At its simplest form, cation exchange capacity (CEC) can be described as the capacity of your growing media to hold nutrients or other chemicals, which is directly related to the amount of negatively charged ions present.
When delving into this subject, it is important to remember that a cation is a positively charged particle and an anion is the analogous negatively charged particle. This is important because we are primarily concerned with our media's capacity to bind or release our positively charged fertilizer nutrient ions.
Some of these positively charged nutrients or "cations" that we are most concerned with include potassium, sodium, magnesium and calcium, all of which play a large role in plant development. The roots of our plants as well as the micro-organisms in our media exchange their free hydrogen ions for these nutrient ions, allowing for the fertilizer to be utilized.
Typically, CEC is measured with the unit MEQ/100g, which stands for milliequivalents per 100 grams; this allows for an easier conversion to real-life units such as grams per square foot or pounds per acre. The lower this CEC measurement, the less capacity the media has to hold onto the positively charged nutrients we mentioned above.
This is where hydroponic and soil media vary and why the nutrient management is different between the two models. Rockwool, a popular media in the hydroponic industry, has a CEC of zero meaning it possesses no negative charge to form bonds with our positively charged fertilizers, allowing for extreme optimization of our nutrient delivery but also leaving little margin for error.
On the opposite side of the spectrum, a soil media that is comprised of mostly organic matter or clay has a much higher CEC; this means that constant fertilization is not a necessity, but in turn is more difficult to re-mediate if something becomes out of balance. Even the type of clay present in your soil can directly affect CEC properties.
There are a couple of ways that we can go about determining what the CEC of our growing media is. The easiest being a visual inspection to determine the levels of each material. By looking at the ratio of inorganic material such as sand compared to organic material such as humus, we can determine if our CEC is high or low.
An exact calculation of CEC can be done by taking a look at the soil pH as well as the amounts of potassium, magnesium and calcium present. This calculation is usually best left to the experts at a soil testing laboratory, which many large universities possess. It is often worth contacting your local extension agent to determine where the best place is to send your sample.
When you receive your CEC lab report, it will often contain a table detailing the base saturation percentages of the elements mentioned above. The remaining percentage of your media represents the unsaturated bonds available to be filled with fertilizer elements. By looking at these numbers, we can determine exactly how much more fertilizer we can add to our media.
This is done by first taking into account the atomic weight and charge of each element we would like to add. For example, potassium has an atomic weight of 39 and a charge of +1. If we remember that hydrogen has the atomic weight of 1 and in this example our media has CEC of 1, it would take 39 times as much potassium to fill up the empty hydrogen sites.
To calculate the remaining CEC that can be filled with our fertilizer elements, we simply multiple our given CEC by the remaining base saturation percentage and then divide that number by 100.
For example, if we have a total base saturation of 80% and a CEC of 40, we have 20% left to fill that we multiply by 40, giving us 800/100 or an available CEC of 8 MEQ/100g. From these basic equations we can conclude that the heavier the atomic weight of our soil, the larger the capacity for heavier cations.
The implications that our CEC values have on our gardens are huge and can influence the type of fertilizer we use, the frequency of fertilization and even the length of our final flush of nutrients.
Because media that have a higher CEC have a larger fertilizer holding capacity, it will take longer to flush the media of dissolved salts which can affect your nutrient management choices. Media such as sand and perlite have such a small CEC that all of our plant's nutrients will need to be constantly supplied. These examples are much different than a soil media that has large amounts of organic material meaning a large nutrient holding capacity.
Coco coir, an interesting hybrid of soil and hydroponic production, has been gaining popularity throughout the indoor gardening industry and poses some interesting management obstacles.
Because it has a very small CEC, it can be treated very much like a normal hydroponic application with a pH adjusted nutrient solution that supplies all of your plant's fertilizer needs. But in its raw form, coco coir possesses negatively charged ions that will bind to some of our nutrients, the most common of which is calcium.
Most commercially produced loose coco is treated with a process that fills up these negatively charged ions, but when using the brick form, it is important to keep this fact in mind. By boosting your calcium levels early in the growing cycle, you can effectively fill up these negative ions yourself.
In reality, not all of us are going to run out and get a CEC analysis done of our growing media, but by keeping these basic principles in mind we can further optimize our growing environment.