Q. Can I reuse soil from a pot in which the plants were infected by something that looked like white dust.
Whatever the trouble was that your impatiens and ajuga encountered, we really can't recommend reusing the soil or trying to sanitize it for reuse. So many pests and diseases are perpetuated in the soil and it would be such a shame to have another generation of plants succumb to the same problem despite your efforts. The best advice is to dispose of the soil and to clean your pot thoroughly, first removing any soil and debris with a stiff brush and then washing with a 10% bleach/ 90% water solution.
Good luck with your containers!
Courtesy of NYBG Plant Information Service