Commodity trading: the onion exemption

Jury selection was held Tuesday in the case of three former El Paso Corp. natural gas traders accused of conspiracy, false reporting and wire fraud in connection with efforts to manipulate natural gas prices. Opening statements are expected Wednesday morning.

Farmer John Peterson smelling onions from the film The Real Dirt on Farmer John. Photo by Taggart Siegel.

While conducting voir dire Assistant U.S. Attorney Belinda Beek noted as an aside that the Commodity Exchange Act that is being used to bring the false reporting charges excludes onions as a tradeable commodity.
Why should onions growers get such a break while carrot and soybean farmers are not? Is the Big Onion lobby really that powerful?
Craig Pirrong, head of the University of Houston’s Global Energy Management Institute (and general wonk about all things related to commodity markets or Russia) has an answer:

Futures trading in onions was prohibited by PL 85-839, passed in August, 1958. There were several occasions where huge quantities of deliveries were made against onion futures contracts, depressing the price of onions in Chicago. At times, the price of onions fell below the price of the bags they were shipped in. Outraged onion farmers, especially in Southwest Michigan succeeded in convincing Congress to ban onion trading. Hence, onions are not (legally) a commodity.

Update: Pirrong has since expanded on the onion issue in a post on his blog, explaining the roots of pork belly futures and energy futures.
Pirrong cites Brokers, Bagmen, & Moles, by David Greising as a source for some of this info.