ISO 21263:2017

Overview

ISO 21263:2017 - Slow-release fertilizers - Determination of the release of the nutrients - Method for coated fertilizers - defines a laboratory method to measure the slow-release properties of coated fertilizers. The standard covers physical (non-biological) release mechanisms only; pH-dependent hydrolysis and biological or microbial degradation are explicitly excluded. The method is a direct elution test in water (25 ± 0.5 °C) and is intended as a reference method for longevity, initial release and cumulative nutrient release from coated fertilizer products.

Key topics and requirements

• Scope and exclusions: Applicable only to fertilizers that release nutrients by a physical mechanism; microbial attack on coatings (e.g., sulfur coated products) and its effects are not measurable with this method.
• Test principle: Elution of a defined test portion (typically 10 g in 500 ml water) with controlled stirring and temperature; periodic sampling and analytical determination of dissolved nutrient concentration.
• Sampling and sample handling: Follow EN 1482 for sampling; avoid damaging coatings (do not crush or grind test sample unless for total water-extractable determination).
• Timing and intervals: First measurement at 24 h, another at 28 days, with about ten determinations up to the stated release time (longevity); examples given for multi-month release profiles.
• Expression of results: Initial release (percent of total water-extractable nutrient), released mass fraction over n days, and interpolated longevity (stated release time).
• Release criteria (Annex A):
  • ≤ 15% mass fraction released within 24 h
  • ≤ 75% released within 28 days
  • ≥ 75% released at the stated release time (longevity)
• Accelerated methods (Annex B): Elevated-temperature accelerated tests are allowed if correlated with the standard method (consider solvent/coating stability and compound solubility differences).
• Precision: Precision established by interlaboratory trials; repeatability and reproducibility limits are provided to support quality control and method validation.

Applications and who uses it

ISO 21263:2017 is used by:

• Fertilizer manufacturers for product development, quality control and labeling of slow-release or controlled-release coated fertilizers.
• Analytical laboratories performing release testing and validation of accelerated test correlations.
• Regulators and certification bodies assessing claims of product longevity and release performance.
• Agronomists and researchers evaluating product behaviour in irrigation or controlled environments.

Practical benefits include standardized comparability of coated fertilizer performance, supporting product claims (longevity, initial release) and enabling correlation of faster screening methods to a recognized reference.

Related standards

• EN 1482 - Sampling of solid fertilizers and liming materials (referenced for sampling procedures)
• ISO 3696 - Water for analytical laboratory use (specifies water quality used in tests)

Keywords: ISO 21263:2017, slow-release fertilizers, coated fertilizers, release of nutrients, longevity, initial release, accelerated methods, Annex B, fertilizer testing, release criteria.