What's mud got to do with it?

In the natural environment, particularly in the Amazonian region of South America, the terrestrial environment has a significant impact on the aquatic one. Soils and other substrates in the region of the blackwater Rivers  such as the Rio Negro, are characterized by very low nutrient concentrations – a byproduct of the low nutrient content of the substrates from which these rivers drain.

The Rio Negro is like THE prototypical example of this dynamic.

Studies have determined that the very low nutrient concentrations of the soils that drain into it have arisen as a result of of weathering, erosion, and sedimentation of the years. All nutrients are found in very small amounts with calcium being particularly low (hence the extremely soft water so characteristic of this habitat.).

Most dissolved organic matter in these soils is bound with aluminum.  Scientists will tell you that, generally, these soils have severe acidity, high aluminum, and low "chemical fertility."


Now, I am absolutely not a scientist, but I have taken a bit of an interest in the geological influences on the blackwater habitats we admire so much. I've tried to grasp some of the ideas about the natural soils in tropical regions and how they interact and influence the aquatic habitats we all obsess over. As a result, I have done "just enough" research to make me sort of dangerous. That being said, as most of you know, I've had this sort of fascination with substrates in our aquariums and their influence on our systems, so trying to learn a bit about this aspect of the natural habitats might help us understand how to more realistically construct our captive ecosystems. And this is where it gets both confusing and interesting!

Scientists have identified a number of different soil classes throughout the world. In the Amazonian region, a type of soil known as "Podzol" is associated with with black water rivers and lakes. "Podzols" are soils characterized by a whitish-grey subsurface, bleached by organic acids. They have an overlying dark accumulation of brown or black illuviated humus.These soils support the rainforests surrounding blackwater streams, yet are the most infertile soils in Amazonia. Now, this makes a certain degree of sense, right, because we've long been told how "nutrient poor" blackwater systems are, and it starts with the substrate, right?

Now, here's what's interesting:  Even though it's nothing like the super fertile "garden forests" we imagine, there is a lot of vegetation over this soil.  Locally called Varrillal which translates to “land of twigs”, it's a "stunted" forest, comprised of abundant thin, relatively short trees. A significant root mat covers the soils, and it's thought that most of the nutrient exchange must occur in this root mat, which keeps the remaining nutrients held within the system- hence the low nutrient levels. And it's another explanation for the relatively nutrient poor water in blackwater systems, right? The terrestrial plants are "hogging" all of the good stuff! (what little of it there is, anyways)

So, this is all well and good, but what does this mean for us as hobbyists?

Well, for one thing, it's interesting to note that, even though we've talked a lot about muds and nutrient-rich planted-tank soils in our blackwater/botanical systems, the reality is that a more realistic representation of these natural blackwater habitats might be achieved (from a substrate standpoint) by utilizing inert or otherwise less nutrient-dense soils. So, those planted-tank substrates with the extra minerals and such, although very interesting to me, are probably not the most accurate representations of natural blackwater habitats!

Now, if we're intent on growing lots of plants in our blackwater tank, these substrates are certainly more appropriate. An "intermediate" substrate could be a shallow covering of muddy,-silt-like materials, such as "Miracle Mud", CaribSea "Refugium Mineral Mud", etc. (these materials will be much more significant in our brackish-water work with "Estuary", for sure!)

All in all,  I personally feel that bathing substrates to our overall aquarium type is an important practice. So, it's a matter of utilizing the material that is best suited for the tank you want to create, as opposed to simply finding some sort of "catch-all" substrate an using that in everything. And that's the beauty of today's aquarium marketplace- there are all sorts of substrates that can be utilized in virtually any type of tank you choose to create!

For a typical blackwater aquarium, different types of inert sands and other commercially available substrates would probably be the most realistic representation. Of course, if you're "doing" a flooded forest, you would likely be best served by keeping a very thin layer of inert sand or silt materials, covered by a more significant layer of botanicals, such as leaves and seeds pods, etc.

In our quest to create more realistic representations of the natural habitat in our aquariums, it's never a bad idea to think about this stuff.

And yes, it CAN make your head spin a bit, huh? Sure was a lot easier when we used to simply empty a bag of classic "Number 3 Aquarium Gravel" into our aquariums and move on to the next thing, huh? 

Well, maybe not...

However, it is making MY head spin a little, that's for sure! 

Today's little serving of "food for thought", courtesy of the sands, silts, muds, and soils of the tropical world.

Stay curious. Stay open-minded. Stay creative. Stay humble.

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


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