Determination of Carbon and Nitrogen in Soil

Importance of Topic

Soil carbon and nitrogen contents are fundamental indicators of soil health and fertility. Carbon reflects organic matter levels and influences nutrient retention, microbial activity and soil structure. Nitrogen, a key component of proteins and enzymes, often limits plant growth and yield when deficient. Accurate, routine measurement of total carbon and nitrogen enables agronomists and producers to optimize fertilization strategies and manage sustainable cropping systems.

Objectives and Study Overview

This application note presents a validated procedure for simultaneous determination of total carbon and nitrogen in agricultural soils using dry combustion. The goals are to demonstrate instrument setup, sample preparation, calibration, and analysis workflow on the LECO CN828 carbon-nitrogen analyzer, while achieving ISO 13878 compliance and ensuring high precision and throughput.

Methodology and Instrumentation

Samples are air-dried at 105 °C for one hour, ground to uniform consistency, and stored in a desiccator prior to weighing. Drying corrects for moisture variability and reporting is performed on a dry-mass basis. Pre-analysis, certified reference materials are prepared per their certificates.

The LECO CN828 employs a vertical quartz furnace under pure oxygen for complete combustion. Combustion gases pass through a thermoelectric cooler and equilibration ballast before a controlled aliquot (3 cm3 or 10 cm3) is extracted. Inert carrier gas (helium or argon) transports CO2 to a non-dispersive infrared cell and N2 to a thermal conductivity detector. Helium offers superior sensitivity for nitrogen at low concentrations; argon may be used when helium is unavailable. Key parameters include a 950 °C furnace temperature, 850 °C afterburner, three purge cycles, and optimized flow rates and timing programmed via Empower software.

Instrumentation

• LECO CN828 Carbon-Nitrogen Analyzer
• Vertical quartz combustion furnace with thermoelectric cooler
• Non-dispersive infrared (NDIR) detector for CO2
• Thermal conductivity detector (TC) for N2
• Aliquot loops (3 cm3 and 10 cm3) and sample carousel

Key Results and Discussion

Typical reproducibility was evaluated using LECO Soil LCRM and NIST SRM 2709. Using helium with a 10 cm3 loop, carbon results averaged 3.75 % ± 0.01 % and nitrogen 0.320 % ± 0.002 %, demonstrating excellent precision. Lower-level reference materials (0.72 % C, 0.10 % N) yielded standard deviations under 0.01 % for carbon and under 0.003 % for nitrogen. Argon carrier with a 10 cm3 loop provided comparable results with slightly higher N variance, confirming method robustness across gases and dose volumes. Atmospheric blanks and matrix effects were effectively corrected through routine blank determination.

Benefits and Practical Applications

• High precision and accuracy aligned with ISO 13878
• Rapid throughput with minimized reagent consumption
• Flexible carrier gas options and aliquot loop selection
• Automated blank, calibration and drift correction routines
• Direct applicability to soil fertility assessment, environmental monitoring and agronomic research

Future Trends and Applications

Advances may include integration of automated sample feeders, real-time data analytics and connectivity to laboratory information management systems. Development of miniaturized, field-deployable dry-combustion analyzers could enable in situ soil testing. Enhanced chemometric calibration models and gas recycling systems will further reduce operating costs and environmental impact.

Conclusion

The LECO CN828 dry-combustion method provides a reliable, high-throughput approach for determining total carbon and nitrogen in soils. With rigorous sample preparation, standardized instrument parameters and proper blank and calibration procedures, users can achieve precise, reproducible results that support informed fertilization and soil management decisions.

Reference

• ISO 13878: Soil Quality – Determination of Total Nitrogen Content by Dry Combustion
• LECO CN828 Series Operator’s Instruction Manual