I suspect if you used soapy water to wet the weeds and dusted potassium chloride on them, the weeds would die, too. Or you could dissolve potassium chloride in water and spray that. You can find potassium chloride next to table salt in the grocery. It only kills the topgrowth, like the Guardian stuff, so it has to be reapplied to kill the roots.

I don't think you can urea something to death. You can buy stronger fertilizer than that almost anywhere.

I have accidently spilled a fairly large quantity of Blood Meal and seen the plants under that readily available Nitrogen source die, but this looks like a pretty expensive means of unwanted plant control. A much less expensive and much more effective means of unwanted plant control are your two hands.

This is for lawns and claims to be selective so you can just spray your lawn to feed it and kill the weeds. It's the only product I ran across that claims to be an organic weed and feed. I haven't been able to find any place that sells it. The website doesn't look like it's been updated and calling the number went right into voicemail.