In a typical garden you can simply topdress the soil. Same for cannabis, I'm just topdressing and using a mulch with water only using blumats for irrigation. Its almost boring, but if you are running perpetual, you should be busy every day.
In a way you are still buying inputs for your soil, they just aren't engineered for cannabis, or nearly as concentrated.
I cheat a bit, I use a premium soil mix like black gold organic and topdress and incorporate into the soil, It mainly consists of vermicompost, kelp meal, gypsum, lime, epsoms, and compost containing guanos, oystershell, crab shell and other things. Stick a blumat in it.. walk away and just trim off suckers.
I like organics, they give a little more aroma to it. I did a side by side of hydro and organics, of the same strain a few years ago of the forum cut GSC of 60 plants. It was easy to tell the difference. Organic had a less chunky structure, and less color and resin, but the smell was more complicated, it wasn't as sweet as hydro. The hydro looked better but the GSC in organics, just had more soul to it.. for lack of a better description.