I have several large pots (like, 18" across or larger) in my back yard, that are full of plants. This being Arizona, we don't so much have planting seasons (there's no time when everything dies off), and at least 2 of the pots have actual trees in them. So, I have pots that I never simply empty out and refill, and for the most part they have live plants in them all the time. Even in the ones without actual perennials, I usually want to plant new things before the previous things have quite died off.
I've noticed that the soil in these pots seems to get lower and lower. I'd filled them up to within an inch or so of the brim, and now they're more like 3 or 4 inches down. I'm not sure if the stuff is just settling, or if the plants are using
it up in some way, or if it's washing down the bottom, but it's gone. I'd like to add some more soil, but I know just dumping more soil in can harm the plants still in those pots, if their stems get buried.
Is there some way I can do this without having to entirely repot my plants, and without harming the plants still in those pots?