Choosing the right soil substrate

[TimeLord]

For more on this topic, check out the Ecology of a Planted Aquarium. I personally like to soak my soil to get the ammonia out of the system.

[desertstream]

I've tried the soil route for a "dirted" tank and learned a thing or two. First off, steer clear of anything with wetting agents - they're a no-go. Look for an organic soil and sift out as much of the chunky stuff as you can. I'd avoid soils with poop in them, trust me, it's just asking for trouble. Some folks bake their soil in the oven, but I just leave it out in the sun for a bit - seems to do the trick.

Before adding the soil, throw down some lava rock - the kind you'd use for a BBQ. It's inert, so it won't mess with your water, but it'll give your soil some room to breathe. Then, sift that soil into the tank and make sure it's nice and wet, like a thick mud. Cap it off with about two inches of sand or gravel. The nutrients will leach into the water, but it's not fish-friendly for a couple months. And let's be real, in two years you'll be ripping it all out and starting over because it'll be anaerobic and basically useless.

My plants loved the dirted tank, but honestly, it's more hassle than it's worth. If I had to do it again, I'd just grab some play sand and root tabs. I'm not a fan of those terrac balls either - the iron in them isn't really accessible to plants. Root tabs, on the other hand, are made from ferrous gluconate, which is way more plant-friendly. So, grab some play sand, root tabs, and a good liquid fert, and you're golden. A 50-pound bag of play sand is like 7 bucks, and a pack of root tabs is 10 bucks - you can't go wrong.

I'd go for bigger lava rock pieces, about the size of walnuts. This way, you get decent spaces between the sand. Crushed lava that's too fine might as well be gravel. The idea is to keep the soil from compacting. You don't need to cover the entire bottom with lava rock, just spread out a few of the bigger pieces with some space in between for the soil to fill in. It's not essential, but it does help prevent compacting.

[boomer]

I agree with Colin that using soil in an aquarium doesn't offer any real advantages. In fact, it can lead to significant issues, like ammonia spikes that can persist for months and pose a risk to fish and invertebrates. The potential benefits just aren't worth the risks.

The fertilizer in potting soil is also a concern, as it's formulated for terrestrial plants with different nutrient requirements. Terrestrial plants need nitrates, which aquarium plants don't require. Using this type of fertilizer in an aquarium can lead to algae problems.

Aquatic plants will thrive just as well in inert sand or fine gravel, with or without substrate fertilizer tabs or liquid additives, depending on the specific plant needs, lighting conditions, water hardness, and fish load.

I'm not convinced that using soil as a nutrient layer is entirely without merit. Providing plants with a rich source of nutrients in an otherwise barren tank has to be beneficial, right? I mean, sure, you can add these nutrients in other ways, but that doesn't change the fact that plants will thrive with a soil substrate beneath the sand or gravel.

I've done my research on this topic, having read Diana Walstad's book and articles, as well as participated in her forum. I've even discussed this with Tom Barr. According to Diana herself, the only real benefit of using soil is the initial CO2 release it provides. However, this benefit is short-lived, and after a year, there's no difference between soil and inert sand substrates in terms of plant growth.

Most of us can easily provide the necessary light and nutrients for our plants through substrate tabs or liquid fertilizers. But CO2 is a different story. It relies on the naturally produced CO2 within the aquarium, which comes from the decomposition of organics in the substrate, as well as the respiration of fish, plants, and some bacteria. Once the tank is established, this is relatively easy to balance.

Using soil with organics can get this CO2 production going faster, but I've never seen any problems with this. On the other hand, the initial ammonia surge that can last up to six months is a serious detriment to using soil. I've torn down and replaced substrates many times, and I just don't see the point in risking the health of my fish.

As for other nutrients in the soil, it really depends on what's in the soil to begin with. Most organic soils are designed for terrestrial plants, which have different requirements than aquatic plants. I'm skeptical that substrate-rooted plants will do any better in soil versus sand with substrate tabs. In my experience, I've seen incredible plant growth in tanks with play sand and substrate tabs.

Plant nutrients ultimately come from the water, regardless of the substrate. The nutrients in the soil must dissolve into the water to be taken up by the roots. It doesn't matter to the plants how the nutrients get into the water. I just don't see the point in using soil when there are risk-free methods that are just as effective.