Summer-induced chlorosis is commonly observed on cool-season grasses during the summer months in the Midwest. This chlorosis generally does not occur in the cooler periods of spring and fall. The problem is often associated with grass grown on sand-based media, but is also observed on clay-loam soils. Preliminary studies conducted on chlorotic Kentucky bluegrass turf at the Iowa State University turfgrass research area showed a visual response to iron applications, but not to other nutrients. Further studies are being conducted to determine why the grass becomes chlorotic only during the summer months. Our hypothesis is that high root zone temperatures are not allowing the roots of the Kentucky bluegrass to acquire enough iron to avoid deficiency during high temperature periods. To test this hypothesis, plugs of grass from the field study area are being subjected to a range of soil temperatures in greenhouse studies using water baths to heat the rooting media. Field plots established over a soil heating system will also be evaluated in the spring of 2007.