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In regard to potting, there has been much material written on the pros and cons of clay pots versus plastic pots. The main thing to remember is that the clay pot tends to overheat and therefore dry out quicker, which could lead to rotting when frequently watered. The plastic pots retain the moisture longer, which means the roots remain moist and are not injured so readily, also, there is less watering to do.
When potting, choose a pot big enough for the plant to grow in. The ideal size for globular plants is one that is slightly larger than the plant, including its spines. This allows protection for the spines if the pots are placed close together. For cylindrical plants, choose a pot big enough to allow the roots to be contained comfortably when spread out. When actually potting, cover up the drainage hole with gravel, or crock up to a third of the height of the pot and cover with a thin layer of compost.
Hold the plant in position, and pour the compost around it, then bump it in the pot to settle the compost in amongst the roots. Make sure that the plant is not planted above the previous soil level, since this will encourage rotting, then top with a thin layer of fine gravel to prevent the soil from splashing up against the plant. Despite all the care taken in potting, some damage to the fine roots is inevitable, so these must be given time to heal.
The compost should be dry to just moist when potting, and the plants should be left in a warm dry place for about a week before commencing watering. If watering is done immediately after repotting, you are running the risk of rotting your plants.
When it becomes necessary to repot, the plant should be carefully removed, and as much as possible of the old compost removed without damaging the roots. Check for any pests, etc., then pot into a size larger container, filling in the space around the old ball with fresh compost and treat as a newly potted plant. However, although spring is the best time to repot, never hesitate in tipping out and inspecting any cactus which is not looking healthy, even in the middle of winter.
The result of the examination may reveal.
Mis-management in watering resulting in the roots rotting.
Some plant malfunction, not due to the grower's management.
One of the various pests or diseases.
Inability to grow due to old age or weak constitution.