After researching, gathering data and a recent discussion on cation exchange capacity (CEC) found here: https://www.plantedtank.net/forums/29...iscussion.html
I have put together some results of CEC ratings from various planted tank substrates. Before I dive into those, I should disclaim that I have no bias towards what substrates anyone uses, if it works for you, fantastic, there is no “one size fits all” substrate in my opinion. I think we have all seen absolutely stunning planted tanks regardless of which substrate was used! I should also disclose that by no means are these CEC ratings a definitive answer to “what substrate is best.” I simply wanted to put some values to the CEC discussions I am seeing all over threads, regardless of which forum I was reading. Some of this is my educated opinion, so remember that when reading, feel free to correct me or discuss any matter you feel necessary.
What is CEC – CEC measures the amount of nutrients, more specifically positivity changed ions, a substrate can hold onto/store for future plant use, the most common being potassium, calcium and magnesium. The CEC rating is generally shown in milliequivalents per 100 grams of substrate (meq/100g). The higher the number, the higher the CEC.
What contributes to CEC – Generally organic material (OM) and clay content are the greatest contributors to substrate CEC. So as the OM and clay content increase, so does the CEC. This puts sandy substrates on the low end, and clay substrates on the high end.
Surface area – Higher surface area generally results in a higher CEC. The more surface area there is, the more potential bonding sites there is along the CEC. This is where the results below are interesting, clay based, planted tank substrates have a very poor rated CEC.
Organic potting soil sources:
humus - 200
vermiculite - 150
sphagnum peat moss - 100 - 200
fine clay soils - 55-65
2:1 v/v) bark:perlite - 24
(1:1 v/v) peat:vermiculite – 141
Tripepi, R. R. (2014). What Is Your Substrate Trying to Tell You Part II.
Common planted tank substrates:
Special Kitty Cat Litter – 27
Seachem Fluorite – 1.7
Seachem Onix – 1.3
Caribsea Tropic Isle Laterite (similar to eco-complete) – 6.7
Calcined Montmorillonite Clay (Safe-T-Sorb) – 30-40
Play Sand - < 0.1
Johnson, J. S. (2000). Substrates for the Planted Aquarium. Planted Aquaria. 17-23.
Substrates for the Planted Aquarium
ADA Aquasoil/other related substrates – Because this is likely formulated from organic matter, clay and binding agents, we can expect a high CEC. I have read on various posts claiming this product is similar to rice paddy soils resulting in a CEC rating of 50 meq/100g. I would take this one step further given the true, tested CEC rating of organic matter mentioned above. I believe a CEC rating of 50-100 meq/100g would be more suitable.
Blasting Sand/Pool Filter Sand – Assuming CEC rating of <0.1 meq/100g (estimated via play sand rating)
As you can see, a dirted tank should have the highest CEC, so is “burning out” a dirted tank a myth? I think we aren’t feeding the CEC of the dirt, so eventually is will burn out. Feed the dirt folks!!
Fluorite and Eco-Complete style soils have poor CEC ratings: If we look at the physical structure of these substrates, we can see that there is lots of “room” between each granule. This corresponds to lots of macro pores which bring the CEC rating per 100 grams down: there is no CEC in macro pores, so more macro pores per 100 grams of sample will result in a smaller CEC rating. But, looking at the product page on each of these substrate’s websites, they don’t claim a high CEC, I think us as hobbyists adopted the term because the materials these substrates are made of are thought to have a high CEC. Not the case.
So, what can these macro pores offer? Well, anyone with Fluorite/Eco-Complete, go look at your tank and you’ll see lots of detritus and mulm that occupies these macro pores. The detritus/mulm is slowly being broken down into plant available compounds via our microorganism populations. This can be compared to a CEC because the CEC stores plant available nutrients to be released at a later date, while decomposing detritus/mulm is simply stored nutrients being released at a later date via the decomposition process. This is however, not as sanitary as proper stored plant available compounds in the CEC from let’s say, dry fertilizers. As the organics decompose, this drives up TDS, releases non-plant usable compounds, could drive algae growth, could cause high point sources of nitrates and generally fouls the water more so than supplying plants with traditional fertilizer. Of course, lots of these compounds can be stored in the CEC, but many are released into the water column. It is my belief that CEC in the planted aquarium will fill up rather quickly, and be of little use, instead we mistake the CEC for decomposing organics within macro pores in our substrate.
What about “heavy root feeders?” – While this is a hot/touchy subject, heavy root feeders in my mind are plants that are simply better suited/more efficient at taking up nutrients via their roots instead of shoots/leaves. There will be root interception of both detritus/mulm/root tabs and CEC nutrients, but I believe mass flow and diffusion from water (both above and below the substrate) is the main way of nutrients getting into our plants. So in theory, detritus/mulm/root tabs would have to be released into the water to be up taken into the plant. Some of the nutrient dense water will remain within the pores of the substrate, easily taken up through roots, while lots will leach into the water column. For this reason, I believe it is too complicated to place a term such as CEC as being a sufficient single source of nutrients for plants.
Overall, the CEC of planted tank substrates in my opinion, is an over estimated source of nutrients, and really shouldn’t be praised when talking about substrates. The bulk nutrients our plants use are likely leached into the water column anyway, with the CEC remaining maxed out at all times. If you skip fertilizing, I believe the decomposition of detritus/mulm is a larger source of “stored” nutrients than the CEC of your substrate. High CEC of course helps and is useable, but within aquaria is it only a small building block of nutrient availability.