Stones, pebbles, cracked terra at the bottom of pots worsens the drainage. Solved

[Bonsai2]

Hello

I read that putting stones, pebbles, cracked pots, etc, in the bottom of a container actually worsens the drainage, rather than aids it.

Two universities in the USA have also suggested that putting stones or cracked terra pots at the bottom of containers does actually worsen the drainage as it can lead to a build-up of moisture just below the roots.

I currently use John Innes 3 with 50% RHS Alpine grit in each pot and that has been working well for me with some small stones at the bottom of the pot. However, if when repotting, the practice of adding stones or pebbles is not used resulting in the drainage being even better, this seems to be the way forward.

Do any members have experience of both?

[kohinoor]

soil and medium are different.
Water behave drastically different.


in the soil, particle are very small,water in soil forming a capillary web.Water move thru soil via cohesive capillary.


if you put Stones, pebbles, it will form big air bubble and pocket.
these air bubble is too big for capillary to take place.There for worsen drainage.

here is a youtube video explain and demonstrate this effect

https://www.youtube.com/watch?v=ego2FkuQwxc


But we are talking about cactus and succulent.The growing medium we use already has a lot of big air bubble .
The particle size is so big that it has 0 matrix potential.The only law apply here is gravity.
The water move thru medium via simple flow.So the pebble rule don;t apply.


in short:
water behave differently in Fine soil and Coarse medium.

[rodsmith]

There was a thread on this subject several years ago. It highlighted experiments that showed, as you have indicated, that having a layer of large grit/pebbles/stones etc at the bottom of the pot actually inhibits drainage rather that encouraging it.

[el48tel]

Just been discussing this with the outside garden expert aka my wife. She has come to the same set of conclusions by trial and error over many years.
The grit and crock will have a large voidage in which finer materials such as sand will become trapped thus blocking the drainage and the downward water flow becomes poorer with time. The capillary action of grit differs from that of finer sand and the upwards flow is different.

[Terry S.]

You might need something over the drainage holes to stop the soil falling out. Plastic mesh (e.g. shade netting) that is easily cut with scissors is good or even a piece of paper in the bottom as shown in the Beechgrove Rebutia piece on TV last Sunday morning.

[el48tel]

Is paper suitable to retain the compost? I seem to recall in the not too distant past, comments being made about paper attracting insect larvae.

[ragamala]

A piece of broken clay pot used to do the trick in the old days. A single piece, not a layer, just as Terry S says. But that was in the days of large drainage holes at the bottom of clay pots. Today I doubt whether the small multiple drainage holes in plastic pots warrant the same attention.

[el48tel]

A piece of broken clay pot used to do the trick in the old days. A single piece, not a layer, just as Terry S says. But that was in the days of large drainage holes at the bottom of clay pots. Today I doubt whether the small multiple drainage holes in plastic pots warrant the same attention.

And many large plastic pots don't have holes .. just an indication of where to apply the drill

[Tina]

I use kitchen roll at the bottom of pots, after a while it does degrade but by then the soil doesn't normally fall out.

[rodsmith]

If using plastic pots I use a piece of kitchen paper towel to retain the soil. With clay pots, the traditional piece of broken pot seems to work ok.