A judgment for alimony, post separation support or child support shall not be a lien on real property unless the judgment expressly so provides, sets out the amount of the lien in a sum certain and adequately describes the affected real property. G.S. 50-16.7(i) and G.S. 50-13.4(f)(8). This rule applies to future payments. However, past due periodic payments may, by motion in the cause or by separate action, be reduced to a judgment with effect and priority of a general judgment lien, including being a lien on real property. Id. For additional reading, see E. Urban and G. Whitney, North Carolina Real Estate, Secs. 21-67 and 21-69 (Harrison Co. 1996).