When a plant is grown in a pot, it will initially use the soil around it to feed however, there will eventually be insufficient nutrients available to sustain it. The addition of fertilizer will only work for a while, it is therefore necessary at some point to change some or all of the soil, this is known as repotting.
The right time
Repot a plant just before it starts to form new shoots. Often the best moment is towards the end of winter. For hardy plants that spend winter outside in their pots, autumn is also a good time to repot them. Plants that fear the cold such as cactus should be repotted in the summer.
What you need for repotting
For repotting, you should use a soil mix that is suited to the needs of the plant being repotted: a good quality compost for example. Generally, a plant is repotted into a pot that is slightly bigger than before, by about 2cm in diameter for example. Something is also needed to untangle the plant’s roots (this is important): a hand fork or even an old kitchen fork with the handle bent will do.
Begin by removing the plant from its old pot. If the neck of the pot is narrower than the rest, the root ball will not come out in tact and it will be necessary to cut some of the roots, unfortunately you have no choice! With the hand fork try to comb the soil out from the roots as if you were grooming a dog’s coat. Do not be frightened to leave the roots bare, remove about 3cm or even more of soil all the way around.
At the bottom of the pot put some drainage material and a small amount of soil as if you were planting a new plant. Position the plant whilst still holding it with one hand. With the other hand, add the new soil making sure to fill all the space between the root ball and the walls of the pot. Make sure the soil goes all the way down in this narrow space and then firm it in. There must not be any air pockets. Add compost to within a few centimetres of the top of the pot (never fill it entirely level with the top edge).Then water the plant well around its base (this helps to eliminate any remaining air pockets). Do not add any fertilizer for several months.
An alternative method. Changing the surface soil
Rather than repot entirely, it is possible to just change the first 10cm or so of soil near the surface, without either changing the pot or removing the plant. Lie the pot on its side and remove as much soil as will fall out using a hand fork or similar tool. Stand the pot upright again and add the new soil filling it up to where it was before, then water. That is it!