White clover, an indicator of low mowing height and low fertility soil.
Spurge, an indicator of compacted soil.
Prostrate knotweed, an indicator of compaction.
Red Sorrel, an indicator of low pH, low fertility soil or dry soil.
Plantain, an indicator of low fertility soil.
Nutsedge and Oxalis (woodsorrel), indicators of wet soil.
Nimblewill, an indicator of excess shade.
Goosegrass, an indicator of compacted soil.
Crabgrass, an indicator of low mower height, wet soils, or high fertility. 
Carpetweed, an indicator of dry soil.
Black Medic, an indicator of dry soil. 
Bermudagrass (wiregrass), an indicator of low mowing height, compaction, or high fertility.
All Weed Pictures by Gordon Johnson, Extension Agent, Agriculture and Commercial Horticulture, University of Delaware, Kent County