China has already stopped importing large amounts of US agricultural products, including soybeans. The US gov is now subsidizing farmers to make up for this loss to the tune of $28 bil in direct payments to US farmers so far...
I don't really understand why people think most large nations can't feed their own people. China can easily do so, albeit with some changes to their diet. China is the top producers in the world of many crops: wheat, rice, potatoes, grapes, apples, tomatoes, watermelons, hogs, sheep, etc. and #2 or #3 in many others: corn, bananas, beef, chicken, milk, etc. Sure they import a huge amount of food as well. But so does the USA.