Grocery store.

Assume that all the foodstuffs that people normally eat have the same potassium content as these nettles. Then you'd need to consume 1.25kg of your normal food per day to get enough potassium. Someone who consumes less than 1.25kg of those normal food stuffs would not get enough potassium. If normal food stuffs have a significantly lower potassium content, then you'd expect widespread potassium deficiency.

Maybe, but it doesn't seem to be a serious public health issue. So common food can't have significantly lower potassium content than those nettles. What with averages being averages, some foods will have more than others.