Cation Exchange Capacity

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The constituent parts of a soil determine how the soil functions within a landscape. There are 3 mains divisions in division a soil ingredients: living matter, dead/decaying matter and matter derived from geological processes. Each of these groups affects the functioning of the soil, as is partly explored in the soil texture modelling exercise.

Organic matter comprising of the decaying remains of plants, soil organisms and above ground organisms which have been deposited within the soil. Organic matter [OM] comes in many shapes and sizes as it undertakes the complicated and often long process of decomposition by bacteria, fungus and many other life forms. These chunks of OM have many negatively charged sites which bind with cations entering and travelling through the soil in solutions. In this demo we use a soil that is high in OM and a soil that is low in OM, it is useful to use the same soil with added compost so that different clay contents do not overshadow the impact of OM. A dye called Methylene Blue is used to demonstrate the cation absorption capacity of the soils and can be bought from a number of lab providers, as it is commonly used as a cell stain.

The dye dissolved in water is poured through the soil, where the dye binds with the negative receptors of of the soil. A soil with the most bonding sites has a higher cation exchange capacity and thus is a better filter for many potential pollutants that travel through soils. The soil with a higher CEC will remove more of the dye, producing a clearer liquid run off. It is an easy visual demonstration of CEC.

Specific learning outcomes:

    • The mechanism of CEC
  • How soils act as water filters within a landscape

Equipment for a group of 5:

    • Methylene Blue dye (powder form)
    • Water
    • Measuring cylinder x2
    • Teaspoon
    • Plastic funnel x2
    • Kitchen roll
    • Pint glass x2
    • Sample soils x2
  • Tablespoon


Estimated time ~10 minutes.

    • Gathering soil samples and mixing with compost if required.
    • Folding kitchen roll -see images in pdf but essentially to create a coffee filter style cone of paper, to act a block of the soil falling through into the glass with the dye.
  • Mixing dye.


Introduction 2-5 mins

Demonstration 5-10 minutes

Total timing ~15 minutes.  

Background learning needs:

    • Understanding of constituent parts of soil
    • Some level of basic chemistry
  • Understanding of soil function as a filter of freshwater


    1. Mix ½ teaspoon of Methylene Blue dye within 500ml of cold water.
    1. Divide this into 2 lots of 250ml within the measuring cylinders.
    1. Place the pre-folded kitchen roll into the funnel and place these in the pint glasses.
    1. Take an equal volume of soil ~8 tablespoons of crumbled soil of both samples into the kitchen roll.
    1. Pour the dye through each of the soils, the dye may take a while to be absorbed, be patient and top it up when there is space, try not to let it run over the edge or escape down the side of the kitchen role.
  1. Observe both the time it takes for the dye to move through the samples and the difference in the clarity of the liquid that appears within the pint glass. The clearer the liquid the more of the dye has been absorbed onto the soil particles.