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).