Iron and crabgrass.
I applied iron to crabgrass at about 6 ounces iron sulfate per gallon. I applied it at a high rate --about 6 times over--to crabgrass near the road in a stripe about 3 feet by 8 feet. The crabgrass turned much darker green in 24 hours, and then almost black, and was severely injured--some of it killed. See photo: leaf was included for contrast (basswood, I think.)
A few yards away, I sprayed Drive 75 WP in a strip about 2 feet by 15 feet. Drive was effective. Crabgrass was mostly brown and dead. Unsprayed crabgrass at right in the photo. Photo was taken at 4 days after treatment. Soil was fairly moist from recent showers and temperature highs were about 82.