This document specifies two complementary methods (method A and method B) that allow lignosulfonates to be indentified as soluble complexing agents in fertilizers.NOTE Lignosulfonate, as a complexing agent, is a natural polymer produced as a by-product of the sulfite method for manufacturing paper from wood pulp in the paper industry. As a natural polymer, it presents a poorly defined and variable chemical structure. It is an intricate mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule, and diverse complexing capacity. The methods are applicable to EC fertilizers covered by Regulation (EC) No 2003/2003 [1]