If its an indoor planter, you will need to amend the soil with the proper nutrients for the plants you are growing equivalent to if they are heavy or light feeders & if they are producing fruit or seeds for you. I usually start in general with G&B Blue Ribbon or Organic potting mix or Nectar of the Gods Potting Mix & then amend for the type of plant I'm growing. I often feed them 3 to 4 times a year indoors with liquid Nectar of the Gods fertalizer, compost tea, worm castings & aged chicken or pig maneur. Indoor potted plants & even porch plants in pots or raised beds in the right sized pot that accounts for their estimated growth can usually go about 3 to 4 years before they need a soil change or uppotting. If your soil is sunken, depleted & seems to be disapearing because its being eaten you could have a pest, or you are growing heavy feeders that need new nutrition. You could also be watering too much of your soil & nutrients away out of too large draining holes
