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Recently we relocated a cactus into a new flowerpot. When it was planted the soil would evenly occupy the flowerpot and adjoin the flowerpot side walls. The soil isn't cactus soil, but just regular soil that we bought from a local shop.

Now the soil shrank, formed a stiff cylinder and that cylinder no longer adjoins the flowerpot side walls - there's about 10 millimeters of air on one side of the soil cylinder and the other side seems to have slanted onto the pot side wall.

The cactus looks just fine.

Is that a problem? How do I address it? Do I loosen the soil or should I just leave it untouched?

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You may want to find a better medium. It doesn't look like there is an article on replanting cactus here, but you might ask what is the best soil mix for replanting a cactus. –  wax eagle Aug 18 '11 at 12:08
    
Following on from bstpierre's answer: you should be able to find a soil mix, specifically formulated for cacti, at your local garden center; peat-based multipurpose compost is unsuitable for cacti - they prefer a loam-based one with a fairly high grit or sand content. –  Mancuniensis Aug 18 '11 at 13:03
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3 Answers 3

up vote 7 down vote accepted

I've seen this happen with soil mixes containing peat that dry out. It may happen with other potting media too. The problem is that when you want to water, most/all of the water is going to just run down the side and not be absorbed by the soil. The preventive measure is to never let peat dry out -- which is possibly more water than your cactus wants. Backing up a step, it would probably be better to use a more appropriate soil mix.

I know nothing about cacti, so I'm not sure if this will be good for the plant, but re-wetting soil that has dried out that much can be hard. Assuming the pot has drainage holes in the bottom, you can place it in a shallow tray of water until the soil has reabsorbed enough moisture to expand back to normal.

If possible, you may want to repot into a potting medium that's more appropriate for cacti -- i.e. soil that can get dried out without contracting.

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and for references, I usually mix a lot of sand in with something like fine compost, for cactus soil. Should be well draining (which just peat, isn't) –  winwaed Aug 18 '11 at 13:19
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If you don't want the bother of repotting it I would suggest that you just take some sand (builder's, sandbox) and fill the areas where there is no soil. The sand will replace the peat and the cactus likes the sand. Just add sand when the soil level drops.

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Expanding on "bstpierre's" answer & "winwaed's" excellent comment (below that answer), I would go with their recommendations of repotting in a potting-medium that's suitable for cacti/succulents eg

  • 50% horticultural grade sand and grit.
  • 50% finely sieved compost or worm castings.

OR

  • 50% pumice (volcanic rock) or perlite.
  • 50% finely sieved compost or worm castings.

OR

Some of the above information comes via the following references:

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