The Cation Exchange Capacity (CEC) of soil is an intrinsic property of the soil. It determines the soil’s ability to move nutrients from the soil particles to the soil solution, where it is readily available for plant uptake. Knowing your soil’s Cation Exchange Capacity is invaluable when determining your soil’s fertiliser requirements. Nonetheless, in recent years land users have overlooked this test. It is important to note that there are many ways to measure Cation Exchange Capacity, and different tests can return contrasting results; selecting the appropriate test depends on the soil type.
What is 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 Cation Exchange Capacity is not a measure of the number of attached cations, which is often thought of. It measures the number of 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. Put another way, it is a measure of the total quantity of negatively charged sites that are available to cations.
On the soil particle, more negatively charged oxygen atoms will be available for binding than are used up by cations. Generally, when cations are leached from soil by rainwater or taken up by plant roots, they are taken from the store of cations 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 the cations are depleted, the more hydrogen replaces them. The soil gradually becomes more acidic with time and less fertile.
Negatively charged clays tend to have a high binding ability. 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 % is:
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 Cation Exchange Capacity?
There are two main ways to improve your soil’s CEC. Indeed, both solutions will improve the overall health of your soil.
- 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 replaces more bound H over time.
- Adding organic matter to the soil. Organic matter has a substantially greater CEC compared to clays. Therefore adding organic matter to soil will increase your CEC and improve the structure, improving 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 Cation Exchange Capacity, i.e. 0 or 40 meq/100g, little can be done to optimise growth without completely changing the structure of the soil.