Alasdair

My initial thinking was the same as Will's.

If you have the water table at the bottom of the pot the majority of the water is going to escape rather quickly out through the drainage holes. Having a layer of coarse soil on the bottom would bring the water table away from the holes, increasing the water retention of your soil. It would also prevent water logging as gravity would pull away any excess water into the coarse layer where it can drain away. If you have finer soil throughout and the water table against the bottom of the pot the water has nowhere to go unless it is over a drainage hole.

I put a layer of 5mm rounded gravel in the base of my pots, always have as it is what i had read in books. But i never put any thought as to why. Now i have, and i think that a drainage layer is quite sensible.

Al

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I can feel another "I wish that was my tree" moment coming on...
Currently studying BSc Plant Biology at the Universty of Wales, Aberystwyth