Some green manures, cover crops, are known to be accumulators of Potassium. As I recall Buckwheat is one.

Unless I'm missing something I don't see how a plant that accumulates potassium is going to do so when potassium isn't in the soil. Potassium isn't like Nitrogen where there are plants that can acquire it from the air.

I think you would need to grow the buckwheat, comfrey, nettles, etc in a potassium rich soil to "mine" the potassium and then transport those plants to the deficient areas. But at that point you may as well use wood ash or some other K source.

If there is a plant that can collect potassium from some source (air, water, magic, whatever) and put that in the soil I would love to hear about it. Like a potassium fixing plant would be interesting, but I've never heard of such a thing.

You might try to get a really deep subsoil sample and have that analyzed. If there's no potassium in the subsoil, you can't pull it up with alfalfa etc. You just have to bring it in from outside sources.

I plant buckwheat every other year with clover.And I'm planting it as a living mulch between rows.I just got back my soil test and the only suggestion was nitrogen.I think most of that is because the clover had not been turned in and it takes a while to release.But I always add a little alfalfa pellets to the rows anyway as I till.

I can't imagine 'making' potassium if none is there to begin with. Municipal leaves could be a decent source.

Lloyd