The question maybe sounds stupid and the answer obvious, but I have not yet heard an answer that convinced me.
The ground where I live is 90% compact clay - no fertiliser, no oxygenation. When you dig out a tree there is one main root and lets say 5-10 small hair like roots. How on earth is this better than fast draining soil that allows to the roots to grow freely (when you dig out a tree from a canister it's filled with roots), the fertilisers we add, the constant watering – and yet, the trees in the canisters grow, let's say, half slower. WHY?

Winter wrote:

The ground where I live is 90% compact clay - no fertiliser, no oxygenation. When you dig out a tree there is one main root and lets say 5-10 small hair like roots. How on earth is this better than fast draining soil that allows to the roots to grow freely (when you dig out a tree from a canister it's filled with roots), the fertilisers we add, the constant watering – and yet, the trees in the canisters grow, let's say, half slower. WHY?

I think the answer is in the question already. We fertilize, aerate, prune the roots so we get a compact rootball with many fine roots, which makes it more efficient.
When in the soil, the tree has to send out roots to find nutrients, the roots must grow longer and bigger, and the longer and bigger they get, the fatter the tree gets.

Auk wrote:

When in the soil, the tree has to send out roots to find nutrients, the roots must grow longer and bigger, and the longer and bigger they get, the fatter the tree gets.

But what does it find? More clay, a stone ...? The necessity of growing sounds reasonable, but the poor roots and lack of nutrients just seem not to be enough to support such growth.

... its like Im growing fast and big muscles because I need to find food - that I don't have in the first place.

Winter wrote:

... its like Im growing fast and big muscles because I need to find food - that I don't have in the first place.

Or better explained by the second law of thermodynamics.

your clay is full of nutrients. Clay is the best in keeping nutrients. So your plant gets loads of them.
The main trouble is the lack of xygne. However, a plant in a pot it not the same as the plant in the ground. Plants have a way of adjusting to the local environment: The plants will create more tissue that is able to also transport oxygen. In the end, your plant will grow a much larger rootball, with a much more stable supply of moisture and stable temperature, aiding in the quick development.

Pot or full ground.. Very different beasts.

leatherback wrote:

your clay is full of nutrients. Clay is the best in keeping nutrients. So your plant gets loads of them.

... but the tree gets much more nutrients in the canister so should grow even larger.
The theory that they grow bigger in nature because they need to search for food and that they don't grow large in canisters because they have everything they need obviously doesn't apply to vegetables - they grow much larger when you fertilise.
I asked this question to biologists and they do not have a clue.
They have more of everything in canisters, so my theory is, that they can feel they are contained and adjust to this.

Rootspread limitation limits the growth. If roots are stopped from growing, the tree will naturally reduce the growth too.
also see
www.sciencedirect.com/science/article/pii/0040580984900078

leatherback wrote:

Rootspread limitation limits the growth. If roots are stopped from growing, the tree will naturally reduce the growth too.

I think this is exactly it. It has all it needs and more in the canister, but it somehow feels that it has limited space and it stops growing. If only we could bypass this sensor ...
So the widespread theory that the tree grows larger in nature because it needs to find nutrients would be rejected. But I still don't know what it uses to grow let's say on a stone klif.

Winter wrote:

I think this is exactly it. It has all it needs and more in the canister, but it somehow feels that it has limited space and it stops growing. If only we could bypass this sensor ...

Let's try and not make up wild theories... the plant does not have a sensor for that.It is simply the physically limited space. There's no more room. It can't grow any further.