Do Aquarium Plants Need Soil to Grow?

The substrate you use in your tank is one of the most important choices you’ll make. This is especially the case when you’re setting up a planted aquarium.

The best option for your plants and fish will be a soil substrate.

Soil is unbeatable if you want a lush, green, and healthy tank. It’s pricier and a bit more difficult to install, but the pros outweigh the cons by a wide margin!

You’re about to see why. In this article, I’ll cover everything you need to know about soil and aquarium plants, including the benefits, use, and alternatives.

Benefits of Using Soil to Grow Aquarium Plants

When setting up your tank, you can choose between three types of substrates— soil, gravel, and sand. Of all three, soil is by far the most suitable for growing aquatic plants.

Here’s why:

– Soil is packed with micro and macro-nutrients

You can even find nutrient-enriched soil being sold as plant fertilizer. The nutrient density of this type of substrate is the biggest reason why it’s so beneficial.

Plants need lots of minerals like iron, zinc, calcium, magnesium, potassium, manganese, and so on.

Without these nutrients, the plant won’t grow properly, and you might even see deficiency symptoms like loss of color, withered leaves, or holes in the leaves.

Sand and gravel contain little if any nutrition for the plants. Compared to soil, these substrates are sub-par when it comes to plant growth.

If you want lush, healthy vegetation, soil is the best choice.

– Soil is suitable for planting

Compared the gravel and sand, soil is also better for planting. Good soil is naturally moist and malleable.

It gets compacted underwater, but it remains soft enough for the roots to penetrate and grow. It helps the plants stay anchored without suffocating the roots.

Gravel, depending on its size, can be too dense and heavy for the roots to traverse. Gravel also doesn’t get sticky, so the plants aren’t stably anchored in the substrate.

This is a problem with tall-growing plants that tend to get heavier as they grow. Gravel also needs regular cleaning, which might disturb the plant’s stability further.

Sand is a bit complicated. It has a fine grain and absorbs lots of water quickly. Like soil, sand gets compacted.

But unlike soil, sand doesn’t “breathe” Once the sand gets pressed under the weight of the water, it will form “dead zones”.

The sand starts growing anaerobic bacteria, which increases the ammonia levels in the tank.

– Soil can harbor beneficial bacteria

Soil and gravel are perfect for the growth of beneficial bacteria. Of the two, soil has an edge because of its wider surface area.

So, if you want to maximize the biological filtration in your tank, soil is hands-down the best choice.

The nutrient density and the “breathability” of soil make it a haven for the good bacteria you need in your tank.

The soil traps fish waste, and then the bacteria break it all down to convert the dangerous ammonia into nitrates.

Sand doesn’t even deserve a mention here because it usually does the exact opposite of what you want.

Due to compaction and low breathability, sand tends to harbor bad bacteria. This type of anaerobic bacteria generates more toxic ammonia in the tank.

– Soil can positively alter water chemistry

Soil is full of minerals, including magnesium and calcium. These minerals slowly seep out of the soil and into the water, altering the water parameters.

As the mineral concentration in the water goes up, the hardness levels will also increase. The added minerals also help maintain a stable pH level.

So, soil can make your water harder and closer to a neutral pH. This is great for the fish in your tank because most species require a pH of around 7.0, and moderate water hardness. Some active soil substrates come pre-packed with bacteria.

Such substrates also help the water chemistry by maintaining a stable water pH. They do so by supporting the nitrogen cycle.

Soil is pretty cool, as you can see. In comparison, gravel and sand are inert. They don’t alter the water parameters in any way, because they contain no minerals.

How Does Soil Help Aquarium Plants Grow Faster?

Soil is king when it comes to plant growth. This beneficial effect is all due to its nutrient concentration.

A high-quality soil contains a balance of all the necessary minerals and micro-nutrients that plants require to fuel and maintain their development.

When minerals are abundant in the soil, the plants can “afford” to spend extra energy to grow larger, and faster.

Nutrients like calcium, copper, iron, potassium, magnesium, manganese, zinc, cobalt, chromium, nickel, sodium, and aluminum are crucial for plant health.

When a deficiency arises, one of the first symptoms is stunted growth. Commonly, the appearance of the leaves and stems is also affected.

So, not only do nutrient-poor substrates not fuel plant growth— they might even inhibit it.

Some soils are also supplemented with active bacteria that support the nitrogen cycle in the aquarium.

These bacteria also contribute a lot to the plant’s nitrogen intake. Proper nitrogen concentrations in the water will also lead to faster growth.

What Type of Soil Is Best for Aquarium Plants?

A good aquarium soil will have to meet a few requirements. The soil should be nutrient-rich and include the concentrations of minerals on its label.

The soil shouldn’t alter the pH of the water. You don’t want water that’s too alkaline or too acidic. The closer you can stay to a neutral pH of 7.0, the better.

The best soil will also contain added bacteria to support the nitrogen cycle in the tank. Using such a substrate will help you cycle your aquarium faster if necessary.

The grain size and shape also matter. You want soil with spherical grains and high porosity.

This will let the roots grow and absorb nutrients easier. This type of soil is also less likely to compact.

It’s going to flatten under the water pressure, but not enough to suffocate the roots.

Finally, a good soil substrate should be free of chemicals and colorings. In summary, the best soil is nutrient and bacteria-rich, porous, non-compacting, and pH neutral.

Also, look for soil that’s not been chemically treated or dyed.

