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Cation Exchange Capacity: the forgotten test of Irish soil analysis

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The Cation Exchange Capacity (CEC) of a soil is an intrinsic property of the soil. It determines the soils ability to move nutrients from the soil particles to the soil solution where it is readily available for plant uptake. Knowing your soil’s CEC is invaluable when determining your soils fertiliser requirements. Nonetheless, in recent years land users have overlooked this test. It is important to note that there are many ways to measure CEC and different tests can return contrasting results; the selection of the appropriate test is dependent on the soil type.

What is the cation
exchange capacity CEC?

In soil, nutrients such as Ca, Mg, Na, K and others are
constantly moving from the surface of the solid soil-particles to the soil
solution. This movement is caused by a weak attraction that binds the
positively charged cations to molecules, like oxygen, on the solid soil-particle;
these weak attractions are called electrostatic charges.

The CEC is not a measure of the number of attached cations,
which is often thought. It is a measure of the number of available negatively
charged oxygen atoms that can bind with positively charged cations. It is the
maximum number of cations that can potentially attach if they are present or
rather, it is a measure of the the total quantity of negatively charged sites
that are available to cations.

On the soil-particle, there will be more negatively
charged oxygen atoms available for binding than are used up by cations. Generally,
when cations are leached from soil by rainwater or are taken up by plant roots,
they are taken from the store of cations that are attached to the exchange
sites. When a cation leaves a site, it is replaced by hydrogen which is also a
positively charged cation. The more of the cations that are depleted the more
hydrogen that replaces them. This results in the soil gradually becoming more
acidic with time and less fertile.

Negatively charged clays tend to have high binding
ability and therefore it can be difficult for a plant to acquire these
positively charged nutrients from the soil solution (as they are bound tightly
to the clay lattice).

What does the
result tell me about my soil?

  • A low CEC (0-10 meq /100g) would suggest that
    the soil has a poor binding capacity. Typically, sandy soils with little clay
    or organic matter. This soil would be prone to leaching.
  • A high CEC (30-40 meq/100g) would suggest that
    the soil has a strong binding capacity, typically seen in heavy soils with high
    clay or organic matter content.
  • Leaching may be observed with soils of a CEC up
    to and including 15/16 meq/100g. CEC value higher than 16 meq/100g should have
    considerate clay content.
  • An ideal soil has a CEC between 18-27 meq/100g.

Base Saturation percentage

The data
collected from the CEC can be manipulated to determine a ‘base saturation’.
Base saturation expresses the percentage of potential CEC occupied by the
cations Ca2+, Mg2+, K+ or Na+

The optimum
base saturation % are:

Na <10%

K 2-7%

Mg 15-20%

Ca 65-75%

The ratio of Calcium to magnesium is critical for plant
nutrient uptake. As a rule of thumb, the Ca to Mg ratio should be 2:1.

What can I do to
improve my soil’s CEC?

There are two main ways to improve your soil’s CEC.
Indeed, both solutions will improve the overall health of your soil.

  1. Adding
    lime to low pH soil can help as it exchanges 2 Hydrogen atoms for 1 Calcium. It
    immediately exchanges the easily available H and over time it replaces more
    bound H.  
  • Adding
    organic matter to soil. Organic matter has a substantially greater CEC compared
    to clays. Therefore adding organic matter to soil will increase your CEC and
    also improve structure which improves all aspects of soil health.

Simply adding nutrients to optimize the base saturation can be futile if the soil is prone to leaching.

In cases where the soil has an extremely low or high CEC i.e. 0 or 40 meq/100g, little can be done for optimising growth without completely changing the structure of the soil.

For more information on CEC and soil testing in general, contact Dr. Conor Murphy Conor@southernscientificireland.com