ISO 15604 Fertilizers — Determination of different forms of nitrogen in the same sample, containing nitrogen as nitric, ammoniacal, urea and cyanamide nitrogen

ISO 15604:2016 specifies the determination of different forms of nitrogen present in the same sample: nitric nitrogen, ammoniacal nitrogen, urea nitrogen and cyanamide nitrogen. It was published on 1 May 2016.

The commercial importance of the method comes from the fact that nitrogen form, not merely total nitrogen content, determines how a fertilizer behaves in the field. Nitrate nitrogen is immediately available and mobile; ammoniacal nitrogen is retained by the soil exchange complex; urea nitrogen must first hydrolyse; cyanamide nitrogen behaves differently again. Two products declaring identical total nitrogen can therefore perform very differently.

Declaring total nitrogen alone is no longer sufficient in most regulated markets. Chinese labelling rules under GB 18382 distinguish nitrate nitrogen, and the EU Fertilising Products Regulation sets separate requirements according to nitrogen form. Each of those declarations requires an analytical method that can separate the forms in a single sample, which is what this standard provides.

Determining the forms separately in one sample rather than in separate runs matters for accuracy as well as efficiency: the forms interconvert, and a sample that sits between analytical steps can change composition. The method exists to fix the forms as they were at the moment of sampling.