ISO 11263:1994

Overview

ISO 11263:1994 specifies a laboratory method for determining the content of soil phosphorus that is soluble in a sodium hydrogen carbonate (sodium bicarbonate) solution. The standard defines an extraction procedure and spectrometric analytical conditions applicable to all soil types, and provides two colour‑development options - at room temperature and after heating - for the formation and measurement of phosphate–molybdate complexes reduced by ascorbic acid. ISO 11263 is intended for routine soil quality testing where “available” phosphate measurements are required.

Key topics and technical requirements

• Extraction reagent and conditions: uses a 0.5 mol·L–1 sodium hydrogen carbonate solution adjusted to pH 8.50 ± 0.02 for extracting soil phosphate; reagents must be analytical grade and water should meet ISO 3696 quality.
• Sample preparation: pretreated soils per ISO 11464; typical extraction example in the standard uses 5.00 g (dry weight) of soil with 100 mL extraction solution, shaken for 30 minutes at 20 °C ± 1 °C, then filtered to obtain a clear extract.
• Colour development: two options are specified
  • Room-temperature procedure forming an antimony–phosphate–molybdate complex reduced by ascorbic acid; colour develops at ambient temperature and is measured after a defined wait time.
  • Heated procedure forming a phosphate–molybdate complex with controlled heating (e.g., 90 °C for 10 minutes) before measurement.
• Spectrometric measurement: absorbance measurement in 10 mm cuvettes - typical wavelengths indicated in the standard are 880 nm for ambient-colour procedures and 825 nm for heated procedures. Measurements must be made in the linear range of the calibration curve within specified timing.
• Quality controls and repeatability: the standard defines calibration, blank procedures, and repeatability criteria (tolerance bands for different phosphorus concentration ranges). Detailed reagent formulations and preparation steps are provided.

Applications and users

• Who uses ISO 11263: soil testing laboratories, agronomy and crop‑management consultants, environmental monitoring agencies, fertilizer manufacturers, and research institutions studying soil phosphorus availability.
• Practical value: provides a reproducible method to assess sodium bicarbonate‑extractable (plant‑available) phosphorus for fertilizer recommendations, soil fertility mapping, agronomic trials, and environmental assessments of P mobility.

Related standards

• ISO 11464 - Soil sample pre‑treatment for physico‑chemical analysis (referenced by ISO 11263)
• ISO 11465 - Determination of dry matter content of soil samples
• ISO 3696 - Water for laboratory use - Specifications and test methods

Keywords: ISO 11263, soil phosphorus, sodium hydrogen carbonate extraction, spectrometric determination, soil quality standard, phosphate analysis, soil testing, laboratory method.