The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage. By Kevin McElroy NEW YORK ( TheStreet) - One of my favorite ways for regular investors to invest in the agriculture commodity boom is to buy the Market Vectors Agriculture ETF ( MOO). This fund is liquid, meaning that anyone can easily buy shares at or near the prevailing ask price. With average volume of nearly 2 million units traded daily, you won't ever have a problem buying or selling a large block of shares. This type of liquidity can be a problem for some stocks and ETFs. More on liquidity issues in a minute... On Aug. 6, 2010, I recommended this ETF to readers of Global Commodity Investing , a research service I help write and edit. Since then it's returned a respectable 26%, which is about twice as good as the return you would have received from just buying a broad market index fund like the SPDR S&P 500 ETF ( SPY).