Hmm. This is a difficult question, but what I would personally do is stop worrying about the soil. Whatever comes out is probably going to come out (make sure it's full of soil, though). I recommend giving the plants foliar sprays of water-soluble fertilizer. Then they'll be less reliant on whatever nutrients are in the soil. Miracle Gro 24-8-16 All-purpose fertilizer should be great (but it doesn't include magnesium or calcium, if those get depleted--fortunately, those are less likely to be water soluble, and less likely to leach out as much).
You'll probably want to use a pump sprayer, instead of a hose sprayer (because a hose sprayer would be overkill, and inefficient).
1 tablespoon per gallon (or 1.195 teaspoons per 51oz) is what I use for weekly foliar sprays on plants. Be careful, though, because the Miracle Gro scoop has neither tablespoons nor teaspoons. The big end of the scoop is 1.5 tablespoons, and the small end is a 0.5 teaspoons.
If you just have the one plant to fertilize, one spray bottle full of fertilizer should last you a really long time.