One website says the song is "This Land Is Your Land." Guthrie wrote it in 1940, but didn't copyright it until 1956. Copyright held by Richmond Group.
He borrowed the music from a Carter gospel song, "When the World's on Fire."
I dunno if the information above is correct.

Be that as it may, the Library of Congress says for copyright information on Woody Guthrie songs, contact:
Woody Guthrie Foundation and Archives
250 West 57th Street
Suite 1218
New York NY 10107
Tel. 212-541-6230
email: woodyweb@woodyguthrie.org

The Library website:
http://memory.loc.gov/ammem/wwghtm/wwgres.html