Falling commodity prices aren't bringing prices for farmland down with them. Even as the price of grain goes down, the cost of the land it's grown on keeps going up, leading George Soros and other guru investors to bet big on agricultural land.

The fundamentals are easy to understand: Over the next 40 years the population of the world is projected to grow from 6 billion to 9 billion, hugely increasing the strain on arable farmland worldwide.

The spiking grain prices that caused food shortages and rioting in dozens of countries in spring of 2008 fell some 50 percent by December. Yet even after the correction, grain prices remain above their 20-year average, and food stocks around the world are still near 40-year lows.

"Land is scarce and will become scarcer as the world has to double food output to satisfy increased demand by 2050," Joachim von Braun, director general at the International Food Policy Research Institute, told Fortune Magazine.

"With limited land and water resources, this will automatically lead to increased valuations of productive land. And it goes hand in hand with water. Water scarcity will probably increase even more than land."

"I'm convinced that farmland is going to be one of the best investments of our time," says commodities guru Jim Rogers.

Eventually, Rogers notes, food prices will rise enough that the market probably will be flooded with supply through development of new land or technology or both, and the bull market will end.

“But that's a long ways away yet," Rogers says.