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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.climatescience.gov/Library/stratplan2003/final/graphics/images/SciStratFig7-5.jpg
Losses due tocultivation are
60-80 Mg C/ha/y
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
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