The pressure for ag subsidies in the 1920's arose as a result of the price of wheat peaking in 1919-1922, at about $2/bu, which was a doubling of the pre-WWI price average of $0.70 to $0.90.

By the mid-1920's, the price of wheat had fallen off to $1.30 or so, but a lot of banks had lent a whole lot of money to the ag sector in the US based on the doubling+ of wheat prices post-WWI.