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<this is a "what if" question instead of "fix a problem" one>

Correct me if I'm wrong: the major functions of soil(a bunch of tiny stones with some dirty stuff in between) to plants are:

  • anchoring point
  • a sponge-like substrate for the "dirty stuff" to be stored until the roots take it up
  • said sponge also preserves at least some of the water
  • and despite all those allow the roots to still breathe air.

I recently learned that an estimate on the time charred wood rots fully is 100 000 years. Same(and others) article also insisted the char is actually quite beneficial "for soil texture".

And now for the leap of reasoning. Imagine you take 5kg embers and grind them roughly(so as not to turn to dust). Then mix fully composted organic matter and plant something.

Would this "soil" work?

One problem seems to be anything missing in the compost will need to be introduced with chemical means - the charcoal will contribute no micro nutrients.

Clay minerals are the most reactive inorganic components of soils. source

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    Soil is so much more than the four bullet points listed. For one thing, you're missing the entire microbiome.
    – Jurp
    Commented Jan 21 at 14:09
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    Using 10-20% coarse charcoal is an established addition to soil. Commented Jan 21 at 15:22
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    @Jurp sterilized soil is still called soil, right? And just left to it's own devices it develops said microbial life. So would a bucket of ground char develop the same given the same amount of nutrients are introduced?
    – Vorac
    Commented Jan 21 at 16:48
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    Sterilized soil, when used for potting up plants, requires fertilizers to feed the plant that is potted in it. There will be no microbiome in a pot, whether the pot contains char or basic soilless mix. If your intent is to plant houseplants in pots, then you'll have to watch your drainage and provide all nutrients via fertilizer. If your intent is to use ground char as a soil replacement outdoots, then your plants will suffer.
    – Jurp
    Commented Jan 21 at 17:04
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    Soil is also chemistry (the nutrients), not just a sponge: microbial life helps to decompose minerals into small particle roots can absorb. (reserve and available nutrients are both important stuff in soil analyses). No microbial life? So you need fertilizers with simple molecules. Coal: there are different types, just check online the components: you need C, P, K, N, but also many minerals. But IIRC also a lot of Ca, which may not be ideal for many plants. Often it is just a question of balancing all stuffs. Commented Jan 22 at 10:20

2 Answers 2

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As comments helpfully point out, soil is a lot more than its mechanical properties. However, "a mechanical substrate for plant growth" is basically the definition of hydroponic medium: The stuff soil has which the substrate lacks, is provided in solution, so yes you can grow plants in it even if it's not "soil".

As for the established use of "carbon" hydroponic medium: Quick Google searches turn up "biochar cultivation sponges" that are probably mostly char but also mixed with binders; and, academic articles about growing plants in charcoal / biochar. It looks like a thing but doesn't seem very mainstream.

I would speculate that plain old cheap carbon, as charcoal or similar, has terrible mechanical properties as a growth substrate because it fractures and turns to powder very easily. Typical substrates (clay pebbles, mineral wool, plastic sponges, plant fiber, etc) are mechanically tough.

I would further speculate that "biochar hydroponic sponges" are some kind of climate tax incentive thing because the materials lists I've seen ("biochar plus natural fiber binders") seem to indicate the char is just a bulking agent and could just as well be any other inert filler

Edit: Marketing material from One example of "carbon hydro substrate" says

  • it "can be used as a soilless growing matrix; an excellent Coco-coir and peat alternative": Coco and peat are common hydro substrates
  • "carbon-negative... growing medium": You can get carbon credits for using it maybe
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And now for the leap of reasoning. Imagine you take 5kg embers and grind them roughly(so as not to turn to dust). Then mix fully composted organic matter and plant something.

It really sounds like you're describing biochar. That basically means charcoal used as a soil amendment or even as a primary substrate. Like @jnj1618340 said, it's not completely mainstream but because of its many benefits, it's rapidly gaining popularity, especially in developing countries.

But given your inclusion of compost already mixed in to the soil, then it actually even more closely resembles terra preta, a form of biochar-based soil that has been in use in the Amazon for thousands of years:

enter image description here

Although its exact composition and production isn't 100% known, the primary component is biochar resulting from felled forest material that wasn't completely burned as slash-and-burn agriculture typically works, but slowly charred as embers with vegetable compost, animal manure and domestic refuse -- including animal bones and broken ceramics -- mixed in.

Something about the way it was made led to it fostering just the right kind of microbial community that gives terra preta some fascinating properties. Namely, it's able to take in new organic detritus and make more of itself, expanding and regenerating at a rate of 1 cm per year. You could take a chunk out, replace it with infertile soil, and it would eventually "heal".

And in a region where soil is almost immediately leached out as soon as the trees go (as in modern slash and burn for cattle ranching), terra preta holds fast to the earth and remains fertile for thousands of years. It's really wild stuff.

It's practices like these that turned large portions of the Amazon basin into hyper-fertile, high-biomass, high-biodiversity areas. An example of human beings leaving nature better than they found it. And people around the world are still studying it, trying to more accurately emulate it and perhaps even improve on it.

https://www.iflscience.com/indigenous-amazonian-soil-technology-key-to-restoring-rainforests-worldwide-68785

https://en.wikipedia.org/wiki/Terra_preta

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