The short answer is no, you should not be capping your aquasoil. But let's talk about why.
What make aquasoils so special?
One of the best reasons to use an aquasoil for root feeding plants, over any other commercially available substrate, is for its CEC. CEC stands for cation exchange capacity and is a substrates ability to absorb nutrients from the water column providing it for root feeding plants. Even if your aquasoil comes with a lot of nutrients, over time your plants are going to consume all the nutrients, but you can essentially "recharge" the aquasoil by adding liquid fertilizers in the water column. The aquasoil will absorb the nutrients right up make it avaliable for uptake. You can use this method to feed your root feeding plants for years to come. It used to be that aquasoils would break down after a 1-2 years, but we have had fluval stratum in this tank for 5-6 years. So this 1-2 punch for nutrients is extremely sustainable.
We talk about this 1-2 punch for nutrients, all about CEC and LieBig's Law of the minimum more in depth in the video below:
So why are hobbyists capping aquasoils?
The short answer is they are confused. Before aquasoils were readily available to the hobby, we had to create our own by mixing different things together like clay balls, organic potting soil, peat moss, etc. Hobbyists still use is organic potting soil to this day to avoid the high cost of an aquasoil. Potting soil is very messy and leech a lot of phosphates into the water column which can lead to string algae. To mitigate the messy soil and leaching of organics, we used to add a 1-2" of sand over the organic soil to "cap", or trap in the mess and organics. This sand works as a barrier to prevent exchanges from the soil and water column, and it works great. The reasons hobbyists started capping their aquasoil's is because others hobbyists would talk about capping their soils, but not differentiating between organic potting soil versus auqasoils, confusing others. There are even popular YouTubers advising others to cap their aquasoils due to ignorance of an aquasoils high CEC. Or sometimes they just like the look of it better, but then don't advise other hobbyists one why its a bad idea or the repercussions on doing so, which we'll cover now.
Why we think capping an aquasoil is a bad idea.
By capping an aquasoil one is adding a physical barrier between the water column and the aquasoil, rendering its CEC worthless. So this means when you add liquid fertilizer to your tank, it will not make it down into your aquasoil.
If you choses to cap a nutrient rich substrate, like ADA Amazonia V1, in 6-12 months your plants will use up all the nutrients forcing you to either replace the aquasoil with new soil, which is expensive, or start using root tabs. In our opinion, root tabs are a poor way to provide nutrients as nutrients levels will spike and fall, spike and fall. Inconsistent nutrient levels can stress out your plants causing nutrient deficiencies and invite algae to take hold. We also know from Liebig's Law of the Minimum that when a plant runs out of a single nutrient, it will stop growing.
How do we know that sand creates a barrier?
Many have claimed that sand DOES allow flow through it, thus recharging an aquasoil, but with no data to back it up. So, we had to do a little experiment to see how far water would travel through sand.
Here is how we tested this idea. What took a basin, added aquatic sand to the bottom and filled it with water gently. After the basin was filled with water and aquatic sand, we added blue food coloring to the water and mixed is gently as to not disturb the surface the sand. We used blue dyed to see how far the blue water would work its way into the sand. The idea was that if the blue dye could work its way into the sand, then a fertilizer would be able to as well.
Someone suggested that the blue food coloring molecules could be bigger than a fertilizer compound, which would not be a good analog for a fertilizer. However, blue dye is a smaller molecule size, 1-2nm, than potassium sulfate which is 260nm. So in theory, if the dye is NOT able to penetrate deep into the sand, neither would potassium sulfate. We chose potassium sulfate because it is not provided by natural methods like nitrates from fish waste, or phosphates from dry fish food. It needs to be dosed manually and replenished in a planted aquarium. If potassium sulfate could not make its way through the sand into the aquasoil, this would be an issue for plants.
DAY 1:
Here is our basin with our aquatic sand and water dyed blue. Lets see how quickly the blue dye is able to penetrate into the sand.
DAY 2:
After 48 hours the water had barely started to penetrate the sand. It was not looking good for the idea that water can flow through sand well.
DAY 10:
We left the experiment sit for a full 10 days in hopes that the blue water would penetrate deeper in the substrate. But it seems like after 10 days it would only penetrate a 1/4" to 1/2" in some places and wasn't going to go any further. This was pretty conclusive to us. Is it was going to take 80-100 (1/4"-1/2" over 10 days) days for the dyed water to penetrate a 2" cap, which is suggested, the aquasoil underneath is not going to be replenished quick enough to keep plants fed at all times.
How quickly does water penetrate an aquasoil?
We set up the same experiencement but swapped out the aquatic sand for an aquasoil, which was Fluval Stratum. Since the aquasoils was so dark and it would be hard to see/show the blue water making its way down into the aquasoil, we also added q-utips every inch for 3 inches.
60 SECONDS:
To our surprise, within 60 seconds the color made its way down to the second q-tip.
120 SECONDS:
And within 120 seconds the blue dye hit the 3rd q-tip.
Conclusion
For us at ASG, this is exactly what we thought was going to happen. Unfortunately sand creates a barrier which make exchanges with the water column nearly impossible. If you chose to use a high nutrient aquasoil, like ADA Amazonia, and cap it with sand, just know that within a handful of months, your plants will uptake the nutrients in the soil leaving your aquasoil exhausted. And because you capped the aquasoil, adding a liquid fertilizer isn't going to recharge the aquasoil. Your best bet is to add root tabs, but overtime the sand is going to mix with the aquasoil, making a huge mess of your substrate forcing you to start over. And if you "HAVE" to cap your aquasoil, do so with a larger sized substrate like EcoCompelte, or a small gravel. This will give larger spaces around the substrate allowing water to pass through the cap. PS - This is the only time you'll ever hear us suggest EcoComplete as a substrate.
Again what we suggest is to keep the aquasoil open to the water column and pair it with a holistic fertilizer like Thrive, or the ASG ferritization kit. This creates a sustainable fertilization regimen, feeding your plants for years to come. Remember, consistency is key, and if nutrients delivery is all over the place, your plants are going to be more stressed, leading to algae.
If you'd like to ask questions about nutrient delivery, or our approach, reach out to us on our form. Click the image below to take you there.
