The overall U.S. economy may be struggling, but you wouldn't know it from a visit to the Farm Belt. A boom is under way across much of the rural Midwest, with agricultural land prices growing at a double-digit clip and farm auctions in certain counties fetching record sales.

One question to ponder: Is this boom rooted in genuine economic gains, or is it another Federal Reserve-induced asset bubble? We lean toward the bubble view.

The...