1. Ch 3 Soil Organic Matter
continued

2. 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

3. What Does Organic Matter Do?

4. : soilslab.cfr.washington.edu/.../DOMSorption.html

6. 0.25 to 1 Mg C/ha/y sequestered in soils as carbonate
0.25 to 1 Mg C/ha/y sequestered in soils as carbonate

7. Losses due to
cultivation are
60-80 Mg C/ha/y

9. 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.

11. 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

13. Proposed structure of humic acid

14. Computer generated model of humic acid