China has a minimum purchase price of corn that's set by the government in order to maintain food stocks. It's also part of a larger jobs program (that I don't know much about).

China also imports 80% of its soybeans which means it's based on the rising/falling prices of oil and whatnot.

In the US, soybeans are a very important crop that's fed to livestock and also used in biodiesel production. There's enormous soybean "crush" infrastructure in the US to support the biodiesel market and the side effect of this results in tons of extra soybean oil. It ultimately ends up with soybean oil being cheap compared to everything else.