At $1,390 an ounce, gold prices are at an all time. Silver is at a 30-year high. Agricultural commodities like fertilizer are also moving toward new highs.

Economists are warning that these commodities could continue to move higher in price for several months. And the stocks of companies selling these commodities have been surging in the past few trading sessions.

Okay, it's not news that Jim Rogers likes agriculture. He's taken every opportunity to tell anyone who will listen that he likes farmland, corn, wheat, rice - and just about every other agriculture commodity.

But unless you have a margin account on a commodity futures exchange, you probably can't follow his advice to a T.

In other words, if Jim shows up on CNBC and says he likes rice (as he did on August 28^th - you can watch the video by clicking here) it's frustrating to sit on the sidelines if you don't have a futures account.

And I'm not recommending that you should open a futures account and simply follow Mr. Roger's advice every time he's on TV. Even if you have a futures account, you know that it's almost a full-time job keeping track of the complicated options and futures contract strategies that would be considered entry-level for most traders.

But I do think your portfolio should have some exposure to agriculture, for reasons so simple that a kindergartner can understand them:

The biggest reason is population growth. According to a 2004 study from the United Nations, "World population is projected to grow from 6.1 billion in 2000 to 8.9 billion in 2050, increasing therefore by 47 per cent."

Last week fertilizer stocks all surged in unison after the world's largest mining company BHP Billiton (NYSE: BHP) tried, and failed, to acquire the world's largest potassium company Potash Corp. (NYSE: POT).

Right now, there's only one company in the sector that's still (relatively) inexpensive. More on that company in a minute…

First, a little tooting of my own horn: just over a month ago, I recommended you pick up shares of another fertilizer company (now the second largest) -Mosaic Co. (NYSE: MOS).

Here's what I said on July 22:

I hate falling into the trap of simply responding to the hottest news headlines - because very few people get rich by reacting to headlines and pulling the trigger on investments based on "hot" trends in the market.

At this point, computers can wipe the floor with most any day-trader, so if you think you can buy yesterday's news and still eke out a profit, you're probably wrong.

In the past couple weeks agriculture has been the hot topic on everyone's mind. First, fires in Russia caused wheat prices to double in less than a month as Vladimir Putin banned Russian wheat exports. In sympathy, many other crop commodities rose in price as well. Then yesterday, BHP Billiton (NYSE: BHP) the world's largest mining company, put in a failed bid to buy Potash Corp (NYSE: POT) the world's largest fertilizer company.

Does your portfolio have exposure to the most inevitable bull market in the world? I’m talking about a long-term trend that has nowhere to go but up. It’s literally a life and death situation.

The alternative to a bull market in these commodities is something the world’s population has to strive to avoid at all costs. It’s more important than oil, more vital than coal, and way more serious than gold, copper, iron - or any other industrial metal or energy commodity.

Because if there’s not more production of this particular group of commodities every day, in perpetuity, most people on this planet will starve.

Yes, I’m talking about food. And I realize it’s not an “exciting” investment, not like some new oil discovery or a massive gold bonanza. But the world’s population is growing.