Urban expansion, at least in the few areas where Iowa cities are growing, is eating up some of the state’s best farmland. In Ankeny, a central Iowa suburb of Des Moines that a May U.S. Census Bureau report ranked as the nation’s fourth fastest-growing large city from July 2016 to July 2017, much of the land being developed for housing is high quality soil for raising crops, an Iowa State University agronomy department survey shows.