ISO 15959:2016

Overview

ISO 15959:2016 - Fertilizers - Determination of extracted phosphorus - defines a standardized wet‑chemical procedure for measuring phosphorus in fertilizer extracts. The method covers multiple extract types including phosphorus soluble in mineral acids, water‑soluble phosphorus, phosphorus soluble in ammonium citrate, 2 % citric acid, and 2 % formic acid. The principle is precipitation of phosphorus as quinoline phosphomolybdate, followed by drying (250 ± 10 °C) and gravimetric weighing to calculate phosphorus content.

Key Topics and Requirements

• Scope: Applies to all common fertilizer extracts for different phosphorus fractions (acid‑soluble, water‑soluble, citrate‑soluble, citric/formic acid‑soluble).
• Analytical principle: Acidify aliquot (nitric acid), hydrolyse polyphosphates if present (reflux/boil, typically ~1 h), precipitate with a molybdate/quinoline reagent, filter, wash, dry at 250 °C and weigh the precipitate.
• Reagents & apparatus: Uses sodium or ammonium molybdate‑based precipitating reagents prepared with quinoline and nitric/citric acid; standard lab glassware, sintered funnels or filter crucibles (5–20 µm porosity) and an oven regulated to 250 °C.
• Quality control: Includes blank tests, verification against a potassium dihydrogen phosphate standard, and guidance on sample aliquots/dilution factors.
• Calculations: Mass fraction of P (or P2O5) is calculated from the corrected mass of quinoline phosphomolybdate using provided conversion factors or custom dilution/portion formulas.
• Precision & validation: Interlaboratory testing (2008) supports method precision with repeatability and reproducibility limits calculated per ISO 5725‑2.

Applications

• Quality control and production: Routine testing of fertilizer products to quantify available and extractable phosphorus fractions for batch release and formulation control.
• Regulatory compliance and labeling: Generating standardized phosphorus results for declarations and conformity with fertilizer regulations.
• Laboratory accreditation & method harmonization: Used by chemical testing laboratories to demonstrate adherence to an international, validated procedure.
• Research and agronomy: Comparative studies on phosphorus availability, solubility and fertilizer performance in product development or soil amendment research.

Who uses this standard?

• Fertilizer manufacturers and QA/QC laboratories
• Accredited testing labs and conformity assessment bodies
• Regulatory agencies and inspection laboratories
• Agricultural researchers and agronomists

Related Standards (applicable)

• ISO 14820‑2 - Sampling and sample preparation for fertilizers
• EN 12944‑1 / EN 12944‑2 - Vocabulary related to fertilizers and soil improvers
• EN/ISO extraction standards referenced for specific extract procedures (EN 15956, EN 15957, EN 15958)

Keywords: ISO 15959:2016, fertilizers, determination of extracted phosphorus, phosphorus analysis, quinoline phosphomolybdate, fertilizer extracts, analytical method, precision, repeatability, reproducibility.