This Statesman article talks about the migration of people living in rural areas to the larger cities. It covers this issue on a larger, worldwide scale (mentions legal and illegal migrations, cross-border migrations, etc…) as well as in China. In the fourth paragraph it talks about the migration specifically in China. It also talks about how the migration affects the farming industry.
The full article is here.
This Xinhua article covers a similar subject and talks about how the government is working to help the rural workers that are being treated unfairly.