Cation Exchange Capacity (CEC) is probably the most important measure of soil's fertility, which is directly related to yield and quality of cannabis. If CEC is low, plants cannot really hold onto their cations, even if we pour the fertilisers in bulk. And if it's high we should consider cutting back instead, cause the soil stays charged for a very long time like a good battery.
Here's the breakdown
Soil is composed of different particles, which are negatively charged, and thus attract positively charged molecules called cations. These on the other hand come from our fertilisers, amendments and water.
CEC is basically a measure of the soil’s ability to hold them, which also directly influences its structure, nutrient availability, PH and its reaction to every treatment we can come up with.
That's how cations are absorbed by the root system
As CEC is defined as the amount of positive charge that can be exchanged per mass of soil, usually measured in moles of electric charge (cmolc) per kg, a cation exchange capacity of 10 cmolc/kg could hold 10 cmol of Na+ cations (with 1 unit of charge per cation) per kilogram of soil, but only 5 cmol Ca2+ (2 units of charge per cation).
This in practise can go two ways. If the CEC is low, very few cations can effectively bind to the soil particles, but if the CEC is high, then the opposite is true, and soil particles can hold onto a lot of cations.
For the sake of argument it's really like comparing a Chevrolet pickup truck with Fiat Cinquecento. First one has all the space you may possibly want if you frequently drive loaded up with gardening supplies. And second one gets full after you dropped two gallons of soil on the back seat.
This has consequences for the grower as fertilisation practices have to be adjusted for your soil or particular medium. High CEC soil is for example much more stable in terms of PH and resilience to nutrient leaching (meaning it needs lower rates of nutes and applied more rarely), while low CEC soil will perform worse in these areas and has to be kept in check all the time.
But how do we raise it if we have CEC problem? Usually by adding organic matter such as humus or EWC, which greatly increases total surface area and provides more binding sites. And this leads us to a trivial, but important conclusion, that the best soil is always an organic soil.
And this table shows what CEC of popular growing mediums is, so you can better understand what you're dealing with
Photo credit: Nice bud above is #highbrix Slurricane (In House Genetics) grown by my buddy The Green Yeti. Courtesy of 420Magazine