Disclosed are compositions and methods of making a liquid fertilizer additive of biodegradable polymeric and/or oligomeric nitrification inhibitors comprised of utilizing a non-aqueous polar, aprotic organo liquid (NAPAOL) as the reaction medium for the reaction of aldehyde(s) with cyano-containing nitrification inhibitors that have one or more aldehyde reactive groups selected from the group consisting of a) primary, b) secondary amines, c) amides, d) thiols, e) hydroxyls and f) phenols.
