I know this is an old thread, but is this why some folks give an 'epsom salts' treatment to perk up their dahlias? I know it's high in magnesium but might it have some potassium too? When I did it one year it made the foliage color up nicely and not look so anemic.
I'm interested in finding a source of potassium or some source of micro nutrients for the dahlias, and I'm pretty sure that our hardware stores don't carry azomite or potasium sulfate; i'd have to mail order for either of those,. Would it be too late to use it now anyway? August is half over already.