Cation Exchange Capacity (CEC) is a value given on a soil analysis report to indicate its capacity to hold cation nutrients. The CEC, however, is not something that is easily adjusted. It is a value that indicates a condition or possibly a restriction that must be considered when working with that particular soil. Unfortunately CEC is not a’ packaged’ product.

Clay or Humus is the KEY

The two main colloidal particles in the soil are clay and humus and neither are practical to apply in large quantities to large areas, without a mechanical spreader, and on clay soil, a tractor with disc or tiller attachment may be necessary to break up the ground the first time.

The CEC of the soil is determined by the amount of clay and/or humus (well-aged compost) that is present. If dependent on clay, you only have soil. If humus is present, you can call it Dirt. These two colloidal substances are essentially the cation warehouse or reservoir of the soil and are very important, because they are primarily responsible for nutrient and water holding capacity. Sandy soils with very little organic matter (OM) have a low CEC, but heavy clay soils with high levels of organic decayed (compost is recommended) matter (OM) would have a much greater capacity to hold cations.

Disadvantages of Low CEC

The disadvantages of a low CEC obviously include the limited availability of mineral nutrients to the plant and the soil’s inefficient ability to hold applied nutrients. Plants can exhaust a fair amount of energy (that might otherwise have been used for growth, flowering, seed production or root development) scrounging the soil for mineral nutrients. Soluble mineral salts (e.g. Potassium sulfate) applied in large doses to soil with a low CEC cannot be held efficiently, because the cation warehouse or reservoir is too small.


Water also has a strong attraction to colloidal particles. Organisms such as plants and microorganisms that depend upon each other’s biological functions for survival are inhibited by the lack of water. Where there is little water in the soil (i.e., too sandy) there is oftentimes an abundance of air which can limit the accumulation of organic matter (by accelerating decomposition) and further perpetuate the low level of soil colloids.

High Levels of Clay are BAD

High levels of clay with low levels of OM would have an opposite effect (i.e. a deficiency of air), causing problems associated with anaerobic conditions. The CEC in such a soil may be very high, but the lack of atmosphere (air) in the soil would limit the amount and type of organisms living and/or growing in the area, causing that immediate environment to be deficient in productive plant growth. If a soil has a very low CEC, adjustments can and should be made, but not solely because of the CEC.

No OM is BAD

A soil with a very low CEC has little or no clay or humus content. Its description may be closer to sand and/or gravel than to soil. It cannot hold very much water or cation nutrients and plants cannot grow well. The reason for the necessary adjustment is not for the need of a higher CEC but rather because the soil simply needs conditioning. A result of conditioning treatment is higher OM which automatically results in a higher CEC, depending on the humus content of the OM.

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What Does It All Mean?

SO – What does all that mean to the average organic gardener? Nothing. OK – an ion is an atom or molecule with a net negative or positive charge. Like a magnet is ‘charged’. Like charges repel, and opposite charges attract. That’s basically all you need to know.

CEC on a Soil Analysis Report

CEC should be listed on a soil analysis report – which you need, if your plants are not growing as expected. And if you don’t know what to expect, consult organic gardening magazines and look at pictures of your plants. Ever notice that very seldom do you ever see photos of scraggly, unhealthy-looking plants in the gardening magazines? Yours are supposed to look like the good example photos in the magazine. If not, you need to figure out why not. That’s the key to making changes that improve plant growth.

Soil Testing

You should get some of your dirt tested every year, even if your plants are doing great. Why? Find out which way your dirt is headed – improving, or getting worse in some aspect? Stay ahead of it, and don’t let it ‘catch you’ from “behind” (know what I mean)? Much easier to fix before actually goes bad – and that doesn’t take long in very sandy or heavy clay soils.

Understanding CEC

If you asked for a CEC rating on the soil report and didn’t get one, change soil analysis laboratories. If you got a CEC rating on the report and don’t know what it means, call the laboratory and ask for explanation. If you don’t get a satisfactory answer, change soil analysis laboratories and try again, until you do get an understandable answer, in relation to all other factors that pertain to YOUR analysis report. Ask: what do I need to do to make it better? Laboratory technicians should be able to speak layman English too. Answer should be: add more clay or aged (high-humus content) compost. Also consider adding agricultural (finely powdered – not pelletized) gypsum to clayey soil. Spread compost on top of the ground and work it in as best you can, with a strong garden rake. If your soil is heavy clay, break it up with a power tiller, ONE time. Rake it in after that. Too much tilling is VERY bad for dirt. If sandy soil – do NOT till compost. BAD to till compost into sand. Breaks it up much too fine, which causes it to decompose much too quickly. And remember – ALL compost decomposes to almost no volume in time – with the exception of the percentage of humus that has developed in it (due to microbial activity).

Maximizing the percentage of Humus? is why composting is SO MUCH MORE EFFECTIVE for permanently changing soil into Fat Dirt - because the chances of OM decaying to make a sufficient amount of humus after its IN the ground - is not as good. It is the Humus in compost that most affects CEC and soil/dirt structure/texture.

So composting your organic matter (OM) as a feedstock with the hot, aerobic method (to 131oF+), and letting it finish/age enough before adding it into soil/dirt as an amendment, is highly preferable.

Yes, there is value of OM as a raw soil/dirt amendment. Yes there is value to allowing grass clippings and leaves to slowly rot into the ground. Yes, cut branches and stout yard clippings will eventually decay. But those items would be MUCH MORE VALUABLE if collected and composted - THEN added to the ground with a considerable microbe population and nutrients already available to plants.

© Robert C. Moore ~ All Rights Reserved

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