ISO 15958:2019

Overview

ISO 15958:2019 specifies a standardized laboratory method for the extraction of water-soluble phosphorus from fertilizers. The method is applicable to all fertilizer types, including compound fertilizers, where water-soluble phosphorus content is to be determined. The principle is simple: extraction in water by shaking under defined conditions to produce a filtrate suitable for subsequent phosphorus determination.

Key Topics

• Scope and applicability: Method applies to all fertilizers where water-soluble phosphorus is required.
• Principle: Extraction in water by shaking under specified conditions.
• Sampling and sample preparation: Sampling is not part of the method; recommended sampling and preparation are given by EN 1482 series. Grinding of the laboratory sample is recommended to improve homogeneity.
• Reagents: Distilled or demineralized water is specified as the extraction reagent.
• Apparatus: Key equipment includes:
  • 500 ml graduated flask (e.g., Stohmann)
  • Rotary shaker operating at 35–40 turns per minute
  • Dry pleated, phosphate-free filter
• Procedure (summary):
  1. Weigh 5 g of laboratory sample to the nearest 0.001 g and place in a 500 ml graduated flask.
  2. Add 450 ml of distilled or demineralized water at 20 °C to 25 °C.
  3. Shake on the rotary shaker (35–40 turns/min) for 30 minutes.
  4. Make up to the mark with water, mix, and filter through a dry pleated phosphate-free filter.
• Test reporting: The standard lists required report elements (see below).

Applications

The ISO 15958:2019 method is intended for practical laboratory use where consistent, reproducible extraction of water-soluble phosphorus is required. Typical applications include:

• Quality control in fertilizer production to verify declared nutrient content.
• Regulatory compliance and reporting where water-soluble phosphorus must be quantified for product registration or market surveillance.
• Comparative testing and inter-laboratory studies due to the method’s standardized procedure.

This standardized extraction is intended to produce a representative aqueous extract suitable for subsequent phosphorus determination by whichever analytical technique the laboratory uses.

Related Standards

Key normative and related references cited in ISO 15958:2019:

• EN 1482-1 - Fertilizers and liming materials - Sampling (recommended sampling method)
• EN 1482-2 - Fertilizers and liming materials - Sample preparation
• EN 12944-1 / EN 12944-2 - Vocabulary for fertilizers and related terms
• Regulation (EC) No 2003/2003 - Fertilisers (referenced method in Annex IV)
• Historical: ISO 15958:2019 cancels and replaces ISO 5316:1977 (technically revised)

Test Report Essentials

The test report shall include at least:

• Identification of the sample
• Reference to the method (ISO 15958:2019)
• Test results obtained
• Dates of sampling and analysis (if known)
• Statement on fulfillment of any repeatability requirement
• Details of any optional operating parameters or incidents that could affect results

This concise, prescriptive procedure helps laboratories achieve reproducible extraction of water-soluble phosphorus and supports consistent downstream analytical determination and compliance activities.