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.
