Not a lot really. Water, light, carbon dioxide, a few mineral ions and away you go.
So why all the fuss about soils? Many peoples first introduction to plants is growing cress on a damp piece of paper. No soil required*.
Apart from conveniently holding the plant in place and providing a relatively stable environment what function does soil have?
What about the essential minerals? True soil has minerals in it, but not in any great quantity and even if they are absent then the result is generally poor growth, which may ultimately lead to less disease resistance but not necessarily a direct cause of death.
So do we need soil? No. But it is convenient and provides a stable environment.
So if plants don’t need soil why all the fuss? Or is this of our own making? Has our own hybridisation and production of cultivars produced over sensitive plants making us ‘helicopter’ gardeners. Growing over needy plants that have been tweaked from an original natural specimen to produce something unnatural.
All plants have been introduced and sourced by us, having been removed from growing naturally in their own environment. The challenge being to produce and own specimen plants form exotic locations.
I have a garden, I have plants in it. Occasionally I cut them down and tidy them up. Generally I let them do their own thing. I don’t water them, feed them or do anything else and mostly they all seem happy enough. Those that are suited to this survive, as they would naturally in the wild.
Growing exactly as they would have done in their native environment for centuries before being dug up by a tweed suited Victorian plant hunter. Maybe it’s time to back off from the garden and worry less, in the end they are just plants.
*Historical note: In 1902 the medical officer of the ‘Discovery’ expedition to the Antarctic, led by Capt. Robert, successfully grew mustard and cress as a source of fresh food. Possibly a gardening first, using blotting paper and water. However he did report that the mustard and cress grew more successfully on the small amount of Antarctic soil that he managed to collect.