i think you might actually be better off investing in the agriculture commodities themselves. and maybe if the stock market has a big correction, you could pick up some cheap shares in agriculture stocks also.... or maybe get a bit of both.
i know there are agriculture ETFs like DBA ... but if it were me, i might spend the extra money and put my money in one of Jim Rogers funds because i trust Rogers and I think that he knows what's going on and i would have more confidence having my money with him.