What is soil organic matter

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What is soil organic matter ?
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Soil organic matter (SOM) is a small fraction of
the mass of mineral soils
SOM
Mineral particles
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SOM is a complex mixture
Living organisms
Biologically active SOM
Recent residues
Humus
Humus
Adapted from Magdoff and Weil (2003)
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What is humus ?
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• While it is unlikely that any 2 humus molecules
  have exactly the same composition, humic
  molecules share the following common
  characteristics
• Extreme chemical complexity
• Resistance to further decomposition
• High specific surface and negative charge
• Dark color

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Soil organic matter is dynamic !
http//www.extension.umn.edu/distribution/cropsyst
ems/components/7402_02.html
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The metabolic potential of soil microbial
communities greatly exceeds organic inputs to
soils
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Why does organic matter accumulate in soils ?
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3 main factors regulate decomposition in soil
systems

• Environmental conditions
• Quality of organic resources
• Interactions with the soil matrix

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Climate affects SOM dynamics
25o C
Brady and Weil (2002)
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Landscape position affects SOM dynamics
Poorly drained
Interstream divide
Somewhat poorly drained
Moderately well drained
LANDSCAPE POSITIONS
Well drained
Poorly drained
Interfluve
Shoulder
Valley floor
SOIL DRAINAGE CLASSES
Backslope
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Blackland soils of North Carolina
Lily (1981)
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Organic resource quality affects SOM dynamics
(Giller, 2000)
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Prairie vs. Forest
Prairie
Forest
Modified from Brady and Weil (2002)
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Geographic distribution of SOM
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The soil matrix affects SOM dynamics
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Location of SOM within the soil matrix
?
Free POM
Sensitivity to management
Intra-aggregate POM
?
Mineral associated OM
Adapted from Carter (2002)
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Relationship between fine mineral fraction and SOM
Adapted from Magdoff and Weil (2003)
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Organic coatings on soil particles
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Blechh !!! Tastes Bad !!!!
Biochemical recalcitrance
How do you expect to live off this stuff ???
Mineral protection
I cant get if off, you try !!
Fe
Al
We already are !!!!!!
Yuck !!!!! Sure is gritty !
Physical protection
Theres gotta to be a way inside !!!
Hey !! Theres good stuff in there !!!!
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How does SOM affect soil properties and processes
?
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Chemical effects
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SOM adsorbs ions and molecules
Adapted from Brady and Weil (2002)
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SOM increases buffer capacity
Havlin et al. (2001)
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SOM chelates metals
SOM
Adapted from l (2000)
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Physical effects
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SOM increases plant available H20
Adapted from Magdoff and Weil (2003)
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SOM reduces bulk density
Adapted from Magdoff and Weil (2003)
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Macroaggregates often form around particulate
OM. Processes that impact aggregation impact SOM
dynamics
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Microbial activity
Sollins et al (1996)
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More OM is needed to stabilize fine textured soils
16 clay
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Adapted from
(19??)
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Biological effects
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SOM energizes biologically mediated processes
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Plant growth promoting rhizobacteria (PGPR) feast
on root exudates
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Crop uptake of fertilizer N and soil N
Magdoff and Weil (2003)
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Potential for poor synchrony
Available for loss
Adapted from Robertson et al. (200?)
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Impact of high CN ratio residues on min-N
www.agagadsgasdhg (2003)
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Measuring SOM
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There are many ways to measure SOM
Adapted from Strek and Weber (1985)
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Soil size fractions

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24.9
12.0
5.6
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Seasonal changes in mineralizable N
Adapted from Magdoff and Weil (2003)
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Weil et al. (2003) recently proposed a
permanganate oxidizable C method for routine
assessment of active soil C.
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Conceptual pools vs. Measurable fractions
http//www.life.uiuc.edu/plantbio/wimovac/soilcand
.htm
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N mineralization rate calculator
http//www.qpais.co.uk/nable/minrate.htm
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Managing SOM
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On-farm sources of OM
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Off-farm sources of OM
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The many benefits of organic mulches
Control erosion
Nourish soil fauna
Suppress weeds
Improve soil structure
Increase infiltration
Moderate soil temperature
Supply nutrients
Adapted from Magdoff and Weil (2003)
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Crop Rotation
High residue crops
Cover crops
Forages
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No-till or Minimum Tillage
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Erosion Control Practices
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4 Organic Matter
2 Organic Matter
Effect of 20 years of contrasting management
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Effect of longterm hay on organic N
Richter and Markewitzl (2001)
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Soil Organic Matter
How much is enough ?
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Organic matter functions mainly as it is
decayed and destroyed. Its value lies in its
dynamic nature. W. Albrecht, 1938