Stock index futures fell on Wednesday as commodity prices dropped for a second day, pressuring global equity markets.

The U.S. dollar rose against a basket of currencies <.DXY>, sapping investor appetite for commodities. Oil prices were off 1.4 percent, adding to a 2.4 percent slide Tuesday, while copper dropped 2 percent from a record in the prior session.

Shares of Dow component Alcoa Inc dropped 2.2 percent to $16.15 in premarket trading.

Investors will scour the December ADP employment report expected at 8:15 a.m. EST for clues about the state of the labor market. Economists in a Reuters survey forecast 100,000 jobs were created last month versus 93,000 new jobs in November.

The market could turn around if you saw good (jobs) numbers, said Rick Meckler, president of investment firm LibertyView Capital Management in New York.

People are looking for a floor in commodities. That also could provide some support if (prices) turn around, he said.

You'll see increased volatility, but the trend is moving higher.

The Reuters/Jefferies CRB commodities index <.CRB> posted its largest daily percentage drop since mid-November on Tuesday, after hitting a 26-month high on Monday.

Global food prices rose for a sixth straight month in December, with the United Nations reporting that a key food price index hit a nominal record high.

S&P 500 futures fell 8.8 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures dropped 68 points and Nasdaq 100 futures declined 12.5 points.

At 10 a.m., the December ISM non-manufacturing index is expected to show the services sector continued to grow last month, adding to recent evidence that the economy is on the mend. Economists forecast a reading of 55.6 versus 55.0 in November.

European stocks <.FTEU3> were down 1 percent as this week's sharp commodities retreat prompted investors to take a breather after an almost 2 percent rise in stocks in the first two sessions of the year. <.EU>

Portugal cleared its first funding hurdle of 2011 on Wednesday, keeping short-term borrowing costs within expected limits, but investors remained uneasy over euro zone debt.

On Tuesday, investors abandoned commodity shares while fears of lower supermarket profits hit food retailers, sending the S&P 500 and Nasdaq lower.

(Reporting by Rodrigo Campos; editing by Jeffrey Benkoe)