Ch 3 Soil Organic Matter
continued
Pools of Organic Matter (OM) in Soils
• ACTIVE (labile, ever-changing, mutable)
– Plant litter
– Light fraction
– Microbial biomass
– Water soluble organics
• STABLE (Humus) recalcitrant, long term,
products of weathering of labile OM
– Humic acid
– Fulvic acid
– Humin
What Does Organic Matter Do?
http://www.soils.umn.edu/academics/classes/soil5611/content/OrganicMatter/
: soilslab.cfr.washington.edu/.../DOMSorption.html
http://www.chm.bris.ac.uk/ogu/images/carboncycle.jpg
http://www.chem1.com/CQ/Carbonate.png
0.25 to 1 Mg C/ha/y sequestered in soils as carbonate
Losses due to
cultivation are
60-80 Mg C/ha/y
http://www.climatescience.gov/Library/stratplan2003/final/graphics/images/SciStratFig
7-5.jpg
www.farmscape.cse.csiro.au/.../mod1/fig1-05.gif
The Changing Forms of Soil Organic Matter
Additions. When roots and leaves die, they become part of the soil
organic matter.
Transformations. Soil organisms continually change organic
compounds from one form to another. They consume plant residue
and other organic matter, and then create by-products, wastes,
and cell tissue.
Microbes feed plants. Some of the wastes released by soil organisms
are nutrients that can be used by plants. Organisms release other
compounds that affect plant growth.
Stabilization of organic matter. Eventually, soil organic compounds
become stabilized and resistant to further changes.
http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html
http://www.ar.wroc.pl/~weber/kwasy2.htm
Composition of SOM
• SOM = humic + nonhumic substances
• Mostly C, H, O ~ 90% on a dry weight
basis
• Remaining 8-10% is mostly N, S and
Cations
• C/N ratio ~ 10
Proposed structure of humic acid
http://www.rsc.org/ej/gt/2000/b001869o/
Computer generated model of
humic acid
http://www.rsc.org/ej/gt/2000/b001869o/b001869o-f3.gif