Aqua soil has been around for a good 30+ years in the aquarium hobby. But in the past couple of years, houseplant hobbyists have started experimenting with aqua soil like Fluval Stratum for indoor plants - and with great results (especially for Alocasia corms and hard-to-root cuttings) - as long as you know what to do, and what NOT to do. Although it looks similar to leca or lechuza pon, aqua soils like Fluval Stratum are VERY different.
Coming up are the do's and don'ts of using aqua soil to grow houseplants, the pros and cons, answers to aqua soil FAQ's, along with comparing different aqua soil brands, such as like Fluval Stratum, Tropica Aquarium Soil and APT Feast. There are also lots of beginner tips and tricks if you're trying aqua soil for the first time for propagating or growing indoor plants.
As for me, my background covers both industries. Before starting my own plant store (Love That Leaf) 5 years ago, I worked in the pet industry for 20+ years. Over that time I owned a large pet store which included a good 40 or so fish tanks. I also worked for NZ's largest specialist pet supplies wholesaler for 10+ years. In my time there, I trained retailers on aquarium science. Yep, I'm a plant-geek and a pet-geek.
So when the worlds of the aquarium hobby and houseplant hobby cross-over, I'm in my element. Let's dig in...
What is Fluval Stratum?
Fluval Stratum is a brand of aqua soil. Fluval is the company, Stratum is the brand name they call their aqua soil. Fluval also make filters, lights, fish tanks and a lot more in the aquarium hobby.
Other products from Fluval are available in NZ but Fluval Stratum (the aqua soil), is unfortunately not available in NZ. The good news is there are many brands of aqua soil that are available in NZ that are similar, maybe even better. I compare a few other options for you coming up below.
What is aqua soil?
If you've only ever seen it but not yet used aqua soil like Fluval Stratum before, the first thing you might think is it looks like tiny, dark leca. But there are multiple differences.
The obvious one is size. Aqua soil is far smaller. Leca averages 8 to 16mm and aqua soil is more like 3 to 5mms (depending on the brand you buy, you can get aqua soil right down to a coarse sand grade also). Being so small means aqua soil might not work in pots with drainage. More about that coming up also.
You DON'T want to approach aqua soil the way you grow in semi-hydro substrates like leca or lechuza pon, as aqua soil (like the name says) is soil. Leca, pon, perlite and more are soil-free, sterile substrates. They don't break down or include any nutrients (to be accurate, lechuza pon does have temporary nutrients added but they run out and you can keep using Lechuza Pon with fertiliser after that, or you can get fertiliser-free pon like Bio Pon). More about the differences coming up, and what that means for your plants.
How is aqua soil different from other substrates?
LECHUZA PON is soil-free and nearly sterile, made from a mix of volcanic mineral stones (zeolite, pumice and lava rock). Once the slow-release fertiliser runs out, it's nutrient-free, so fertiliser is required after about 6 months. It's hard when dry or wet, won't break down over time, and can be reused indefinitely.
PS: You can also get fertiliser-free pon alternatives like Bio Pon by Bio Leaf which comes in two grades, fine or medium (the fine grade is a similar size to Lechuza Pon).
PERLITE is soil-free and sterile, made from heated volcanic silica glass. It doesn't provide any nutrients so fertiliser is required. Perlite doesn't degrade or break down over time and can be reused.
LECA is soil-free and sterile, made from a mix of kiln-fired clay and albite (a mineral found in volcanic rock). It does not provide any nutrients so fertiliser is required. It does not decompose or break down over time and is reusable indefinitely.
AQUA SOIL (like Fluval Stratum) is made from volcanic earth. It's a type of soil, is organic, and is not sterile. It includes nutrients, although not at a level that replaces the need for fertiliser. It also contains humic and fulvic acids and will buffer pH into the 'Goldilocks Zone' (not too acidic, not too alkaline) for better nutrient uptake. It will decompose and break down over time into a fine dirt / sludge.
Inorganic substrates like leca, perlite and lechuza pon are popular alternatives to common houseplant substrates for those looking to minimise pest and disease problems. That isn't a benefit of aqua soil since it is organic.
Why use aqua soil for houseplants?
The success rate and speed of rooting in aqua soil like Fluval Stratum is pretty impressive for some plants compared to other substrates, especially for corms and for rooting stubborn cuttings. Aqua soil can be a game-changer for rooting difficult plants.
It's also used to rehabilitate plants after root rot. You can use aqua soil for established plants also, but because it softens and breaks down into a fine dirt over time, I mix it with other substrates for longer-term use (perlite is the most popular to mix it with). More about that coming up.
