What's the best salt to kill plants?

Rock salt? Water softener salt? Ugh, 50lbs of table salt? 

I cleared a bunch of brush and can't use chemicals because of how close it is to my chickens, so I'm thinking salt the earth. 

Up there against the shed. It's far enough away I'm not worried about runoff reaching my garden, and if it kills the grass below that would actually be a bonus. 

I just don't want that E36 M3 to grow back in a couple months, or ever really

I wouldnt use salt. You can maybe do high percentage vinegar to burn them, but i would use cardboard, plywood scraps or carpet scraps to just cover the area and stop everything from growing. 

Sodium chloride is sodium chloride.  Whatever is cheapest is the best salt to kill plants.

High percentage vinegar and black plastic will kill a lot. 

I use straight vinegar in a spray bottle. Safe and cheap.

These are thick, woody bushes and vines moreso than grass or weeds. We have to get in with pole saws and grabbers to get it all out. 

I'll try putting salt down then soaking it with vinegar so the roots can absorb it.

You can buy super high concentration vinegar. Like this  https://a.co/d/gqKNTuF

you can dilute or spray on straight. It'll work well. 

Salt is virtually forever. Once it gets into the soil it never leaves especially in the concentrations you are talking about.

Look up "brine spill remediation" and don't let the EPA catch you.