News and magazine sources provide information about current events and social issues, which can help inform academic arguments you are making in your papers. These sources can also be used in a historical context, as well. Also remember that information isn't static; news changes as more information is gleaned.
I ran a simple search in the U.S. Major Dailies database for california AND farmwork to see what kinds of newspaper articles I might find.
Although not solely about farmwork, my search led me to doing some reading about the 1986 Amnesty. This 1987 New York Times article, "Tense Times for Aliens as U.S. Clamps Down," provides some brief background about the law and the second phase of implementation, which relates to employers being fined for hiring undocumented immigrants.