这会成为一个问题，一个简单的现实原因:主导农业政策的群体，正在远离农村环境和粮食生产劳动。

Several years ago, China became the biggest soybean and corn importer in the world.
The main exporters are from south America.

Based on fertile soil and high efficient agriculture technology,
the price is much lower.

You may consider it as a presentation of globalization.
Since we produce many stuff in Wal-mart,
why other nations couldn't distribute their crop to us?

Don't ask me "what are we going to do if they cease to support us? "
First, I really don't the answer.
Second, I couldn't find motivation to do so.

I hope this helps.

谢谢了

You are very welcome buddy.
Have a nice night. :)