The idea that there is a clear distinction between typical factors of
production and land (and other things "fixed in supply") is not
uncontroversial. An interesting article examining the issue is:

Florenz Plassmann and T. Nicolaus Tideman
Frank Knight's Proposal to End Distinctions among Factors of Production and
His Objection to the Single Tax
History of Political Economy, FALL 2004; 36: 505 - 519. 

Jeff Biddle