How has government policy shaped corn production and, thus, in part, what we eat?

Government policy has shaped corn production at every stage since the end of the Second World War. As Pollan points out, in 1947 the American government had a surplus of ammonium nitrate, the main ingredient for making explosives. It also happens to be an excellent source of nitrogen for plants. So the government, seeing an opportunity to boost food production, instructed a munitions plant in Alabama to make chemical fertilizer from bomb material. Thus modern-day fertilizer was born, which facilitated the exponential growth of corn and other crops.
Soon, farmers realized they could grow vast quantities of corn without exhausting the soil. In turn, this led to a huge glut of corn on the market, which kept food prices low. Ever since the war, successive governments have given massive subsidies to farmers growing corn in order to keep food prices at a sustainably low level. To date, every single bushel of corn enjoys a 50-cent subsidy from the US government.
As corn is cheaper than sugar, high fructose corn syrup has increasingly replaced sugar as a sweetener, to the extent that something like a quarter of all the products sold in an average American supermarket contain corn.