So...I tested iron solution sprayed on grass. I used Bonide ferrous sulfate containing 15 percent iron. I added 8 ounces to a quart of water--normally this would cover about a thousand sqft with my hand sprayer. Like last year, it did not dissolve completely, but left a bit of undissolved residue in the bottom of the sprayer; no problem. I sprayed two 4-foot square areas, with about 18 inches of untreated grass between in the center behind the clump of tall fescue. I sprayed twice over on the left and once over on the right. It rained after about 8 hours, and the photos were taken after about 24 hours. After 7 hours, a dark color was visible--a blackish stain or residue. Brown crabgrass in the area was stained distinctly black, (second photo). Dandelions were stained somewhat black also.