Overview
ISO 14255:1998 specifies a laboratory method for measuring soluble nitrogen fractions in air‑dry soils using a calcium chloride extractant. The standard covers extraction and automated analysis of nitrate (NO3 −, including nitrite when relevant), ammonium (NH4 +) and total soluble nitrogen in a 0.01 mol/L calcium chloride (CaCl2) soil extract. Soluble organic nitrogen is obtained by difference (total soluble N minus inorganic N). The method is intended for air‑dry samples pretreated according to ISO 11464.
Key topics and technical requirements
- Sample type and pretreatment: Air‑dry soil, sieved to 2 mm and water content determined per ISO 11465.
- Extraction: 1:10 mass/volume ratio (typically 10.00 g soil + 100.0 mL of 0.01 mol/L CaCl2), shaken at 20 °C ±1 °C for 2 hours, then centrifuged (≈3 000 g for 10 min). Extracts should be analyzed immediately or stored ≤4 °C for up to one week.
- Analytical techniques: Automated segmented flow analysis (SFA) is specified for best reproducibility.
- Nitrate (and nitrite): dialysis, cadmium reduction to nitrite, Griess–Ilosvay colour development, measurement at 543 nm.
- Ammonium: dialysis and Berthelot (salicylate) reaction with measurement at 660 nm.
- Total soluble N: persulfate digestion (oxidation, UV‑catalysed), conversion to nitrate, then nitrate determination as above.
- Quality control and calibration: Use calibration series prepared in CaCl2, run blanks, and apply the standard calculation to express results as mg N/kg soil. Example calculation:
wN = (a − b) · 10 · (100 + w)/100
where a = extract concentration (mg/L), b = blank (mg/L), w = sample water content (%).
- Normative references: ISO 11464 (sample pretreatment), ISO 11465 (water/dry matter), ISO 3696 (laboratory water quality).
Applications and users
ISO 14255 is used by:
- Environmental and agricultural analytical laboratories
- Soil scientists and agronomists performing nutrient and fertility assessments
- Research groups studying nitrogen cycling, mineralization and soil N availability
- Regulatory and monitoring programs assessing nitrate/ammonium mobility and soluble nitrogen loads
Practical applications include fertilizer management planning, baseline soil surveys, environmental monitoring for nitrate pollution, and research on soluble organic nitrogen pools.
Related standards
- ISO 11464 - Soil quality - Pretreatment of samples for physico‑chemical analyses
- ISO 11465 - Determination of dry matter and water content (gravimetric)
- ISO 3696 - Water for analytical laboratory use
Keywords: ISO 14255, soil quality, nitrate nitrogen, ammonium nitrogen, total soluble nitrogen, calcium chloride extraction, segmented flow analysis, soil testing, laboratory method.