A few things change when you're using fixtures with South.
Much like syncdb, South will load the initial_data fixture when an app has been successfully migrated to the latest migration for an app. Note that the data in the fixture will not be available before then; South only applies it at the end, as it may not match the current database schema.
Fixtures from migrations
If you need to load a fixture as part of your database setup - say, you have a migration that depends on it being around - the best thing to do is to write a new migration to load the fixture in. That way, the fixture will always be loaded at the correct time.
To make such a migration, first make a blank migration:
./manage.py datamigration appname load_myfixture
Then, open the new migration file, and restructure your forwards() method so it looks like this:
def forwards(self, orm): from django.core.management import call_command call_command("loaddata", "my_fixture.json")
(you'll have to leave backwards() empty, as there's not much you can do to reverse this).
Then, when this migration is run, it will load the given fixture.