The US government gives out $25 billion a year in agricultural subsidies through a farm bill that is renewed every five years including this one. Way back in the dawn of time, the original intent of these subsidies was to ensure that farmers could get at least a minimum price for their crops so that they could afford to keep farming. Over time as people realized that they could get more subsidies for growing more, many of the farms got bought out and became part of agri-business conglomerates that today collect the bulk of those subsidies. The money is primarily paid out to support the price of corn, soy, wheat, rice and cotton with the first two being biggest recipients.
The increased use of those first two crops in processed foods is thought to play a large part in the increased rates of obesity among Americans. Those same two crops are also the most common and probably least efficient feedstocks for biofuels. One the reasons so much money is paid out in agricultural subsidies is that the big companies which benefit the most from them spend a good chunk of it lobbying and contributing back to the campaigns of the legislators that push the bill through congress. As a result, big corn and soy producers have an incentive to push corn ethanol and soy diesel production because they get more subsidies.
Ultimately the health and wealth of Americans would probably benefit more if more of that money was turned over to basic research into areas like algae diesel, cellulosic ethanol, and batteries and also to supporting smaller farmers growing fresh produce and products for local distribution.
[Source: New York Times Magazine]