I can't get Fluval Stratum, what other options are there?
In the same way 'Lechuza Pon' is the best known brand of pon in the houseplant hobby, Fluval Stratum is one of best known brand of aqua soil in our hobby (although aquarium enthusiasts might not agree), but there are LOTS of other excellent options for Fluval Stratum alternatives if you can't get the Fluval Stratum brand in your country.
Fluval Stratum is one of the lighter aqua soils, making it more likely to move when submerged in water, and meaning it doesn't hold some plants as securely as some other brands. Not a biggie, but if it bothers you there are other, heavier options.
Fluval Stratum is also known as one of the softer aqua soils. That's good for plant roots to get into, but not as good for plant parents as it can cause it to break down faster than harder aqua soils.
Golden Tree Aqua Soil vs Fluval Stratum
One benefit of Fluval Stratum for aquarists is it leaches less ammonia (a source of nitrogen). Good for aquarium inhabitants like fish and shrimp, not as good for plants who need the nitrogen the ammonia provides. However, high nitrogen is less important for initial rooting (it matters more to support foliage growth for longer-term growth).
Two close matches to Fluval Stratum if you prefer low ammonia for propagating are Golden Tree Aqua Soil and Tropica Aquarium Soil. If you prefer the standard aqua soil size (around the same as Fluval Stratum), then the closest matchs looks to be Golden Tree, which is also heavier so holds onto plants better.
Tropica Aquarium Soil vs Fluval Stratum
Tropica is reportedly more nutrient-rich than Fluval Stratum, but still low in ammonia, and a much smaller size. Tropica is reported to start root growth faster for propagating. As far as size though, Tropica is a lot smaller. Around 1 to 2mms vs Fluval Stratum which around 3 to 5mms. The smaller size is reported to provide more support for unrooted cuttings and delicate new roots.
APT Feast vs Fluval Stratum
APT Feast is another popular Fluval Stratum alternative. Made by The 2 Hr Aquarist, APT Feast is richer in nutrients than Stratum, with around twice the nitrogen of most aqua soils. APT Feast also provides nutrients for longer (the brand reports at least 12 months). If you want the extra nutrients, but not the smaller size of Tropica, APT Feast is the way to go offering the standard aqua soil size, around 3 to 5mms.
How long does aqua soil last?
For best results, start with fresh aqua soil every growing season. To help extend how long it lasts physically, keep handling to a minimum and water gently, with a spray-mister or watering bottle rather than the tap or pouring water in. Gentle treatment helps extend the time before it fully breaks down into a sludge.
Aqua soils are made of earth (often a mix of volcanic soil, volcanic ash, clay and peat depending on the brand). Aqua soil does decompose and break down. It usually lasts longer physically than nutritionally. Physically, you should get around one to two years use for most aqua soils before they break down into a fine sludge if you take care with handling and watering (as aqua soil becomes softer once wet). Some aqua soils are softer than others (such as Fluval Stratum).
On the nutrients side, the nutrient and pH buffering benefits of aqua soil can run out sooner than the substrate breaks down. Expect around 3 to 12 months. The nutrients levels from aqua soil are not enough for plants. You still need to fertilise.
A lot of factors can change how soon the nutrients are depleted from aqua soil. Fluval Stratum is reported to be at the shorter end of the scale vs APT Feast which is reported to provide nutrients for longer than most aqua soils, but because of so many variables it's hard to get exact comparisons.
Using a good fertiliser extends the time aqua soils keeps providing nutrient benefits to plants. Aqua soil is great at holding onto nutrients and releasing them to the plant's roots.
Depending on the aqua soil you choose, you'll likely want to add fertiliser from the first signs of root growth, then after about 6 months you might like to add a source of fulvic and humic acids into your nutrient routine also, like GT Rootzone. More about fertilising coming up.
Does aqua soil need to be rinsed before use?
No, you don't need to rinse aqua soil before using it for your plants. In fact, most manufacturers recommended not to because what you'll be rinsing out may look like dust but includes the aqua soil itself and some of the nutrients along with it, going down the drain!
Is aqua soil like lechuza pon or leca?
No, they are very different. Being inorganic, pon and leca do not decompose like organic substrates do, so they can last indefinitely. They can be re-used over-and-over. They don't break down or degrade.
Substrates like leca, lechuza pon and perlite retain a lot of air even when wet and don't compact over time. But as they are sterile, leca and perlite don't offer the pH buffering, humic acids and fulvic acids, or extra nutrient boost that aqua soil does.
