Native milkweed grows in my garden beds wherever it sprouts since I garden to sustain pollinators--bees, butterflies & hummingbirds. They're essential to sustain our natural food supply. Like you, I've seen no butterflies the past couple of years; only moths. If your soil is healthy*, normally you shouldn't need to do anything for it.

* mine is slightly acidic sandy loam

The only fertilizer I use is organic seaweed extract. I don't fertilize established perennials, only newly-planted additions to help them establish their root systems.

Just a tip, but folks might offer more help if you included your USDA zone and/or region in your identification. I'm in Zone 6, southern New England. There's an ap (of course) that will give you your zone based on your ZIP code.

Don't get me started on chemical fertilizers...