The compost in your soil is what your plants are feeding off of. Just be aware that by flushing you are not only flushing out salts but nutes and micro nutes as well. I'm not saying all will be gone but just keep that in mind. If your new growth starts to look lime green, they are asking for nutes.....

You want to use two to three times the amount of water you container is made to hold. In your case, you need to use 10-15 gallons per 5gal container in order to properly flush.....