Aqua soil also becomes soft when wet. Leca and lechuza pon remain hard whether wet or dry. When making leca, the clay is fired at high temperatures in a kiln. The clay expands in the heat, forming a hard ceramic shell on the outside and honeycomb texture on the inside, full of holes. As the kiln turns, the leca ends up a round or oval shape. Aqua soil is relatively hard and firm as long as it stays dry. Once wet, aim for minimal handling as with only a little pressure the pallets turn into a fine sludge.
How do you water plants in aqua soil?
Like soil, aqua soil is a water-loving substrate. It absorbs and holds on to water well. Great for propagating cuttings and growing plants that prefer not to fully dry out, with Alocasia corms a favourite for growing in aqua soils like Fluval Stratum. As it dries, aqua soil changes colour from black to a dusty charcoal (or from a dark-chocolate to more of a milk-chocolate colour, depending on the brand). If you grow in a clear pot, it's also easy to judge the water level.
Leca, lechuza pon and perlite maintain a higher percent of air vs water even when fully saturated. Self-watering pots or using a water reservoir, is recommended with leca and lechuza pon, but something to be wary of with Fluval Stratum and other aqua soils as it can lead to rot. Being always-wet can speed up the rate aqua soil softens and breaks down into a sludge.
When watering aqua soil it's best to spray with a mister or use a squeeze watering bottle suitable for seedlings that has a gentle flow, rather than pour water in using a jug or use the tap or shower. As well as reducing the risk of over-watering, watering gently also reduces disturbing the aqua soil pallets and washing away the soil and nutrients. Keep spraying until you see a little excess water just start to pool at the bottom (another reason I'd consider clear containers a must with aqua soil). You ideally want to avoid the bottom of the pot sitting in water.
What pots are best for Fluval Stratum?
It's easier to use Fluval Stratum and other aqua soils in a container without drainage holes, maintaining either a very low or ideally no water reservoir at the bottom (see the advice about watering above).
Although you can use aqua soil in pots with drainage, aqua soil is so small you'll likely lose it out the drainage holes of most commercial pots. If growing corms, those little clear pottles that dipping sauce comes in are a great space-saving option. Smaller pots are a smart idea for saving money too since Fluval Stratum and most aqua soils are expensive.
Can you grow established plants in aqua soil too?
Yes, but because aqua soils break down, compacting into a 'sludge' over time, using aqua soil for corms and cuttings is better as a temporary substrate for propagation until rooted. As it breaks down, the lack of air space can create an anerobic (low-oxygen) environment, conditions ideal for stem and root rot.
I'd save growing in 100% aqua soil like Fluval Stratum for your corms and cuttings, transferring them into leca, lechuza pon, potting mix or your substrate of choice, when they have enough root growth to be potted up. Another option is to pot up into a combo of aqua soil and perlite.
You absolutely can grow established plants in aqua soil also, but because it breaks down, it's best to add something hard and airy that won't compact. Three popular combos are aqua soil and perlite, aqua soil and lechuza pon, or aqua soil and leca.
The addition of perlite, leca or pon helps maintain air space, reduce water-retention and resist compaction. Due to the smaller size of aqua soil I find it better when mixed with other smaller size substrates like perlite or lechuza pon. With leca you'll normally find the aqua soil just works its way down to the bottom, falling through the gaps in the leca.
For growing already rooted plants in aqua soil, a higher-nitrogen aqua soil brand like APT Feast might be a better choice than the low nitrogen Fluval Stratum. For propagating unrooted cuttings or corms, a lower-nitrogen (low ammonia) aqua soil like Fluval Stratum, Golden Tree or Tropica would be good choices. However because the nutrient levels are not enough to replace fertiliser, a higher-nitrogen foliage fertiliser or lower-nitrogen root fertiliser is still required even when growing in aqua soil.
How are aqua soils like Fluval Stratum made?
Aqua soil, like the name suggests, is soil. Usually volcanic earth and peat which offers a natural benefit of lowering pH to around the slightly acidic 'Goldilocks Zone' ideal for nutrient availability and plant growth. Peat is also a rich source of fulvic and humic acids. Different brands of aqua soil may be a mix of volcanic ash, volcanic soil, peat and clay.
Once collected, the larger granules of volcanic earth are filtered out and kept, sometimes enriched further with nutrients like ammonia and additives like coal, before being lightly roasted or burned to help the granules hold their shape better, forming what we know as aqua soil, basically soil in a pellet.
