Determination of nitrogen content in fertilizers - Elemental analysis for solid and liquid fertilizer samples

Dumas method application - Based on the standards AOAC 993.13 and DIN EN 13654-2:2001

The importance of fertilizers in agriculture

Nitrogen is a critical macronutrient for plant growth. Because of the continuous intensive agriculture, the natural regeneration process of the soil is impossible, and thus, the nutrients have to be added artificially using fertilizers. The addition of nitrogen increases crop yields but too much nitrogen can have a negative impact on the environment and on the quality of the products. A good balance must be found, and this is possible only by knowing the exact nitrogen content of the fertilizers used.

In the Dumas method, the nitrogen contained in the sample is oxidized to nitrogen oxides in an oxygen atmosphere, at high temperatures and in the presence of a catalyst. Then the nitrogen oxides are reduced to nitrogen with the help of copper. The side products, water and carbon dioxide, are separated in specific traps. Last the nitrogen is detected by a thermal conductivity detector (TCD) and its amount is determined using a calibration previously performed by analysing a suitable substance with known nitrogen content.

The determination of nitrogen in fertilizers

Sample preparation (Solid fertilizers)
The sample is homogenised with a variable speed rotor mill. Immediately before weighing in, it is again thoroughly mixed by hand. Then the sample (50-100 mg) is weighed with tin foil. After sealing the tin foil, the sample is placed in the transfer tray.

  • App note: Depending on the sample matrix, the sample weight can be reduced to increase the lifetime of the consumables.

Sample preparation (Liquid fertilizers)
The sample must be representative for the analysis. Immediately before weighing, the sample is shaken again thoroughly. Tin foil (e.g. DumaFoil) is tared and some Super-Absorber (ratio 1:10) is added. After the balance has been tared again, the sample (50-100 mg) is added with a syringe. After weighing, wait a few seconds until the sample has formed a gel with the Super-Absorber. After that, the tin foil can be sealed and the sample can be placed in the transfer tray.

  • App note: Depending on the sample matrix, the sample weight can be reduced to increase the lifetime of the consumables.

Calibration
The selected calibration must cover the working range. For a recommended sample weight, a calibration of 1 mgN to 10 mgN carried out with EDTA is usually sufficient. The minimum requirement for the correlation factor R2 is a value ≥ 0.9999.

Exemplary results for solid fertilizer samples - Humus

Sample amount [mg]Nitrogen (N) [%]Average nitrogen content [%]Standard deviation nitrogen content [%]
80.2302.4252.4270.073
80.2432.427
80.2422.526
80.1142.455
80.4042.429
80.4302.301

Exemplary results for liquid fertilizer samples – Fertilizer NPK (5-3-7)

Sample amount [mg]Nitrogen (N) [%]Average nitrogen content [%]Standard deviation nitrogen content [%]
53.3735.1414.1170.015
53.7265.123
50.7395.110
56.2135.106
54.9565.097
50.8735.122