Wikipedia has a decent article on CEC here:
en.wikipedia.org
As does Cornell University here:
And here, for the more tolerant and patient of you, is my interpretation of what I have found on CEC - the OP did ask about why:
CEC comes primarily from two different soil or growing medium components. In normal outdoor soils, a significant portion of the CEC comes from the clay component of the soil, and importantly,
the CEC of clay is not affected by pH. The other major contributor to CEC is organic matter, and, unlike the CEC from clay,
its CEC changes with pH.
Most of the growing media us indoor growers use have little clay in them, so the CEC's of the stuff we use is usually controlled by organic material,
and will change with pH. Hence availability of calcium, magnesium, and potassium especially will vary with the pH of our media, so managing pH and allowing it to vary within an appropriate range is generally a good idea.
Hydroponics is special. Pure hydroponics without growing medium has no CEC, but it is perhaps more accurate to say that CEC is irrelevant - all nutrients are in immediately available ionic form. Their availability is not affected by the ability of a growing medium to hold and release cations - whatever we put in the nutrient solution is available, period. This allows tailoring and controlling ratios of nutrients but it means that the grower has complete responsibility for doing this. No medium, no interference with nutrient availability, but
also no help from the stabilizing influence of soil or growing medium. Ph still needs to be controlled for plant health in pure hydroponics, but not because of its influence on CEC, but rather because of other direct influences on nutrient absorption from solution.
"Hydroponics" with coco is different again. There is a good discussion of this medium and its CEC here:
The widespread use of calcium and magnesium supplements in the indoor gardening industry i
www.maximumyield.com
Bottom line for "hydroponics" with coco is that it has special CEC behaviour - namely it starts with too much potassium and/or sodium attached to its CEC sites, and very little calcium and magnesium. If it's CEC sites are not loaded up with calcium and magnesium by initial buffering, the coco will bind up the calcium and magnesium while releasing the potassium. Result, calcium and magnesium deficiency, and/or lockout due to excess potassium. According to the Advanced Nutrients blurb on their coco line, iron is also part of this CEC issue with coco. Also, since coco's CEC behaviour is due to organic CEC sites, it behaviour is dependent on pH. Bottom line is that coco needs to be buffered with calcium and magnesium, and coco specific nutrient design and pH managegment is a good idea. The notion that coco is hydroponics reflects the need for detailed management of nutrients, but this management is different than hydroponics with no medium.
As if all this were not complicated enough, stuff we add or don't add to our soils or growing media change CEC behaviour. Vermiculite has very decent CEC, for example, expanded clay balls, specific CEC behaviour as well. If we mix coco into our peat mix, we proportionally change the CEC behaviour of the combined medium. And so bloody on. The bottom line in all of this is that the specific behaviour of nutrients in our grow will vary with media components we put in there, and with how we manage our nutrients and pH. Given the complexity, trying to reinvent the wheel is not necessarily a good idea, It is best to
stick with the specifics of a proven approach, especially when dealing with so called "soil-less" or "hydroponic" media.
If you got this far in this blurb, congratulations, you likely have more than enough patience to grow nice weed.
