So seeing I bothered to get a BSc in Agricultural Economics I may as well use it for something, goodness knows I have never been an agrologist!
So let us start with some basics. Observable facts already mentioned. Farmgate prices have been falling in real terms or inflation adjusted terms or however you care to measure it. What this means is that for the consumer food has been becoming cheaper and cheaper as the food surplus grows and as incomes have increased faster than food prices. Of course, this means comparing likes to like. That means carrots, peas, corn, potatoes, meat, etc. and not canned goods, processed foods, pre-packed meals ready to eat, which contain value added. So just like you cannot compare a combine in 1955 for $10.000 to a fully loaded combine for $200.000 in 2005 you have to have a standard of measurement.
So far so good.
Farmgate prices have been falling, meanwhile inputs have been going up in price. This reflects the fact that fewer farmers can farm larger tracts of land more efficiently. The rural population has fallen from 50% to less than 5% of the overall population since the end of WWII not counting jobs in food processing. What goes for farming goes for raising chickens, pigs, cattle,etc. However, those rising input prices have squeezed a lot of farms, and not just the stereotypical family farm, but large farms as well. This reflects diminishing returns. Ever higher inputs to produce slightly larger surpluses. Eventually, you hit a wall wether it is not enough arable land, not enough irrigation, not enough rainfall or something like bird flu.
Incomes on the land are stagnating. Farmers complain. Historical electoral maps give farm votes more weight than urban votes in many cases. Plus, food security is an issue. And the family farm is like motherhood and apple pie. So governments decide instead of letting the number of farms decrease, the number of acres decrease, they will subsidize production.
If they did not subsidize then crops would only be grown on the best land (forget about urban sprawl for a minute). However, if the subsidies are tied to production, it will pull in more marginal land. Land not suitable for production. The larger the farm the larger the subsidy. By the time you make grandma & grandpa's family farm viable again, the large farmers and factory farms are making a handsome profit growing such staple American crops such as rice and sugar cane.
So subsidies increase the amount of food grown if they are tied to production (conversely you can pay farmers to take land out of production if you want to subsidize the farmer and not production). Now, America, Europe, Australia and Canada can afford to subsidize more than poor countries can. Only 5% of the people live on the land, but produce an agricultural surplus. However, this suppresses price world wide, as if we cannot consume it in the home market it ends up being sold internationally or given away as food aid (commonly known as dumping). We end up selling food cheaper than Africans, Asians and S. Americans can produce it (with exceptions - I am trying to keep this short, so I cannot go into every exception). so developing farmers cannot grow locally grown crops which would compete with our staples.
Take away production subsidies and the amount of surplus food grown goes down. This will increase food prices in other markets while barely making any impact at home unless food production actually falls below consumption. However, at the moment it is so ridiculuous that they pay farmers to grown corn and then they pay to subsidize the manufacture of ethanol or in some cases corn is so cheap you can burn it as a source of fuel. This is not sensible. A waste of taxpayer's money and it hurts third world farmers.
In western Canada the government withdrew freight rate subsidies on grain for export. A lot of farmers were very upset. However, in the past 20-years we have seen a lot of marginal farmland turned into mixed farming or ranchland. There is more beef farming. The grasslands are less susceptable to erosion. The countryside looks better. The water quality improves because you have less fertilizer run off. And the economy does better because beef and cattle farming adds value to the grain instead of shipping it somewhere else to be processed. Now with the beef ban in the US, W. Canada has also invested more in local beef processing, so again more value added stays close to home. The transition was painful, but the policy was right. Take away market distorting subsidies and let the market develop. If you need rural aid, or regional subsidies, then make them transparent, income support for example, but don't subsidize production. It is wasteful and counterproductive.
I am sorry, not enough time to write more, so I hope not too many contradictions. if so, just let me know. Thanks.
The organized state is a wonderful invention whereby everyone can live at someone else's expense.