Repotting- what to put at the bottom/changing dirt

[sparkman123]

I am looking repot several of my cacti. My question is, what and how should I arrange the bottom? Up until this point, I've been putting a few layers of newspaper at the bottom; the local greenhouse suggested coffee strainers; and these posts

https://www.cactiguide.com/forum/viewto ... 25&t=40641

https://www.cactiguide.com/forum/viewto ... 25&t=40641

seem to recommend mesh screens at the bottom with some mixture of larger rock on top of that.

For reference, my soil mixture is 70% cactus potting soil, 30% pummice.

When I water the cacti, I usually give them enough water until it runs out the bottom. Then I let the water sit, allowing the cacti to slowly drink it up from the dish. If I use a system where I put rocks at the very bottom, it would appear to me that they would prevent the cacti from sucking the water? Would that be a bad thing?

Another reason I ask is for the purposes of future repotting. If I use a hard clay pot, it would make sense that a layer of rock at the bottom would make pushing the cactus out easier.

Let me know what you think.

[ElieEstephane]

Newspaper is a good idea but i'd definitely skip on the rock layer. Coffee filters drain slowly so don't use them. Let the mix be uniform all around the pot. I found that no matter how big the drainage holes are, the soil will not fall out after the initial spillage so i don't use anything
Leaving the cacti stand in water is a very bad idea!

[DaveW]

You really need to let the water coming out of the pot drain to waste occasionally in order to flush out any build up of salts. If you use fertiliser the plant does not take up equal amounts of each chemical in it so that in surplus can eventually build up to toxic levels in the soil if you let the plant soak it up again. Maybe OK on a couple of occasions, but they need a flush out with clean water occasionally (called leaching) and let it run to waste. Obviously if the plants are in the house you have to do that outside or in a sink.

These days I use plastic pots which usually have many smaller holes in the bottom so do not leak soil as much as did the old clay pots with a large single round hole did, so do not need anything in the bottom. However it depends how fine your potting soil is.

Stratification (different distinct layers) in a pot is not a good idea as it impedes rather than aids drainage. A drainage layer in a pot bottom recommended by the old gardeners seems logical, but is not born out by scientific tests. Water does not pass easily between a finer layer of soil to a sandy or gritty layer until the fine layer is sodden wet and then it only soaks away the surplus water with the fine layer still staying wet. Ideally if you want to improve drainage add more grit or coarse material to the soil and mix it in thoroughly so the whole pot is filled with soil of a similar texture, with no different distinct layers.

This video explains the science and disproves the old gardeners myth of drainage layers in pots. Note the coarser sand layer in the video impedes drainage from the fine layer above rather than speeds it up as would seem logical at first.

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

[Charlie VIII]

I use bonsai potting mesh, a4 sheets, do a search on ebay for it. Easy to cut to the required size and get maximum usage out of it