Soils 205- General Soils

Lecture 25- Soil Organic matter

I.  Soil organic matter- complex mixture of organic substances

    a.  importance

    -source of nutrients

    -CEC is generally >> clays

    -water holding capacity is 4-5 times greater than clays

    -enhances aggregation

    -role in global C cycle

    b.  imbalance

            i.  causes of imbalance

                -tillage

                -deforestation

                -fire

                -artificial drainage

    c.  primary source of SOM- plant tissue

    d.  Formation of SOM

        -process includes degradation and synthesis

        -involves three basic processes

        1) carbon compounds are oxidized producing CO₂, water, energy and decomposer biomass

        2) essential nutrients are released and/or immobilized

        3) compounds very resistant to microbial action are formed through modification of the original tissue or

        by microbial synthesis

    SYSTEM AT EQUILIBRIUM

        FORMATION OF "NEW" ORGANIC MATTER

II.  Humus- complex and resistant mixture of black or dark brown amorphous and colloidal organic substances that results from microbial decomposition and synthesis.

    a.  rate of decomposition and humus formation- controlled by

    1)  climate

    2) quality of organic material- the C:N ratio and lignin content are often used as measures of organic matter quality

    3)  soil properties (texture and drainage)

    b.  C:N and decomposition

        i.  typical C:N ratios

           in soils    8:1 to 15:1

           in plants 20:1 to 400:1

           in microbes  4:1 to 10:1

        ii.  competition for available N

III.  amount and distribution of OM in soils