Determination of Nitrogen in Soil and Plant Tissue (TruMac N)

Importance of the Topic

Nitrogen plays a critical role in plant growth and crop productivity. Accurate measurement of nitrogen in soil and plant tissue supports efficient fertilization strategies, helps to avoid deficiency or excess nutrient application, and aids in monitoring the nutritional status of high-value crops such as tobacco, cotton, and fruit.

Objectives and Overview

This study demonstrates the use of the LECO TruMac N macro-combustion nitrogen determinator as an alternative to the traditional Kjeldahl method. It evaluates system performance for soil and plant tissue analysis, compares helium and argon carrier gases, and examines the impact of different aliquot loop sizes on precision, sensitivity, and operational cost.

Methodology and Instrumentation

Samples of soil and plant tissue are dried (soil at 105 °C, plant at 80–85 °C) and ground to below 0.5 mm to ensure homogeneity. Approximately 1 g of soil or 0.5 g of plant tissue is weighed into ceramic boats and combusted in a horizontal furnace under pure oxygen. Nitrogen gas released during combustion is collected and measured by a thermal conductivity cell using helium or argon as the carrier gas. The system incorporates a TE cooler to condense moisture, an autoloader for handling crucibles, and exchangeable aliquot loops (3 cc or 10 cc) to optimize sensitivity and reagent consumption. Calibration is performed with certified standards such as EDTA, phenylalanine, and nicotinic acid.

Main Results and Discussion

Typical results obtained for certified reference materials show that:

• Helium with a 3 cc loop delivers highest sensitivity and precision at low nitrogen levels (%N ~0.18, s=0.001).
• Helium with a 10 cc loop and argon with a 10 cc loop yield comparable performance for mid-range nitrogen levels (s=0.01–0.003), though argon models exhibit slightly reduced sensitivity due to smaller thermal conductivity difference.
• Macro-sample capability (up to ~1 g) enables analysis of heterogeneous soils without compromising precision.

Operational considerations such as soot formation in the primary filter can be managed by adjusting sample mass. The argon model offers an alternative when helium supply is limited, with minimal impact on routine analysis.

Benefits and Practical Applications

• Rapid, automated analysis with no wet chemistry reagents, reducing labor and waste.
• Ability to handle larger, heterogeneous samples improves representativeness in soil testing.
• High precision and low detection limits support both agronomic research and quality control in fertilizer manufacturing.
• Versatility for routine laboratory work across agriculture, environmental monitoring, and plant nutrition diagnostics.

Used Instrumentation

• LECO TruMac N macro combustion nitrogen determinator
• Ceramic horizontal furnace and large ceramic sample boats
• Thermal conductivity detector with helium or argon carrier gas
• TE cooler (5 °C) for moisture removal
• Autoloader and 528-203 crucibles
• Aliquot loops: 3 cc and 10 cc options

Future Trends and Potential Applications

Advances may include enhanced detector technologies for improved sensitivity, integration with isotopic and multi-element analysis, further automation for high-throughput workflows, exploration of alternative carrier gases, and development of portable or field-deployable systems for on-site nutrient monitoring.

Conclusion

The LECO TruMac N provides a robust and efficient macro-combustion approach for nitrogen analysis in soil and plant tissues. Its ability to process larger sample sizes, combined with optional carrier gases and loop configurations, offers flexibility to meet diverse analytical needs. The method enhances laboratory productivity and supports precise nutrient management strategies.

Reference

• LECO Corporation. Determination of Nitrogen in Soil and Plant Tissue. Application Note, Form No. 203-821-394, Rev. 3, September 2022.