A free market is one in which prices of goods and services are arranged completely by the mutual consent of sellers and buyers. In a free market economy, individuals, rather than governments, make the majority of decisions regarding economic decisions and transactions (Encarta), i. e. free competition (Websters'). Almost a lost concept.

International markets, however, are always controlled to some extent, by agreement between the exporting and importing countries, and moreover by international trade regulations (WTO talks presently stalled).

My statements do not accuse the United States alone; all major producing and trading nations share in the current situation.
Other countries provide subsidies as well. The EU countries Germany and France, Australia-New Zealand, and Brazil etc., all extend agricultural subsidies.

Subsidies enable a corn (or whatever crop) growing country to sell at a price lower than the world price, thus displacing growers that cannot compete at the low price. Subsidized farmers have the advantage in the market.

The U. S. has good crop land, so do a number of other countries playing the same game- Argentina-Brazil, Australia, China, India, etc.
A good article, or slide show, is "Foreign Crop Subsidies and Tariffs," (Texas Tech, Texas A&M, Missouri and Iowa State Univ. and others) which deals with cotton primarily but has a good summary of practices by various countries. Crops included are corn, sorghum, rice, cotton, soybeans, sugar and wheat. Twenty-one countries are included.
The developed countries protect their agriculture by income supplements and subsidies. Developing countries use tariffs or restrict imports.
Other countries have substantial outlays to assist agriculture (2007-2008 figures)-
India $52 billion
China $43 billion
Brazil $26 billion
Pakistan $2.7 billion

Nigeria, mentioned by pdq, has the highest tariffs on agricultural imports, followed by India, Pakistan and Egypt (The U. S. does not keep Nigeria "healthy and fed." They are a major supplier of oil to the U. S., in spite of occasional disruptions, and it is not one of the starving countries).

The article-slide show absolves the developed countries, like the U. S., of blame, a conclusion which I think is flawed. The effect of the subsidies hurts specific crop growers in the 21 countries considered, but more importantly the poorest nations are not considered at all.

Guide to foreign crop subsidies and tariffs, a power point presentation at the Texas Tech. website.
www.aaec.ttu.edu, and click on the link to download "Cotton economics research ..." or
http://www.aaec.ttu.edu/ceri/policy/Crop%20Subsidies%20Handbook/T&S_LayoutFinal.pdf

I know it makes work in getting the websites, but if I make the link, then one cannot decide whether they want to visit that site or not.

(Hemp growing was made illegal in the U. S. in 1938. Canada recently allowed production of industrial hemp, which just makes a bad smoke).