Iron on grass and crabgrass
I tested iron sulfate as an iron supplement for turfgrass using about 6 ounces per half-gallon of water, which covered abut 1000 sqft.
Iron applied to green grass (bluegrass plus fine fescue in light shade), caused very slight darkening of the color. Even when sprayed 8 times over, there was only a slight visible darkening effect. Dandelion appeared to be injured. There was no injury to the grass.
When the solution was applied 6 times over as a 30 inch stripe to crabgrass, near a road, a distinct darkening of the color was visible 8 hours later, probably due to some black staining. After 48 hours some of the crabgrass appeared to be injured or dead.