The substrate you stick with? The idea of Cation Exchange Capacity...

The botanical-style aquarium "movement" has begun to really evolve. It's been interesting to see the fine work of so many hobbyists really push the envelope out a bit.

One of the things you might have noticed lately is that we're looking at more and more ways to create realistic, functional, and aesthetically unique aquariums. It's not just about having a large selection of botanical's about experimenting with them and other tangential natural materials to help recreate some of the aspects of the natural habitats which we admire so much.

Like, substrates, for example.

As you know by now, we've been developing (seems like forever) a line of substrates, designed to help you experiment at recreating various types of aquatic habitats, such as the igapo and varzea of South America, among others. And when you develop new products like substrates, you have to consider how the end users will embrace them (or, IF they will embrace them!), and how they will apply them to their work.

Inevitably, there are questions.

The genesis for today's piece was a message that we received from a member of our community, who was curious about some interesting aspects of botanicals and of our substrates- specifically about their "CEC" factor- something which aquatic plant fans consider when evaluating substrates for growing their plants.

What is "CEC?"

Cation Exchange Capacity (CEC) is the ability of a material to absorb positively-charged nutrient ions. This means the substrate will hold nutrients and make them available for the plant roots, and therefore, plant growth. CEC measures the amount of nutrients, more specifically, positivity changed ions, which a substrate can hold onto/store for future use by aquatic plants.

Thus, a "high CEC" is important to many aquatic plant enthusiasts in their work.  While it means that the substrate will hold nutrients and make them available for the plant roots. it doesn't indicate the amount of nutrients the substrate contains. 

For reference, scientists measure cation exchange capacity (CEC) in milliequivalents per 100 grams ( meq/100g).

To really get "down and dirty" to analyze substrates scientifically, CEC determinations are often done by a process called "Method 9081A" of EPA SW- 846. CEC extractions are often also analyzed on ICP-OES systems. A rather difficult and expensive process, with equipment and methods that are not something hobbyists can easily replicate!

As you might suspect, CEC varies widely among different materials. Sand, for instance, has a CEC less than 1 meq/100 g. Clays tend to be over 30 meq/100 g. Stuff like natural zeolites are around 100 meq/100g! Soils and humus may have CEC up to 250 meq/100g- that's pretty serious!

What nutrients are we talking about here? The most common ones which come into play in the context of CEC are iron, potassium, calcium and magnesium. So, if you're into aquatic plants, high CEC is a good thing!

Of course, this is where the questions arise around the substrates we play with.

It makes sense, right?

Our "Nature Base" substrates that we are going to release soon do contain materials such as clays and silts, which could arguably be considered "higher CEC" materials, because they're really fine- and because higher surface area generally results in a higher CEC. The more surface area there is, the more potential bonding sites there are for the exchange to take place. Alas, nothing is ever exactly what we hope it should be in this hobby, and clays are often not all that high in their CEC "ratings."

Now, the "Nature Base" substrates are what we like to call “sedimented substrates”, because they are not just sand, or pellets of fired clays, etc. They are a mix of materials, and DO also have some soils in the mix, too, which are also likely higher in CEC. 

Promising, from a CEC standpoint, I suppose!

However, they were really created to replicate the substrate materials found in the igapo and varzea habitats of South America, and the overall habitat- more "holistically conceived"-not specifically for plant growth. And, in terrestrial environments like the seasonally-inundated igapo and varzea, nutrients are often lost to volatilization, leaching, erosion, and runoff..

So, it's important for me to make it clear again that these substrates are more representative of a terrestrial soil, and are not specifically formulated to grow aquatic plants luxuriously. Interestingly  the decomposition of detritus and leaves and such in our botanical-style aquariums and "Urban Igapo" displays is likely an even larger source of “stored” nutrients than the CEC of the substrate itself, IMHO. 

An added benefit of these types of substrates is that they will provide a home for beneficial bacteria- breaking down organics and helping to make them more available for plant growth. 

That being said, the stuff DOES grow aquatic and riparian plants and grasses quite well, in our experience! Yet, I would not refer to them specifically as "aquatic plant substrates." They're not being released to challenge or replace the well-established aquatic plant soils out there. They're not even intended to be compared to them!

