While the U.S. Federal Reserve’s plan to buy more than $1 trillion in debt has helped unfreeze the credit markets, it has also effectively capped U.S. Treasury yields and undermined the dollar.

And that’s caused commodities to soar as currency speculators and safe-haven investors head for higher ground.

At the culmination of the policymaking Federal Open Market Committee’s (FOMC) two-day meeting Wednesday, Fed Chairman Ben S. Bernanke revealed that the central bank would purchase up to $300 billion in longer-term Treasury securities, as well as an additional $750 billion of mortgage-backed securities. The central bank also said it would buy debt issued by government-sponsored agencies such as Fannie Mae (FNM) Freddie Mac (FRE).

“To provide greater support to mortgage lending and housing markets, the Committee decided today to increase the size of the Federal Reserve’s balance sheet further by purchasing up to an additional $750