In Cleveland there are 18,000 vacant lots (or ~3,500 acres of vacant land)
"Ohio State University researchers Sharanbir and Parwinder Grewal conducted a study to determine if Cleveland could achieve self-reliance in the provision of several key foods. In this city of ~400,00, there are more than 18,000 vacant lots,or about 3,500 acres of vacant land."
Masi, Brad; Fiskio, Janet; and Shammin, Md Rumi. "Urban Agriculture in Rust Belt Cities".Solutions. January-February 2014, Volume 5, Issue 1
Original source: Rokakis, J, Thriving Communities Institute. Interview, February 8, 2013.