I just came across a well-written background paper on farm subsidy policy by Troy Dumler, who is an extension economist with KSU.

"The Case For and Against Farm Programs" was published in August, 2006 but hasn't lost any relevance. You can access it here as a PDF file.

Here's Mr. Dumler's summary, and we heartily agree with his closing thought:

This paper discusses five of the most common economic justifications for farm subsidies. When analyzed in depth, those justifications are not always as valid as they may seem at first. Certainly, there are challenges facing U.S. farmers. Problems of variability of income are real to many farmers and rural communities. From the 1930s onward, the reaction of the U.S. government to these challenges has been to subsidize selected farm commodities. Those subsidies, however, often have unintended consequences that mitigate their intended purposes. Likewise, as time goes by, programs can become outdated and ineffective. So while the goal of farm subsidies may be noble, their actual effect may be limited. Therefore, the question rising out of this discussion may not be, Should we eliminate farm subsidies? Rather, the question may be, Are there farm policy options that would better serve U.S. agriculture, taxpayers, and consumers?