Overview
ISO 11265:1994 specifies an instrumental, routine method for determining the specific electrical conductivity of an aqueous extract of air-dried soil. The measurement provides an indication of the content of water-soluble electrolytes (a proxy for soil salinity) and is applicable to all types of air-dried soil samples. Results are reported as electrical conductivity (mS/m) corrected to 25 °C.
Key topics and technical requirements
- Sampling and pretreatment: Use the <2 mm fraction of air-dried soil prepared according to ISO 11464.
- Extraction: Weigh 20.00 g soil and extract with 100 ml water (extraction ratio 1:5 m/V) at 20 °C ± 1 °C. Shake (recommended ~30 min), then filter and measure the filtrate. The 1:5 ratio is chosen to obtain an extract from soils with high organic matter.
- Measurement: Use a conductivity meter with an adjustable range, automatic temperature correction and preferably cell-constant control. Measurements are corrected to 25 °C and recorded to 0.1 mS/m.
- Calibration and reagents: Use analytical-grade reagents and grade 2 water per ISO 3696. Calibrate/check cell constant with potassium chloride solutions (examples in the standard: 0.100, 0.0200, 0.0100 mol/l KCl with specified conductivities at 25 °C). Calculated cell constant shall not differ by more than 5% from the manufacturer's value.
- Precision and repeatability: The standard defines repeatability limits (e.g., accepted variation: 0–50 mS/m → ±5 mS/m; >50–200 mS/m → ±20 mS/m; >200 mS/m → ±10%). An interlaboratory trial (26 labs) is reported in Annex A to illustrate precision and reproducibility.
- Interferences and precautions: Watch for electrode contamination, air bubbles, and atmospheric CO2/NH3 (particularly affecting measurements
