In article ,
kay wrote:

'Nine gardening myths debunked | Life and style | The Observer'
(http://tinyurl.com/clpbejp) -

-9 +2.

No, water does not "sit above gravel" in pots. Physics is not mocked.

I think what they're trying to say is that water doesn't drain until the
soil is saturated; until that point pretty well all the water is in the
soil (ie above the gravel). Just as if you put a damp sponge on top of a
layer of gravel, it would stay damp, and wouldn't drain all its water
down into the gravel. The gravel wouldn't get appreciably wet until the
sponge was sopping wet and the water started falling out of it. So all
you're doing by adding a layer of gravel is making the pot smaller.

Very likely. That's still wrong.

The point is that the drainage hole area is small, and it often
gets clogged with a layer of humus held up by larger particles.
A layer of gravel in the bottom means that the pot will clog
only when the layer of humus covers the whole area. It's exactly
the same principle used to strain the wort through the mash in
brewing. A layer of gravel works, at least with John Innes style
composts.

However, there is a second factor, too. Most soils have TWO
saturation levels: the maximum they can hold without draining,
and the maximum they can hold in the short term. In my (fine)
sandy loam, they are very different, and the former doesn't
cause any trouble to growing plants (not even ones that hate
waterlogging) because there is still plenty of air. Gravel
provides somewhere for the water to go when that level is
reached in larger pots.

Where I fully agree is that there is a myth that adding a layer
of gravel will necessarily make a pot well-drained. That is
complete nonsense. But gravel in pots works, if used properly.


Regards,
Nick Maclaren.