How big does a container need to be to develop happy soil micro-biome?

So all of my gardening has been in pots thus far.  I do things like rotating what I plant in there, adding compost, wood chips, etc. in order to try and foster the development of micro biom and good microbes, but I've heard that a pot, even a large one, is too small to really succeed.  Is this true?  How big of a container would be big enough to foster life and balance like that?  Would a small raised bed be big enough?  Etc.  Am I wrong about a pot not being big enough?

Good question! There are so many variables that it's hard to give a definitive answer.

For example, we grow our tomatoes in half barrels. But we have no illusions that these are self-sustaining soil systems. Rather, they are fed with nutrients from a large and far-flung system that might be 1000 times the volume, or perhaps much more. That's okay, we do this by design.

Based on my observations in a cold climate, I would speculate that a 6x12x1 ft. raised bed, which is about 2 cubic yards/metres in volume, might be just enough to form a productive soil ecosystem. External inputs would still be needed, but 2m3 volume might just be enough to build a soil community. My 2c.

I would assume that most soil, even not in pots or garden beds, benefits from compost, biochar, other things from time to time.  I guess I wondered if a large pot was enough soil for those happy microbe chains I hear about to come about.  Glad that, at least in a raised bed, its possible.

I feel this depends on what you put in the pots.

You mention compost and wood chips, yes those are great choices ...

Adding compost tea and worm casting, too!

Riona Abhainn wrote:I would assume that most soil, even not in pots or garden beds, benefits from compost, biochar, other things from time to time.  I guess I wondered if a large pot was enough soil for those happy microbe chains I hear about to come about.  Glad that, at least in a raised bed, its possible.

I'm sure that happens in really big pots. As Anne suggested, adding the right stuff will feed your happy soil critters, who will in turn feed your happy plants. From my perspective, the challenge with pots is the limited volume of soil and most importantly the moisture factor (it always seems there's too much or too little).

In my other post, I was musing about the soil volume that would create a  sustainable system.

ButI have been thinking further about minimum volumes. Based on what I've observed in my giant compost bins I think a living soil system could be created with less volume. (These are half bladders from IBC totes, 39"x39"x18" 500L/125 USG capacity.) That's still a really big pot, and there's no contact with the subsoil. But with some active management and moisture regulation it could work.

Take this for what it's worth. I'm just a gardener, not a soil scientist.

I think it's relatively easy to momentarily get a pot with a happy soil microbiome -- you could just fill it with healthy garden soil and it's good. The problem is that as things happen, the pot doesn't have the rest of the surrounding earth to draw on as a buffer. If you experience a drought, some big class of microbiota might die off in your garden, too -- but next year it'll be back when it moves back in from farther down or next door or drifts in on the air, or the few surviving members of the clade start to thrive and reproduce again. You could just try to simulate that by periodically replacing a hand-spade of soil in your big pot with a similar helping from the garden or nearby woodlands or whatever. You obviously won't ever be able to compete with the literal entire Earth, but you can sort of simulate it.

If the human microbiome were to be packed together, it would occupy about 3 pints.

The deeper the pot, the better. And to have some spongy natural material , like rotting wood and nettles or comfrey in the bottom to keep worms happy and cool in hot times. Watering from bellow in discs containing water mostly, so roots go down. Let it dry out completely until the plants go limp. Then water from above with like mentioned above with soil garden or from a nice tree you collect some soil from on a walk. ( always carry a bucket wherever you go,all the time hihi.). That will fill all layers of the pot with much needed microbiome. Rinse repeat.

In my opinion, all you really need is roots and rain and the rest will follow.
I've left a 5 gallon bucket with some clay subsoil sitting out,it eventually sprouted a catalpa tree and some smartweed.
The holes in the bottom of the bucket allowed bugs in, and in they came.
It still wasn't great soil but it was living and definitely better than it was.
It would have continued to get better if I hadn't interrupted the process.

The important ingredients are life and time.