ISO 21263 Slow-release fertilizers — Determination of the release of the nutrients — Method for coated fertilizers

ISO 21263:2017 specifies a method for determining the slow-release properties of the nutrients from coated fertilizers. It was published on 15 December 2017 and is the analytical counterpart to the product requirements in ISO 18644.

The method is deliberately restricted in scope. pH-dependent hydrolysis and degradation by biological or microbial mechanisms are excluded, so the standard applies only to products that release nutrients by a non-biological, physical mechanism. Attack on the coating by soil microorganisms — including the case of sulfur-coated fertilizers — is explicitly not measurable by the technique described.

The standard acknowledges its own practical limitation: it involves a lengthy process that may not be appropriate for routine day-to-day testing. Accelerated methods are permitted provided they are correlated with this document, and an example of such a method is given in Annex B. Regression analysis may be used for the same purpose.

That structure reflects the commercial reality of the category. Labelling a product as releasing over 90 or 120 days is a claim about soil behaviour over a full season, and only a reference method performed at real time can establish it. Routine production control then runs on the accelerated method, with the correlation to the reference method as the link between the two. Without a shared reference, a release claim is untestable by the buyer and therefore worth little in a contract.