How Much Soil Do Aquarium Plants Need to Grow?

As a general rule, your soil substrate should be at least 1.5-3 inches deep. This is the bare minimum to sustain optimal root growth.

Without enough soil, the plant won’t anchor itself properly. You might find that taller, heavier plants start to lean sideways. In some cases, larger fish might even uproot and kill the plants.

Note that plants with large root systems would benefit the most from a deeper substrate.

There’s no upper limit to how thick the substrate can get. However, more isn’t necessarily better.

The thicker the substrate gets, the more you risk the lower layers getting compacted. Avoid piling the substrate higher than 4 inches tall.

You don’t have to make the substrate completely level, by the way. You can get taller portions in the back, where you’re more likely to add deep-rooted plants.

Smaller plants with less intricate root systems can thrive in substrates that are just 1.5 inches deep.

You can place these plants in the foreground for a better view.

How to Use Soil in Your Aquarium?

Using soil is not largely different from other substrates. You’ll have to prep the soil beforehand. Other than that, installing it isn’t too complicated.

Here are the steps you should follow for the best results:

1. Measure the substrate.

You’re going to prep the substrate before adding it to the aquarium. But you need to get the quantity right first.

This will depend on the length and weight of your aquarium, as well as the desired depth of the substrate.

Let’s take an example! You want to achieve a 2-inch depth for your substrate. You have a standard 30-gallon aquarium measuring 36 inches in length and 18 inches in width. In this case, you’re going to need 21 liters worth of soil.

I suggest looking for an online substrate calculator. Input your aquarium measurements and desired substrate depth.

You’ll get a precise measurement to work with.

2. Rinse the soil before installing.

This will help reduce some of the murkiness you’ll get when adding soil into the water.

Rinsing will get rid of some of the extra dirt like the smaller granules that float up into the water column.

Just fill a container with water. Add your soil and stir well. If you get any floating debris, pick that up with a strainer. Let the soil set before dumping the murky water.

Discard as much of the water as possible without throwing away the soil.

You can repeat the process a few more times until the water is clear.

3. Moisten the soil.

Skip this process if you’ve already rinsed the soil. If you’re using a no-rinse substrate, this step is crucial. It will make your work easier when setting up the tank.

Put the soil in a container and pour just enough water to cover it. Let the soil soak for a few minutes before pouring the mixture into the aquarium.

4. Add and shape substrate.

Pour the moistened soil into the main aquarium. Let it sit for a few minutes. The loose soil grains will sink to the bottom.

Once that’s done, you can gently press the soil down to shape it. Don’t press too hard! You don’t want to create dead pockets.

This will inhibit the oxygen exchange between the soil and the water.

Just pat the substrate gently to get it into place. Don’t bother about making the entire thing level.

You can get slightly deeper or shallower regions here and there.

5. Add a top cap.

The “cap” is an extra layer of substrate that sits on top of the soil. It helps the soil stay in place and prevents things from leaching into the water.

This will keep smaller soil fragments down on the substrate, preventing that ugly murkiness.

It also protects the plants from fish. If you have fish that love digging through the substrate, this protective layer will deter them from uprooting your plants.

So, choose a “cap” substrate. I suggest gravel because of its high breathability and because it doesn’t get compacted. Add enough of the gravel to get a 1-1.5 inches high layer on top of the soil.

And voila! You’re done! Your substrate is all set and ready.

Can Aquarium Plants Grow in Gravel or Sand?

Soil is superior to any other type of substrate. But if gravel or sand is all you have on hand, you can still grow plants in the aquarium.

You’ll just have to tweak a few things:

1. Choose hardy plants like Hornwort, Java Fern, Java Moss, Amazon Sword, or Vallisneria. These species are more likely to thrive even in suboptimal conditions. Avoid fragile plants with thin roots.
2. If you got to choose between gravel and sand, I recommend gravel. The larger grain size allows water to flow freely and prevents compaction.
3. Choose a gravel substrate with finer grain size. Large gravel is harder for the roots to traverse.
4. Anchor the plants properly. Gravel and sand substrates aren’t gummy and sticky like soil, so they don’t keep the plants in place as well. To anchor your plants, I recommend using either a hollowed cap or any other small recipient. Place the plant’s base there and then plant. The recipient will keep the plant in place, still allowing for the roots to grow and spread.
5. If you’re using sand, sift the substrate periodically. This will get rid of the dead-zones issue you get with fine-grain substrates. Use a small aquarium rake to break up any portions of hardened, compacted sand. Be careful not to disturb the plant roots. Alternatively, you can also use your hand.
6. Don’t forget fertilizer! Gravel and sand are inert. They contain no nutrients, and they don’t do anything. You’ll have to make up for this by adding the nutrients yourself. Any type of fertilizer will do. But root tabs are most common. This fertilizer comes in little pellets. You just have to bury them throughout the substrate. The fertilizer will do all the rest!

Conclusion

Soil is the OG of planted aquariums. It’s a malleable substrate, full of nutrients, and it provides excellent support for plants with heavy roots and stems.

These are all qualities you want from a good planting substrate. Gravel and sand bring none of these benefits.

However, with a bit of diligence, you can also grow plants in these substrates. You’ll just have to add fertilizers and clean the substrates regularly.

But remember, these substrates don’t keep the plants in place very well. You’ll have to limit your choices to hardy or small plants with strong roots.