Determination of Carbon/Nitrogen in Soils and Plant Tissue

Significance of the Topic

The simultaneous determination of total carbon and nitrogen in soils and plant tissues is a critical diagnostic tool in agriculture and environmental monitoring. Accurate C/N analysis informs growers about soil organic matter content, nutrient availability, and plant nutritional status. These insights guide fertilization strategies and management of high-value, intensively cultivated crops such as tobacco, cotton, fruits, and alfalfa.

Objectives and Study Overview

This application note presents a validated method for quantifying total carbon and nitrogen in soil and plant tissue samples using the LECO CN928 macro combustion analyzer. The study aims to demonstrate system configuration, key operational parameters, sample preparation procedures, and typical performance metrics for precision, accuracy, and throughput.

Methodology and Instrumentation

Samples are dried (soil at 105 °C, plant tissue at 80–85 °C), stored in a desiccator, and weighed (~0.50 g) into ceramic combustion boats. The CN928 operates under a pure oxygen environment in a horizontal ceramic furnace at 1350 °C. Combustion gases pass through a thermoelectric moisture trap and equilibrate in a ballast. A controlled aliquot (3 cm3 or 10 cm3) enters a helium or argon carrier stream for detection by non-dispersive infrared (CO2) and thermal conductivity (N2) cells.

• Carrier gas: helium (higher sensitivity) or argon
• Furnace temperature: 1350 °C
• Aliquot loops: 3 cm3 (extended reagent life) or 10 cm3 (optimal low-range N detection)
• Key timing parameters: baseline delay, integration delay, purge cycles

Used Instrumentation

• LECO CN928 macro combustion C/N analyzer
• Horizontal ceramic combustion furnace
• NDIR CO2 detector; thermal conductivity N2 detector
• Ceramic combustion boats and autoloader
• Carrier gases: helium or argon

Main Results and Discussion

Calibration using EDTA (41.00 % C, 9.56 % N) and verification with nicotinic acid demonstrated high accuracy across a range of certified reference materials. Four soil and plant standards (C from ~3.8 % to 47.4 %, N from ~0.33 % to 3.6 %) yielded:

• Precision: standard deviations typically ≤0.1 % C and ≤0.02 % N
• Accuracy: measured values within certified tolerances
• Comparable results with helium and argon carriers; helium provided slightly better sensitivity at low N levels
• 3 cm3 loop extended reagent lifetime with minimal performance compromise

Benefits and Practical Applications

The CN928 method offers rapid, cost-effective determination of total C and N with excellent reproducibility and minimal sample preparation. It supports routine soil fertility assessments, monitoring of crop nutrient uptake, quality control in agricultural laboratories, and research applications in soil science and plant physiology.

Future Trends and Opportunities

Emerging directions include integration of isotope ratio mass spectrometry for ^13C/^15N studies, automation and high-throughput sample handling, miniaturized combustion modules for field deployment, and coupling with data analytics and remote sensing to develop precision agriculture platforms.

Conclusion

The LECO CN928 provides a robust, sensitive, and efficient solution for total carbon and nitrogen analysis in soils and plant tissues. Its flexibility in carrier gas choice and aliquot sizing, combined with streamlined calibration and sample handling, makes it well suited for diverse agricultural and environmental laboratories seeking reliable nutrient profiling capabilities.