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I've looked at some youtube videos, where they say that mediums such as vermiculite allow good drainage, but also have great water holding capabilities.

I am newbie gardener, and to me, this is almost a contradiction. What do they mean when they say that a medium has good drainage, but also holds water well?

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It seems a contradiction, but if you look more in deep, there is a reason:

A soil with a lot of clay (so finer grains of soil) is a concrete like structure: water and clay paste together: there is a lot of surface, which doesn't allow water to move, so it will also absorb less water.

A soil with a lot of sand (so the larger grains, but finer as stones) allow water to move: there is a lot of spaces between grains. But so the water go away quickly.

The ideal soil should have enough surface of grain, to attach water, but also not too much space to allow water to go away. So a mixes soil would help. Note: also a sand pocket could help (and they will act like a well): there is enough space to keep water, but the soil around doesn't allow water to flow away.

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I wonder whether it might help understanding to 'scale up'. Consider a saturated sponge buried in pebbles inside a container that is porous but retains the pebbles. Water will flow straight past the pebbles (good drainage), so roots in the pebbles will stay relatively dry, but the sponge will hold water (that can be sucked out by roots as required). The key may be that you are not expected to use vermiculite on its own – only as an additive.

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