Dirt Primer

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Falun Dafa Hao!
Nov 26, 2023
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Huntsville, Alabama
After a look at thread Here, I thought it might be nice to have a discussion about just dirt since dirted tanks have such a mix of understanding and experiences.

I am not a soil scientist, but I have taken Soil Science and could play one on TV. My first round of undergrad was Horticulture Science at Oregon State University. I was also a grower/propagator at a wholesale ornamental plant nursery for a few years making plants. Now I am an engineer, but still have personal affinity for ecology and natural processes.

Anyhow, if a real soil scientist wants to take over the conversation, feel free, but my effort here is to share a layman's understanding of soil and let the end user determine how it might apply to this hobby. I will try and add in a few bits that do pertain when applicable though.

Soil is generally a mix of some or all of four main components: clay, sand, silt, and organic matter.

Clay is made of very small and dense mineral particles that can hold tons of nutrients. The surface area per volume is many many times that of sand etc. You would want some of this in most soil because it is a permanent nutrient sink. Various chemical adhere and break away from the surface as they are used by the plants or dissolved in the water column. Too much though and the soil becomes dense, compacted and less oxygen is available (even aquatic plants need oxygen in the root zone). Some mineral that are highly desirable are found in clay. Soil that is reddish will usually have iron-based clay in it. Plants need iron, but it is not always readily available even in high iron soil until further processing. Clay is good -- in the right proportion.

Everyone knows about sand. It's gritty and sandy and made of fairly large mineral particles. It doesn't hold a lot of nutrients, but is great for creating a looser soil structure that plant roots can penetrate, fluid and nutrient flow is enhanced in the substrate, and compaction is reduced. It also provides a great physical filter in the right conditions. Aggregate particles of sand that are the same size prevent compaction, while mixed sizes can result in less usability as a filter medium. Neither is good or bad, just different applications.

Silt is a small mineral particle somewhere at the range where if you had it in your hand and spit on it and rubbed it, it would feel somewhat smooth like really fine sand, but not slippery like clay. This spit test is a common way soil scientists (or farmers) get a quick understanding of the relative mixture of clay, silt, sand, and organic matter. Silt is very desirable in an aquatic soil because it still holds some nutrients, but tends not to compact (more on this later).

Organic matter:
Organic matter is just that -- stuff that comes from biological decay. It can be peat, wood, leaf, animal, or other source in origin, but it contains, but doesn't bind/hold lots of nutrient value. It also creates a looseness and almost a "glue" between particles in the soil structure. The good thing is, in your tank, this is continually renewed.

Loam is some mix of the above in reasonably non-extreme ways. In other words, you can have sandy loam, clay loam, silty clay loam, etc and they will still have reasonable amounts of the rest of the stuff as opposed to 90% clay etc. This is the good stuff. You can create loam by adding compost, sand, clay, etc. that are lacking in your base soil.

What does all of that mean? It depends. If I had to take a stab at what makes a good soil as an aquatic substrate, I would say a silty or sandy loam. Say 10-15% clay, 20-30% organic, and the rest silt and sand (by volume). The low oxygen environment of flooded soil is different that that that of terrestrial plants, so requires a bit more granularity and less clay is probably a decent starting point. Silt and sand makes for a non-compacting media. Clay provides a permanent nutrient exchange surface, and the organic matter continually breaks down into more nutrients. Those are loose and definitely SWAG numbers. It just so happens that riverbottom shorline deposits are often in these kind of ratios.

If a person really had some resources and were experiment savvy, a more clay rich base with silt/sand/organic on top and a cap of sand would be a really good starting point. I do want to point out that for most tanks, a sand cap is desired. It would be very difficult to maintain water clarity otherwise. This is an unnecessary, but heavily desired effect in most tank.

A good substrate soil should be thought of as a permanent or extremely long-term solution. Actual soil answers that need if the soil part is reasonably well understood. Nutrients will bind and separate from the clay and silt indefinitely. Bacteria have multiple environments to develop.

Compaction is not generally desired. It can happen in any substrate, but can largely be avoided in true soil so long as there is mechanical and biological activity. Plants with large active root systems are highly beneficial. The large roots continually and safely open the soil and the micro roots do it on the small scale. Microorganisms and burrowing inverts, as well as "digger" fish will help maintain porosity. In the absence of that, it can be an issue, and soil may not be the best choice. The more you can mimic the environment of soil in nature, the better.

Large aggregates:
I won't talk about this too much, but basically small gravel or very large sand is neither harm nor help, but can help create good structure.

Mixing soil:
If you mix soil, do it dry if possible. This will help some of the structure to remain intact and recduce compaction later. Pulverized soil can take quite a while to redevelop structure (grains/globs/etc.) and wet soil tends to pulverize when worked. Also, things like mixing sand into clay etc. just work better if it is all crumbly. One of those pastry things (dough blender) with all the blades would be a good tool to mix with. Think mixing pancake mix -- you want some lumps.

