Those dark spots come from the iron in the fertilizer. When you feed your trees and plants make sure you broom off, or blow off any fertilizer from your sidewalks, driveway, or any block wall boarders you have around your beds. I have used muriatic acid to clean any iron spots on the cement, it works good. Just don't over do it, put a small amount of acid on the iron spots and wash it off with water as soon as you see it is gone. Be careful not to breath any fumes, this stuff can take your breath away. You can purchase this acid at any pool supply, T & D pools over by Colony Plaza or on 466 by Bob Evans should have it, and it is very reasonable to purchase.
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Ron
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