Remember, our substrates are intended to start out life as "terrestrial" materials, gradually being inundated as we bring on the "wet season." And because of the clay and sediment content of these substrates, you'll see some turbidity or cloudiness in the water. It won't immediately be crystal-clear- just like in Nature. That won't excite a typically planted aquarium lover, for sure. And no, we haven't done CEC testing with our substrates...It's likely that in some future, some enthusiastic and curious scientist/hobbyist might just do that, of course!

I can't stress it often enough: With our emphasis on the "wholistic" application of our substrate, our focus is on the "big picture"- not specifically aquatic plant growth. Yet, hobbyists being hobbyists, I'm sure that they will evaluate them based upon this ability, so I felt that I should at least address this topic at this juncture.

High CEC certainly can help grow aquatic plants in these or other substrates; however, it's important to note that it's only a small component of of nutrient availability for the plants. 

And of course, if we go back to the idea of CEC being a "measure" of a material to absorb positively charged nutrient ions, the question comes full circle to, "Do botanicals themselves have "CEC" capabilities?"

The answer is, I'm not really sure!

Yet, the postulating non-scientist in me can't help but wonder if, much like loam or humus, that these plant-derived materials have this unique ability. I mean, we talk about botanicals and leaf-litter beds as attachment points for biofilms, fungal growth, and other organisms, so it makes sense to at least postulate that they have the capacity to sequester some nutrients as they break down, right?

I think that it does.

I mean, at the very least, we've played around with the idea that decomposing leaves and such comprise a sort of aquatic "mulch" for plant growth. Mulch, by definition is: "Material (such as decaying leaves, bark, or compost) spread around or over a plant to enrich or insulate the soil." What works in the garden likely works in the aquarium, in the context of "substrate enrichment", right?

I think that the possibility of utilizing botanicals along with specialized substrates as a means to grow plants and enrich the overall aquarium ecosystem on many levels is simply too irresistible to overlook. Indeed, botanicals are a sort of "substrate" in and of themselves- and that is something that's quite interesting! 

The body of experience that we as hobbyists are accumulating with botanical-style aquariums is not only resulting in greater understanding and refinement of techniques in our sector- it's "trickling back" to other sectors of the hobby, such as aquatic plants, etc.

And of course, for facilitating and elevating the idea of the "Urban Igapo"- a "transitional" aquatic/terrestrial habitat replication...something that's seldom been done before in the hobby!

And we couldn't be more excited to see that! Remember, the "substrate" that you stick with in your aquatic display can have a profound and important impact on the diversity, health, and stability for many years to come!

Here's to some exciting new experiments!

Stay excited. Stay curious. Stay observant. Stay diligent...

And Stay Wet.


Scott Fellman

Tannin Aquatics 







Scott Fellman
Scott Fellman


2 Responses

Scott Fellman
Scott Fellman

August 18, 2020

Yup, you were the inspiration here!

I love your idea on creating a more resilient microbial population! It’s part of a bigger thesis that I have that a bed of botanicals can serve as a sort of analog to the “deep sand bed” I na reef tank- a place for denitrification, nutrient processing and export, etc. It’s a very interesting thing, which I think mimics a lot of the processes which occur in the natural habitats we are trying to replicate. I think the whole idea of botanicals and leaves in a planted aquarium has much, much more to give us, in terms of ideas, information, and even breakthroughs in water quality management. A very interesting time!

Joshua E Morgan
Joshua E Morgan

August 15, 2020

Funny…I just asked about this a few days prior to this post. About as thorough an answer as one would need…thanks :)

I came up with another interesting use for botanicals while reading another one of your recent posts (in this case, discussing hobbyists that rush setting up new tanks…I’ve admittedly been in that category on multiple occasions). From what I have read, unhealthy plants are algae magnets – the sugars and proteins released by damaged or dying plant leaves encourage algae to settle and grow on these unhealthy plants. My idea is that, after cycling a tank normally (in the dark) until no more nitrite or ammonia is detected, I could spend another month or two using botanicals in the tank (which would still be in the dark) to encourage the development of resilient microbe populations that could quickly begin to process those same sugars and proteins before algae can get a foothold; once that happens, I would go about adding plants (and a light) to the tank in question.

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