I have been researching online about what crops grow best in the Grinnell and Fairfield areas (in regards to Timlynn's question relating to the Iowa Produce Market Potential Calculator). Though my online research has yielded no results, Jon Andelson sent me some good information in response to an e-mail:
"Among commodity crops, it's pretty clear than corn and soybeans grow best here (compared to wheat, for example, or rice). Corn can be for humans (sweet corn, popcorn), though that's a small part of what is grow here. Soybenas can, too (tofu, soy yogurt, etc.), though, again, that's a small part of what's grown here.