The organic matter in soil continually breaks down over time, causing the soil to become compacted. Compacted soil means that the soil will lack aeration and loses the ability to drain excess water. This leads to ill-health in bonsai, which is why re-potting, escpecially in organic soil mixes, is so necessary.
There is no definite schedule which one can follow on how often to repot a bonsai. Each individual tree has its own needs. Most articles one reads, will say to re-pot about every 2 years, but personally I do not agree. The most full-proof way of telling when to re-pot a bonsai is by lifting it out of its pot/container- if the roots are still contained within the soil and are not yet visible around the edges of the soil line, then it is not quite time to re-pot yet.
The best time of year to re-pot a bonsai is early spring. The reason for this is that the tree is still coming out of its winter dormancy, thus reducing the potentially damaging effects of root-ball trimming. It also allows for the tree to grow new roots for sustaining itself right at the beginning of the growing season, therefore decreasing the chances of any damaging effects that may have been caused during this process.
"What root-ball trimming?" one may ask. Well in the opening paragraph I mentioned that roots need to be trimmed to prevent a bonsai from becoming pot-bound- this is the most important part of re-potting. Remove the tree from its previous pot and gently remove all the soil, leaving just the roots exposed. NEVER REMOVE MORE THAN 70% OF THE TREES ROOT SYSTEM. You should ALWAYS have 30% of the original root system remaining to ensure the survival of your tree.
If you are worried that you have removed too much of the root system, remove some of the trees foliage, this will help it to cope. Another thing to bare in mind is that the total surface area of those delicate little roots, far exceeds that of the larger roots, thus increasing nutrient absorption, making them more important than the thicker, longer roots. When re-potting always keep the trees roots moist- if they dry out, they will die. Therefore the faster you can complete this process the better. So try to prepare your potting soil mix beforehand and have your selected pot ready ...
The best way to ensure a successfull repotting is by placing a mound of your selected soil medium in the centre of the pot, this helps ensure that there is not a large empty space under the base of the tree. Open up the now-trimmed roots and spread them out around the pot. Work the soil in between all the roots, gently but thoroughly. Upon completion, soak the entire bonsai- pot-and-all in a basin that is deeper than the height of the pot. When all the excess air has stopped bubbling to the surface, remove the tree and place it in your en.