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Geosciences LibreTexts

1.8: Soil Organic Matter Content: Loss-On-Ignition

  • Page ID
    15137
  • Purpose:

    The purpose of this exercise is for you learn proper laboratory techniques to measure organic matter content and aggregate stability and become familiar with how they are related.

    Learning Outcomes:

    Upon completion of this exercise you should be able to:

    • determine organic matter content of a soil sample using the loss-on-ignition method
    • understand how soil organic matter content affects soil stability and erosion

    Background:

    From Brady and Weil, The Nature and Properties of Soils, 13 th Ed. Soil organic matter consists of a wide range of organic substances including living organisms such as bacteria and fungi, organism remains such as partially decayed plant material and soil organisms, and organic compounds formed by metabolic processes. Organic matter comprises only a small portion of a typical soil (~1 to 7% in surface horizons and much less for subsurface horizons).

    Organic matter binds mineral particles, giving soils a granular structure, keeping the soil loose and aerated. Organic matter also increases the water holding capacity of soils and water availability to plants. As organic matter decays it releases essential nutrients to the soil that is taken up by plants. Also, organic matter is the primary source of carbon and energy for soil organisms.

    As organic matter continues to break down and accumulate within the soil, humus forms. Humus is the colloidal fraction of soil organic matter. This gives fertile soils the distinct dark brown to black color. Humus has a surface charge that attracts and binds nutrients ions and water molecules, increasing soil fertility. It is also one of two sources of cation exchange capacity (CEC) in the soil (clay is the other major source). CEC represents the sites in the soil that can hold positively charged nutrients like calcium (Ca++), magnesium (Mg+) and potassium (K+). If CEC is increased, the soil can hold more nutrients and release them for plant growth. To increase CEC, you must increase soil organic matter content.

    Equipment required:

    • Soil samples
    • Convection oven
    • Insulated gloves
    • Soil spatula
    • Top load balance
    • Mortar and pestle
    • Crucibles
    • Sharpie marker
    • Weighing paper
    • Tray (2)
    • Tongs
    • Muffle
    • Furnace
    • Tweezers
    • Muffle furnace map (2)
    • Calculator

    Exercise:

    1. Use the soil samples you dried, ground, and sieved in the sample preparation exercise.
    2. Clean and dry crucibles following cleaning methods outlined in sample preparation exercise.
    3. Label crucibles and weigh on balance, record crucible weight (C1).
    4. Thoroughly pick ALL roots from soil samples.
    5. Place soil samples in clean, labeled crucibles and weigh on same balance, record crucible + sample weight (C2). 
    6. Record sample locations on “muffle furnace map” and place crucibles on the map.clipboard_e672ab04f747b074b2795adc912912a56.png
    7. The following muffle furnace program is necessary to remove organic matter from soil samples: 1) Ramp muffle furnace to 550o C in 1 hour (a rate of ~9.15oC/min); 2) soak samples in muffle furnace at 550oC for 2 hours; 3) allow samples to cool for 4 hours or moreclipboard_e4c6e2496d5ea78e352e7e0940a6b436f.png
    8. Allow samples to cool in muffle furnace until sufficiently cool to handle with gloves, place in convection oven if still too hot to place on scale.
    9. Weigh crucible + no OM sample on balance, record crucible + no OM sample weight (C3).
    10. Calculate the percent organic matter by weight using the following formula: 

    % Organic Matter = [(C2 – C3) / (C2 – C1)] x 100

      Sample 1 Sample 2 Sample 3
    C1 (Crucible Weight)      
    C2 (Crucible + sample weight)      
    C3 (Crucible + no OM sample weight)      
    % Organic Matter      

     

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