
thickened walls
• does not break down very readily, so it accumulates within soils
as large molecules
• acts as an organic “glue” that holds soil mineral particles together,
creating soil structure
Reference: Singer, M.J., and D.N. Munns (1987) “Soils, An Introduction”. Macmillan, New York.
Forms of Soil Organic Matter
Glomalin
• is a glycoprotein (sugar-protein compound) produced by
“arbuscular mycorrhizal” fungi (AMF’s)
• creates a lattice- or web-like coating on soil mineral particles that
holds the particles together and promotes soil structure
development
• inhibits the breakdown of unstable forms of organic soil material by
acting as a protective barrier against decomposition
• may help soils hold on to carbon for long periods of time; perhaps
as long as 100 years
Reference: Comis, D. (July 2008). Glomalin: What is it…and What Does it Do?. Agricultural Research, 20-21.
(htt //
(http://www.ars.usda.gov/is/AR/archive/jul08/glomalin0708.pdf)
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Soil Texture
Sand: 2.0 – 0.05 mm
Silt:
0.05 – 0.002 mm
Clay: <0.002 mm
R f
Reference:
Broderson,
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W
W.D.
D (1991)
(1991). “F
“From th
the S
Surface
f
D
Down”.
” USDA
USDA-SCS
SCS NEDC.
NEDC
Soil Structure Development
Reference: Singer, M.J., and D.N. Munns (1987) “Soils, An Introduction”. Macmillan, New York.
Soil Structure
Granular soil structure contributes to significant
pore space in the surface horizon of this soil.
Reference: Singer, M.J., and D.N. Munns (1987) “Soils,
Soils, An Introduction”.
Introduction . Macmillan, New York.
Organic Matter Effects on Soil Properties
Adding
g organic
g
matter to soils p
produces the following
g results:
• Surface structure becomes more stable and less prone to crusting and erosion
• Water infiltration increases and runoff decreases when soil structure improves
• Soil organic matter holds 10 to 1,000 times more water and nutrients than the
same amount of soil minerals
• Beneficial soil organisms become more numerous and active with diverse crop
rotations and higher organic matter levels
Reference: Soil Quality Technical Note No. 5: Managing Soil Organic Matter. (October 2003). USDA-NRCS Soil Quality Institute.
(http://soils.usda.gov/sqi/concepts/soil_organic_matter/files/sq_tn_5.pdf)
Organic Matter Effects on Air Quality, Water Quality,
and
dA
Agricultural
i lt l P
Productivity
d ti it
• Dust, allergens, and pathogens in the air immediately decline
• Sediment and nutrient loads decline in surface water as soon as soil aggregation increases
and runoff decreases
• Ground and surface water qualityy improve because better structure, infiltration, and
biological activity make soil a more effective filter
• Crops are better able to withstand drought when infiltration and water holding capacity
increase
• Organic matter may bind pesticides, making them less active; soils managed for organic
matter may suppress disease organisms, which could reduce pesticide needs
• Crop health and vigor increase when soil biological activity and diversity increase
• Wildlife habitat improves when residue management improves
Reference: Soil Quality Technical Note No. 5: Managing Soil Organic Matter. (October 2003). USDA-NRCS
USDA NRCS Soil Quality Institute.
Managing Soils to Increase Organic Matter Content
• plant diverse, high biomass crop rotations that will put more residue back
on the soil surface
• use cover crops to protect the soil surface
• try to reduce the number of tillage operations, or use minimum- or no-till
technologies; glomalin production decreases under conventional tillage
• introduce rotational grazing to manage plant growth and health and to add
additional forms of organic matter (manure)
• to promote glomalin production, avoid or limit planting crops that do not
have arbuscular mycorrhizal fungi (AMF’s), particularly members of the
Brassicaceae family, such as cabbage and cauliflower
References: Soil Quality Technical Note No. 5: Managing Soil Organic Matter. (October 2003). USDA-NRCS Soil Quality Institute.
Comis, D. (September 2002). Glomalin: Hiding Place for a Third of the World’s Stored Soil Carbon. Agricultural Research.
(http://www.ars.usda.gov/is/AR/archive/sep02/soil0902.htm)
Using
g Soils to Sequester Carbon
• in general, the only way a soil can sequester or store carbon is if
more organic matter is being added to the soil than is being utilized
by any microbes or plants living in the soil
• soil carbon levels can be maintained under conventional tillage
provided that enough organic matter is added back to the soil to
replace what was lost; however
however, the maintenance level of soil
carbon will typically be lower than that found in a natural soil
• conservation tillage alone will slow the decomposition of soil
carbon but will not stop the loss of it; additions of soil carbon in
carbon,
the form of crop residues, cover crops, and/or manures are
required to increase soil carbon levels
• soil carbon losses are not as great in areas with a cool
cool, humid climate
Reference: Soil Quality Agronomy Technical Note No. 12: Long-Term Agricultural Management Effects on Soil Carbon. (August 2001).
USDA-NRCS Soil Quality Institute. (http://soils.usda.gov/sqi/management/files/sq_atn_12.pdf)
Using Soils to Sequester Carbon
Reference: Soil Quality Agronomy Technical Note No. 12: Long-Term Agricultural Management Effects on Soil Carbon. (August 2001).
USDA-NRCS
USDA
NRCS Soil Quality Institute.
For More Information About
S il Q
Soil
Quality:
lit
USDA-NRCS Soil Quality homepage- http://soils.usda.gov/sqi/index.html
S il Q
Soil
Quality
lit publications
bli ti
- http://soils.usda.gov/sqi/publications/publications.html
htt // il
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Glomalin:
USDA-NRCS Agricultural Research Service (ARS) - http://www.ars.usda.gov/main/main.htm
(note: type “glomalin” in the “Search” to find articles and research on glomalin)