Cation (NH4+, K+, Ca2+, Fe2+, etc...) Exchange Capacity (CEC) is defined as the degree to which a soil can adsorb and exchange cations.
Okay, the 2 year College degree brain muscle is being stretched. Nothing from the Barron's Aquarium plant book.....
Think, think, think...... Come on Pooh Bear! Think, think, think....
Organic Gardening for Dummies? No.
Square Foot Gardening? No.
Seachem Product literature. No.
Seachem H2Grow freshwater planted brochure. No.
Think, think, think....... College and High School Chemistry?
Think, think..... GETTING CLOSER..... think, think, think......
K+ (Potassium maybe??) Ca2+ (Calcium something..) Fe2+ (Good Iron, not the Bad iron Fe3+)
N (Nitrogen) (1) H (4 hydrogen atoms) with a + ion....
Think, think, think..... Cation = Ions???? POSITIVE IONS!!
Think.. Exchange = giving and taking things
Think.. Capacity = How much that can be held by something -
Like how much Grape Juice can be in the Juice Pitcher while the 5 year old carries it across the room without spilling it...... (stains carpets terrible you know..)
Think, think.
Soil = good stuff, natural, source of bio-available carbon and nutrients with good bacteria that helps nuture roots and root hairs to absorb good organic gardening compost soil to grow more good stuff.....
Not Miracle Grow (Bad Stuff) which can burn root hairs and kill off GOOD Bacteria which we want to keep the plants healthy and not waiting for its next fix of (MIRACLE GROW)
So CEC is good organic gardening stuff -
good substrate that you get from Natural Stuff,
that feeds and nutures the plants in our aquariums to make them glow healthy green, red colors right??
Okay, I don't really know what you are talking about - but I am trying like all heck!
Any truckers here?
Cec 
Hybrid Mode
