Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some agriculture stocks to your portfolio but don't have the time or expertise to hand-pick a few, the PowerShares Global Agriculture ETF (NYSEMKT: PAGG) could save you a lot of trouble. Instead of trying to figure out which stocks will perform best, you can use this ETF to invest in lots of agriculture stocks simultaneously.

On your own, you might not have selected PotashCorp (NYSE:POT) or Monsanto Company (NYSE:MON) as agriculture stocks for your portfolio, but this ETF counts them among its nearly 50 holdings. The ETF has lagged the world market over the past five years, but if you believe in long-term agriculture demand, then its future seems promising.

The ETF's basics
ETFs often sport lower expense ratios than their mutual fund cousins. The PowerShares Global Agriculture ETF, focused on agriculture stocks, sports an expense ratio -- an annual fee -- of 0.76%. The fund is fairly small, too, so if you're thinking of buying, beware of possibly large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

A closer look at PotashCorp
Fertilizer giant PotashCorp sports a market cap near $30 billion. Its stock is yielding 3.8%, and its dividend has quintupled over the past three years. The company has struggled a bit in recent years, dealing with an oversupply in the market as well as the breakup of a key Russian potash cartel (now seemingly repaired). In its last quarter, it posted estimate-topping earnings, but they were still nearly 40% lower than year-ago levels. Falling prices for potash hurt the company, though nitrogen and phosphate prices rose a bit.

Management blamed tough winter conditions in North America and transportation slowdowns, among other things. With investors having high hopes for increased demand from China and India, management noted that signs are promising, but demand will likely grow gradually in the near term, not explosively.

Bulls like PotashCorp's wide moat, as it's in a difficult industry to enter. They also like the company's share repurchases. Bears worry about disruptions due to the conflict in the Ukraine, though opinions are divided on the potential impact of that. A drop in Russian potash exports, for example, could reduce supply and boost prices.

A closer look at Monsanto
Agriculture and seed giant Monsanto is about twice as big as PotashCorp, at least in the eyes of the market, as its market cap is around $60 billion. The company is often in the midst of controversy over herbicides, genetically modified organisms (GMOs), and more. Indeed, Russia recently announced that it won't import GMO products. It's not alone, and the list of such companies is likely to keep growing.

Still, Monsanto's last quarter featured impressive numbers, as it topped analyst expectations with revenue up nearly 7% and profit margins rising, too. Bulls like its growing soybean business, though corn is still a much bigger revenue-generator. Its diversification across several crops, as well as its pest-fighting products and investments in biologicals/microbials and precision agriculture, are also appealing. The company is continually innovating, and it sports a rich pipeline. Monsanto stock yields 1.5%.

The big picture
If you're interested in adding some agriculture stocks to your portfolio, consider doing so via an ETF. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Longtime Fool specialist Selena Maranjian, whom you can follow on Twitter, has no position in any stocks mentioned. The Motley Fool owns shares of PotashCorp. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.