Handle with care! Aqua soil is soft(ish)
Unlike leca or lechuza pon, aqua soils like Fluval Stratum become softer when wet, and break apart relatively easily with a little pressure, turning into a fine sludge. Water naturally softens and breaks it down over time even with careful handling. That's one reason it's advised not to rinse aqua soil whether used for aquatic plants or houseplants, and to water gently, as you're essentially washing away the soil and nutrients.
Do you need to use fertiliser when growing plants in aqua soil?
Yes, although aqua soils like Fluval Stratum do contain nutrients, they don't contain enough to replace the need to fertilise. You DO still need to use fertiliser when growing houseplants in aqua soil.
Knowing which brand of aqua soil you're using matters too. Some are higher in nutrients than others. Some have little or no ammonia so the nitrogen level is lower. A low-ammonia brand like Fluval Stratum, Golden Tree Aqua Soil or Tropica Aquarium Soil make them better suited for propagating corms and cuttings.
A higher-nitrogen / higher-ammonia brand like APT Feast is a better match for growing already-rooted plants. However, even at the lower or higher levels, it's only a small consideration as it isn't enough to replace using fertiliser either way. If using a higher-nitrogen aqua soil for propagation, a low-nitrogen root fertiliser like GT CCS would be a better choice.
To make it more difficult, aqua soil typically has no nutrient analysis. It's a natural product that varies in its nutrient profile, which also changes over time as it breaks down and gets used up. If you're starting with fresh aqua soil, straight out of the bag, provided it's a brand known to have high nutrients, I would wait and only start fertilising when roots start to form. Then once roots have started to grow, my go-to is GT CCS, a low-nitrogen root fertiliser made for propagating.
If you're using older aqua soil that's been used before then I'd kick things off with a little fertiliser to reactivate the aqua soil from day one, and every time you water. Bonus points if your fertiliser also buffers pH.
Think of aqua soil more like a growth booster than a fertiliser. Aqua soil typically contains ammonia (a nitrogen source), phosphorus, potassium, iron, boron and other micro-nutrients, as well as humic and fulvic acids. All at beneficial levels but not typically either high enough, or complete or balanced enough, to meet all the requirements for plants all by itself.
As for other substrates like lechuza pon and leca, because they are sterile and do NOT provide any nutrients, you do need to fertilise with them also. Keep in mind fresh-out-of-the-bag Lechuza Pon provides short-term, slow-release fertiliser. If you're using the Lechuza brand of pon, you should NOT fertilise for 6 to 8 months until the fertiliser has been used up to avoid the risk of over-dosing and causing fertiliser burn. You can get fertiliser-free pon these days like Bio Pon.
When growing in soil-free, sterile substrates like leca or pon, you'll want to use a fertiliser suitable for hydroponics like PowerGrow or GT Focus. When growing in Fluval Stratum or other aqua soils, a hydroponics fertiliser is still beneficial but not as vital since aqua soil is soil and is not sterile. PowerGrow offers the added benefit of fluvic and humic acids already included, or you can supplement those separately by adding GT Rootzone to your fertiliser of choice.
What pH does aqua soil create?
Depending on the brand, a big benefit of aqua soil for plant growth is the ability to buffer pH to the sightly acidic zone plants prefer. Fluval Stratum for example buffers to around 6 to 6.5, Tropica Aquarium Soil to around 6.4, APT Feast to around 6.2 to 6.6 pH, Chihiros Aquasoil to 6.2 to 6.8 and Golden Tree Aqua Soil to around 5.5 to 6.5.
The optimal nutrient pH zone for plants - the 'Goldilocks Zone' - is between 6 to 7 with 5.5 to 6.5 preferred, a range that allows for a good balance of both macro- and micro-nutrients to be taken up by plants for maximum growth and health. Aqua soil's pH buffering capacity will reduce over time so using a fertiliser that buffers pH can help extend that benefit, such as the GT Focus range.
What are fulvic and humic acids?
Another big benefit of aqua soil is being a natural source of humic and fulvic acids. These soil superheroes are formed when organic matter breaks down in soil. Peat is naturally rich in both. They are biologically active compounds that boost growth (root growth in particular), support photosynthesis, improve nutrient uptake and create a healthier, more fertile substrate.
But if you prefer sterile substrates, or your aqua soil is old, you can add fulvic and humic acids in. Products like GT Rootzone can be added in with your fertiliser of choice. Or go for an all-in-one like PowerGrow fertiliser, which combines both fertiliser, fulvic and humic acids in one. Some fertilisers like GT Focus also buffer pH to the slightly acidic 'Goldilocks Zone'.