Ants! Please help
So this is my first year gardening, and my two 4'x4' beds are doing really, really well with the SFG system.
But, I've got an ant problem. Two of my squares--one containing cabbage and the other kale--have a lot of ants in them. The ants are tiny and black, and I can observe them crawling up and down the base of the stem, down around the root system. As far as I can tell, it looks like they've chewed away part of the stem at the very bottom, especially where it goes underground. One of the kale has already wilted and died.
I've tried sprinkling peppermint oil around the base of the plants; baking soda; flour; cayenne pepper...I've planted some marigolds and mint plants around my beds...but nothing seems to be phasing them, and soon I think the plants will be dead. I'm also obviously worried about them spreading to other parts of the containers. I've searched all around the garden but can't find any ant hills, either.
Can someone who's had this same problem give me another pest control recommendation? I've done everything organic thus far, and I don't want to use any kind of dangerous chemical pesticides or anything like that. I've seen people mention using Borax, but if their nest is underneath the plants (which presumably it is?), then wouldn't they carry the Borax down into my soil?
Any help would be greatly appreciated. Thanks.