"The commodities bull market has further to run," says Martin Hutchinson. In Money Morning, he looks to four ETFs to play gold, silver, and base metals as well as coal.

"As long as the world's central banks continue to interest rates at these very low levels, speculative interest in commodities will be strong. And since only minor central banks yet show signs of moving rates, the commodities bull market has further to run.

"There are three reasons why commodity prices have been rising, and they’re all still true:

• China and India continue their torrid growth.
• Global stimulus plans are bullish for commodity prices
• And hedge funds and other speculative investors are big commodities players.

"There are a number of ways to play a commodities bubble. It’s probably smart not to restrict your buying to gold and oil alone, but to spread yourself among a number of sectors. Let’s take a look at some of the better plays right now available.

"Powershares DB Base Metals ETF (NYSE: DBB) tracks the Deutsche Bank AG base metals index, thereby allowing you to invest directly in the price movements of non-precious metals. It is reasonably liquid and money has been flowing into it recently.

"iShares Silver Trust (Amex: SLV) invests directly in silver bullion, which has been left behind somewhat in its relationship to gold’s price rise – and which can be expected to move up as gold does, possibly by an even greater percentage.

"Market Vectors Gold Miners ETF (NYSE: GDX) holds gold minin stocks; gold miners benefit disproportionately from a rise in the price of gold because their production costs are fixed.

"This means that miners are a more leveraged way to play gold than the metal itself, particularly as surging speculative demand can increase mining companies’ P/E ratios.

"Market Vectors Coal ETF (NYSE: KOL): China’s power supply is still coal-fired, and demand is soaring, hence global coal prices are likely to be pulled upwards by Chinese demand alone. KOL has a market capitalization of $283 million."