Sorry, the board monkeys won't let me edit my post any more, so here is a revised version:

I don't know anything about ph and weed, or fungus and weed, but most round, irregularly spaced holes that I've seen in other leaves are from fungus. Fungus grows in a circle.

Those plants are definitely N-starved. N deficiency causes the oldest, lowest leaves to yellow and die, while the new growth stays green. The plant actually moves the N from the old to the new.

You might be chasing your tail with the nutrition. What were the initial symptoms that led you to apply the potash? Or the magnesium? Do you see those symptoms in the pics, or are they gone, as though you fixed those symptoms? If you still see those symptoms, point them out, because I don't see signs of either. Did you know that excess potash can cause nitrogen deficiency? (And magnesium deficiency, too, but I don't see that.)

Read this page before you do much more:

Deficiency and Toxicity Symptoms

Keep in mind that the info there is mostly related to field crops, but it's generally relevant to most vegetative plants.

Those holes, though, just don't look to me like they are deficiency-related. That tends to be pretty symmetrical when it happens, like around the leaf margins (edges), between the leaf veins, at the leaf tips or over the entire leaf.

Those look like the leaves were hit by birdshot, holes scattered everywhere. There are fungi called "shothole fungi," and that's what that looks like to me. I just don't know if it affects weed. If I'm right, you should see little yellowish, *maybe* watery lesions on the mostly healthy leaves, then gray dead spots on the somewhat sicker ones, and holes in the sickest. If that's it, I don't know what it will do to your plants, or what the proper remedy is for something you plan to burn and inhale. Google tobacco and shothole and fungicide and see what you find.

Also see:
AZ Master Gardener Manual: Determine Causes

If they were mine, I'd probably flush hard and fertilize with a balanced fertilizer with minor elements and see what happens. If they perk up and start growing, you can probably forget about the holes if you can't find a smokeable fungicide. I'm too lazy to look for one right now.