Soil-less mix:
A lot of "soil" sold in the market (most in fact), is not soil at all, and is what is technically known as soil-less media. Potting soil is a good example. It is typically a peat moss base with other mulch and perlite for aeration. Fertilizer is added in because there just isn't much in the way of natural nutrients present. Top soil used to be actual soil from somewhere that was bagged up. Now, I have only been able to find mixed sand/mulch types. IMO, anything bagged should only be used as an amendment if your actual soil has too much clay, not enough sand, pH needs (peat moss is great), or other specific reasons, but shouldn't be considered as the sole substrate under sand. It will decompose, and lose its original nutrient value over time without being able to leverage the mineral portion of soil that acts as a surface sink to store, recycle, and replace nutrients. Another concern with soil-less mix made from hardwood mulch is that there is little control of what kinds of trees. Some trees contain toxins in the bark, natural herbicide properties, etc.

On this topic, soiless aquarium substrates will naturally come up. I have no real strong opinion about them in general. They can provide a lot of ease and consistency, generally have some lifespan, and can certainly help grow plants without requiring much more knowledge. In many situations, they are ideal.

Anyway, I may add some more, but hope that this at least provides some understanding of "dirt". I am happy to try and answer with more detail, or provide some resources, but want to keep this at the general hobbyist level.
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I thought I would add a few more thoughts on pre-made aquatic plant substrates and how they can be integrated into an actual dirted tank. As you can see above, my feeling is they have a place and I am not anti-aquasoils in any way. But for the purpose of this discussion, the focus is on true dirt.

I can see a lot of value in using them as a soil conditioner if you are at all unsure of the quality of your source. Mixing them in at some rate up to as much as 20-30+ percent could really help in a tank with source dirt that is clay heavy or has known nutrient deficiencies. The nutrients will bind to the original soil and it will ultimately integrate as a proper soil structure with the benefits of the formulated substrate and the permanency of true soil.
I would like to bounce something out to you, @noahm .

I have considered my soil from out back for a tank. I live on a low coastal plateau with pale beige soil - a mix of sand and clay. It isn't great garden soil, and a farmer dreaming of rich black soil would take one look and buy a cow. I think it may once have supported apple trees though. It's never been ploughed and farmed - it was probably pine forest until relatively recently - the past 100 years.

I haven't done a soil tank for decades, but this soil has me thinking. I aquarium-fished in Gabon last summer, and walking up and down those streams I noticed how similar the muck seemed, except there the clay was quite red.

How would I test it?
You can often get it to an university that has a horticulture dept and get a general soil test done. They should be able to tell you what type and a bunch of other nutrient related stuff. Some agriculture extension offices -- I am not sure what Canada does, but surely has something similar. A local farmer supply should be able to direct you to some inexpensive route. A sandy or silty loam would be probably preferred in a tank.

That said, there are a few things that you can do observation-wise. Dig out a scoop and crumble it when it's slightly damp as well as when it's very saturated. If it loses its structure when it is saturated (becomes an uncrumble-able mass of clay), then it may have too much clay content for a good structure and would do well to get some peat or compost added in and possibly a bit more sand. You can also look at how plant roots grow into it. This is a bit more subjective, but look for branching, penetration, how straight things that should be straight are, how much hair roots you can see still attached when you pull it out, how deep the smaller fibrous roots go. As plants fill up the space, they create granules and structure. The soil for an aquarium should probably be smaller grain, but not crumble to dust.

That red clay you see in the tropics (and at home) is almost always due to high iron. That doesn't mean other soil doesn't have enough. It has to be available (chelated) to really be of use. Tomatoes are a great indicator plant. Rhododendrons may be something you have up there too. If they grow nice and green as opposed to slightly yellowed leaves but still green on the veins, you have iron deficiency.

The end goal is to get something with a sandy/silty structure and a moderate amount of clay to give you long-term adsorption surface and bacterial sites. For this, the spit-test is usually pretty telling. Just put a nickel sized blob in and spit in your hand (or add a few drops of water). With a little experience and not too many callouses, you can tell clay (slippery), from sand (gritty), and silt (think ultra-fine river sand), and organic matter (generally whatever is left).

Organic matter and nutrients is generally less of a problem in aquariums because we continually put it in there, but it can't hurt to mix in a bit of compost initially. If the structure isn't so crumbly, mix when very dry and it should work out a bit more easily. It is the mineral structure in an aquarium that is more important long-term as organics will naturally become integrated.

If you are willing to take a picture, find a handful of reasonably dry soil from the top 3 inches, crumble it up to varying degrees and spread it out on a paper plate or sheet of paper. Leave in a few small plants and weeds. I can probably tell you a reasonable bit from there. Also, a somewhat close picture (a couple feet away) of whatever herbaceous plants are growing in it will be useful too.
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