Capital (Money) is hard to comeby unless you inherit it. When prices are low (largely because of subsidies) a farming operation operates at a loss. It takes more capital to farm than the farm pays out in return.

Hence you get more subsidies. Prices get lower as more subsidized grain is dumped on the market, and the death spiral continues...

Add to this the fact that the farmer factors the subsidies into his business plan:
The more land he farms the more subsidies he gets thus he wants to aquire as much land as possible. the subsidies give him the extra insentive and extra capital he needs to aquire more farmable acres. This drives the price of land higher (making it more capital intensive to operate a farm) while at the same time it drives the price of commodities lower (Making it less profitable to farm).

Eventually the farming bubble will burst (just like the internet bubble) and millions of farmers will go broke. The key to watch is the price of farmland. When that goes south, the capital will dry-up and the marginal and highly indebted farmers will go bankrupt. Bush knows this. The Democrats know this. The dance in Washington is kind of like musical chairs: Who's gonna be stuck with a farming crisis when the music stops?

In summary, the farm subsidies only delay the enevitable and ultimatly will make the enevitable worse when it occurs.