CEC, what the hell is that I hear you say! I't something you really need to know if your serious about growing. It means cation exchange capacity. None the wiser? CEC refers a mediums ability to exchange cations between organic matter and the plants roots. Hang in there there's more but it's worth it. Cations are positively charged elements such as calcium, magnesium and potassium. Cations are held by negatively charged particles called "colloids". The defining feature of colloids is that the particles are small and consequently the total surface area is huge. For this reason the negatively charged colloids are capable of holding enormous quantities of positively charged cations. As plant roots uptake cations, other cations in the nutrient replace them on the colloid. If there is a concentration of one particular cation in the medium, other cations will force them off the colloid and take their place. Thanks to my mate G. Low for the info and for confusing us! lol. In laymans terms it means the mediums ability to hold, release and replace nutrients. I know mine. What's yours?Think about this as it is an extremely important part of growing. Getting nutrients to your plants is important in my view. A lot of medium mixes will NEVER give you the maximum out of your grow. Why? because of their CEC. They simply can't release and hold nutrients as well as other mediums. Now you don't know me if you have to ask which medium has the highest CEC. This is a small part of plant mechanics and well worth knowing. If you don't know how it works, you've got a battle in front of you. Hmmm, might have to do a thread on that unless someone else wants to explain how a plant works. Never seen a thread on plant mechanics come to think of it. All trying to do it and don't really know how it works.