What is Cation Exchange Capacity?

Cation exchange is not hard to understand, but the technical jargon used can make it seem like an 'experts only' concept.

A Cation (pronounced “Cat-Iron”) is a particle with a positive charge. For every positive, of course, there must also be a balancing negative - you simply cannot have one without the other. For example, if you took some Calcium Chloride and dissolved it in water the Calcium would be positively charged part and the Chloride ion would be negatively charged.

Cation exchange then is what happens when a negatively charged particle swaps (Exchanges) one kind of cation for another.

An interesting thing about soil is that its very tiniest particles (Clay and Humus) are negatively charged. So they work like anions - attracting and holding cations.

These clay and organic particles are so small that they will remain suspended in water for many hours. Any particle that can do this is called a “Colloid” and in soil there are two main kinds of Colloid – mineral Clay particles and organic Humus.

Each of these particles can attract and hold a number of Cations – the most important of which are Calcium, Magnesium, Sodium, Potassium and Hydrogen. So the more colloids a soil contains the greater its “Capacity” to hold Cations.

Cation Exchange

Imagine you have a jar full of jellybeans. The total number of jellybeans that will fit in the jar is its Jellybean Holding Capacity (JHC).

Now let’s suppose you wanted one of the jellybeans – you have a problem. In this particular jar there is a rule that cannot be broken – The Jar MUST ALWAYS be Full! This means that if you want to take a jellybean out, you must Exchange it for another one.

The result is that the size of the jar not only represents the number of jellybeans it can hold, but also the number available for Exchange so the JHC is also the Jellybean Exchange Capacity (JEC).

Now instead of jellybeans, think of Cations in soil and guess what you have. That’s right – the Cation Exchange Capacity (CEC). That is pretty much all there is to it!

An Important Soil Function

The importance of Cation Exchange in the way soil functions cannot be overstated. It is one of the important mechanisms by which plants obtain nutrients and it has a strong influence over soil structure and productivity. It can also affect on soil texture to some extent.

Having said this, there is no good or bad level for the CEC. A sand may have a CEC of 3 or so, while a clay could be around 30. It is just a characteristic of each individual soil.

The CEC is a defining characteristic of any soil, rather than something to be managed. Just like the Jellybean jar, each jar has a particular size and can hold just so many jellybeans - no matter how much you like jellybeans, you can't really change the size of the jar!

Of course the CEC may change a little over time, if the organic matter level of the soil changes, such changes are usually small.

The Exchange System in Soil

The Cation Exchange Capacity is actually the hub of a dynamic system upon which the entire soil ecosystem depends. It has inter-relationships with plant roots (& other soil organisms), the soil solution and the minerals that are being weathered to form new soil.

The cations that occupy the exchange “sites” in the soil come from both the minerals in the soil and salts dissolved in the soil solution. One in particular – Hydrogen – also comes from the breakdown of organic matter, exudates from plant roots and the use of acidifying chemical fertilizer.

Hydrogen is (in a sense) the main “Currency” of the exchange system. In other words, it the thing that plants use to access cations held by soil colloids.

Remember our Jellybean Jar and how you could only get one out of the jar by exchanging it for another? Well this is precisely what plants do – they exude weak acids from their roots that exchange Hydrogen for cations. Plant roots then absorb the resulting salts from the soil solution.

Logically then, to reverse this situation and improve soil, we need to know how much Hydrogen and other cations are present in order to determine the nature of any soil problems associated with the cation exchange system.

Another consideration is that the effectiveness of this system depends more on the relative proportions of the major cations (ie. Soil Balance) than the actual CEC.

For example, if the soil is high in Magnesium (relative to Calcium) it will tend to be hard setting when dry. Similarly, soils with high percentages of Magnesium, Sodium or both, will tend to be dispersive when wet and form a hard surface crust when dry.

Whether the soil is a Loamy Sand with a CEC of 3 me/100g or a Medium Clay with a CEC of 28 me/100g, the amounts of Calcium, Magnesium and Sodium will differ, but (in similar proportions to each other) their effect on soil structure and soil balance will be the same.

Imbalances like these will also affect soil structure and agricultural productivity. The latter will be especially noticeable, since hard soil will restrict root access and limit growth.

Such growth restriction often leads people to assume there is an effect on plant nutrition. However, fertilizer that is applied to relieve the problem may often have little or no effect.

One reason for this is that cation imbalances affect all organisms in the soil – not just plants – and these other soil organisms often assist in nutrient cycling processes and even plant uptake.

The question now is: “How should we correct this imbalance?”

Ted Mikhail's research revealed one more important aspect of the soil exchange system - Organic Matter!

The organic colloids in soil have their own requirement for exchangeable hydrogen and balancing all this with calcium and magnesium - as we do for the clay colloids - would lead to a loss of soil organic matter.

To avoid this, the Mikhail System includes an adjustment to the measured cation exchange capacity, based on the percentage of organic matter in the soil. This is called the Adjusted CEC and it makes the process of soil improvement all the more reliable, while also protecting the vital organic matter in the soil.

Further Reading

The causes of various soil problems
Once you understand how the relative proportions of various cations affect the soil, you can easily identify the causes of soil problems.

The actions of Lime, Dolomite and Gypsum in soil improvement.
Each of these materials has a different action in the soil, so choosing the right one (or combination) is important to getting a good result with soil improvement.