7,070,719,529.

This was the number of people on this planet as of early March, according to the U.S. Census Bureau.

By the time you read this number, however, it will be completely out of date. Hundreds of people are added to the world population every single minute.

By 2050, roughly 9 billion people will live on the planet, about 30% more than today.

And every member of this growing population needs food to survive.

So, the big question for us today is: How do we, income investors, take advantage of a growing demand for food?

We could buy food producers such as Dean Foods (NYSE: DF), but that stock yields 0%. Meat processor Hormel (NYSE: HRL) isn’t much better, yielding a scant 1.7%.

I think the most compelling opportunity comes from a little-known niche of master limited partnerships (MLPs) that yield more than 7%. These rare MLPs don’t have anything to do with gas and oil pipelines like most limited partnerships. Instead, they produce and sell a good that’s never been more important — fertilizer.

Population growth drives fertilizer demand. Every year, 75 million more people must be fed. At the same time, people in emerging nations are also eating more meat, which increases demand for grains. Today, roughly 35% of the world grain harvest is fed to animals.

In the United States, nitrogen fertilizer — the fertilizer of choice for corn — is in the sweet spot. Last year, U.S. farmers dedicated the most acreage to corn since 1937, according to the U.S. Department of Agriculture.

Drought is also playing a role in increasing demand. The recent drought in the United States was the worst since 1956. Its effect on U.S. fertilizer demand this year is generally seen as positive. Analysts note that corn prices are extremely high and expected to remain elevated through 2013, thanks to the drought. As a result, fertilizer use will be high.

Not all analysts are optimistic. Some argue that because a significant number of corn farmers earned lower income in 2012 and their fields are still parched, farmers will restrict 2013 planting and buy less fertilizer.

But nitrogen fertilizer companies themselves — such as Rentech Nitrogen Partners (NYSE: RNF) — are bullish. Rentech’s CEO says he “expects the drought to have a favorable impact on nitrogen prices and demand…”

Risks to Consider: One more note for when you’re looking into MLPs like Rentech Nitrogen Partners… Because these partnerships can throw off unrelated business taxable income, which is taxable even in a tax-sheltered account, the units are best held outside a tax-deferred account.

Action to Take — > The partnership‘s location in Illinois, the heart of corn country, also provides a key strategic advantage. Trucks can load directly at the Rentech facility and apply the fertilizer at the critical time in the growing cycle. Combine that with a solid yield of 7.3%, and you start to understand why the shares have soared — doubling since first trading in November 2011.

In the long term, the partnership is poised to benefit from increasing demand for nitrogen fertilizer. And income investors looking to profit from global population growth should give the shares serious consideration.

P.S. — Stocks like Rentech are perfect for investors who are near or at retirement age and are seeking a second income. We call these “Retirement Savings Stocks”… they can hand you a “second income” as much as 14 times what you can get with CDs, seven times higher than bonds, and as much as three times higher than brand-name Dow stocks. To learn more about them, go here.