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

http://www.climatescience.gov/Library/stratplan2003/final/graphics/images/SciStratFig7-5.jpg

Losses due tocultivation are

60-80 Mg C/ha/y

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

http://www.rsc.org/ej/gt/2000/b001869o/

Proposed structure of humic acid

Computer generated model of humic acid

http://www.rsc.org/ej/gt/2000/b001869o/b001869o-f3.gif