Prices for commodities fell broadly for a second day after the head of the European Central Bank offered no immediate measures to support the continent's fragile economy.

ECB President Mario Draghi outlined a new effort to buy government bonds to try to ease high borrowing rates that some countries face. He said ECB policymakers will work on a more detailed plan in coming weeks, but provided few specifics and did not detail steps that would be taken immediately.

The announcement fell short of traders' expectations after Draghi said last week that the ECB would do "whatever it takes" to preserve the union of 17 countries that use the euro.

His comments came a day after the Federal Reserve ended a two-day meeting without implementing steps to bolster the U.S. economy. But the Fed noted that growth had slowed over the first half of the year and repeated its pledge to provide additional help as needed to promote economic growth.

Kingsview Financial analyst Matt Zeman called the lack of specific announcements from the two central banks a "big, big disappointment."

"It's given people reason to pull the trigger and take profits and go back into the safe-haven assets of Treasurys and the dollar," he said.

Zeman speculated that more traders will stick with relatively stable assets until there is a better idea of the banks' future plans.

There also is some caution among traders ahead of Friday's unemployment report for July, which will be released by the Labor Department. "If this jobs number is lousy...that's obviously going to accelerate some of this risk-off type of trade," Zeman said.

Gold for December delivery dropped $16.60 to finish at $1,590.70 per ounce and September silver fell 54 cents to $26.995 per ounce.

September copper declined 8.45 cents, or 2.5 percent, to finish at $3.2905 per pound, October platinum fell $13.50 to $1,387.80 per ounce and September palladium decreased $14.75, or 2.5 percent, to $567.85 per ounce.

Energy contracts were mixed. Benchmark oil dropped $1.78 to finish at $87.13 per ounce, heating oil fell 1.65 cents to $2.8423 per gallon and wholesale gasoline rose 3.54 cents to $2.8696 per gallon.

Natural gas prices plunged nearly 8 percent after the government said stockpiles rose last week, putting the inventory level 14.5 percent above the five-year average. Natural gas fell 25.1 cents to end at $2.92 per 1,000 cubic feet.

In agriculture trades, corn for December delivery dropped 4.75 cents to finish at $7.9575 per bushel, November soybeans fell 12.5 cents to $16.165 per bushel and September wheat fell 14.5 cents to $8.65 per bushel.