2004–State-wide soil temperature cooled to below 50F on Nov. 21st. 2003–Iowa soils cooled to below 50F by Oct., 28th. However, a warming impacted the areas south of Highway 20. Soils south of Hwy. 20 did not remain consistantly below 50F until Nov. 21st. 2002–Entire state soil temperature below 50F: October 22; October 13 Nashua, Ames
http://extension.agron.iastate.edu/NPKnowledge/soiltemphistory.html