Total nitrogen and soil C/N ratio

Soil biological activity soil biology Biological indicators of soil quality

Total nitrogen corresponds to the sum of organic nitrogen and mineral nitrogen.

The dominant form of nitrogen in natural and agricultural soils is the organic form (between 1 and 3 g of nitrogen per kg of dry matter in the first 25 centimeters of soil, and only 25 mg/kg of mineral nitrogen, at most).

The mineral form corresponds to ureic, ammoniacal, and nitric nitrogen.

Total soil nitrogen is used to calculate the C/N ratio (where C represents soil organic carbon), which allows assessing the degree of evolution of organic matter, that is, its capacity to degrade more or less rapidly in the soil; however, soil biology must also be taken into account to qualify the degradation kinetics of organic matter.

Standard

NF ISO 13878: Soil quality - Determination of total nitrogen content by dry combustion (“elemental analysis”)
NF ISO 11261: Soil quality - Total nitrogen determination - Modified Kjeldahl method

Sampling and logistics

Analysis on dry soil.

Description of the measurement method

The sample is dried at 38°C, sieved at 2 mm.

Total nitrogen is measured:

either by dry combustion on soil ground to 200-250 µm (Dumas method - NF ISO 13878). This elemental analysis is combined with the measurement of total soil C. The sample is calcined under oxygen at 950-1000°C and the produced gases are purified to isolate nitrogen. The signal measured by a thermal conductivity detector (catharometry) is then converted into total nitrogen content.

or by the Kjeldahl method on soil sieved at 2 mm (NF ISO 11261). This process starts with mineralization of nitrogen through a chemical reaction with sulfuric acid (H₂SO₄). Then, mineral nitrogen is distilled and quantified by colorimetry.

Example ranges of variation with the Dumas method

Indicator	Land use	Number of observations	Min	Max	Median
Total N	Large crops	730	0.28 g/kg	5.09 g/kg	1.44 g/kg
	Grasslands	571	0.46 g/kg	12.30 g/kg	2.31 g/kg
	Forests	578	0.04 g/kg	11.30 g/kg	1.66 g/kg
	Vineyards and orchards	59	0.37 g/kg	4.02 g/kg	0.98 g/kg
C/N	Large crops	817	5.1 g/kg	21 g/kg	9.8 g/kg
	Grasslands	571	7.8 g/kg	17.4 g/kg	10.3 g/kg
	Forests	578	8.8 g/kg	52.7 g/kg	15.8 g/kg
	Vineyards and orchards	59	5.2 g/kg	18.3 g/kg	10.6 g/kg

Source: Data from the RMQS network on 0-30 cm (total nitrogen, Dumas method)

Interpretation

The measurement of total nitrogen allows estimating the potential supply of mineral nitrogen by the soil, making it an indicator of soil nutrient supply.

The multiplication of the soil organic nitrogen stock (total nitrogen content x quantity of fine earth/ha) by the mineralization coefficient K2 (annual mineralization rate calculated from physico-chemical characteristics and cropping system) allows estimating mineralizable nitrogen over the year.

The soil C/N ratio is a primary indicator of the quality and evolution of organic matter (OM). Except for forests and special cases, this proportion is around 10 (10 times more carbon than nitrogen).

A low C/N (<8) indicates rapid decomposition of organic matter, while a high C/N (>12) indicates slow and difficult decomposition (the rate of organic matter degradation depends on the C/N ratio but also on the soil biological composition!). It thus informs about the nutrient supply of the soil, as well as the stock and transformation of carbon.

Advantages and limitations

Advantage

Fast and inexpensive analysis
Between 10 and 20€ as a complement to a soil analysis (for total nitrogen)
Usable in advisory situations

Limitations

Slow evolution over time; at least 5 years are needed between two analyses to start seeing a significant change.
C/N is a state indicator that remains general regarding organic matter properties. It will be insufficient to guide sustainable soil management practices.

Soil C/N only discriminates extreme situations (90% of arable land plots have a C/N between 8 and 12). It should be complemented by other indicators, such as OM fractionation, for a deeper understanding of soil OM quality.

Source

RMT Bouclage. 2025. Biological functioning indicators of agricultural soils. [03/11/25]. https://www.rmt-fertilisationetenvironnement.org/moodle/pluginfile.php/5041/mod_resource/content/3/Recueil%20indicateurs%20de%20fonctionnement%20biologique%20des%20sols%20agricoles_vf251009.pdf
Boiffin J., Kzlizagbahi I., and Sebillotte M. 1986. Cropping system and organic status of soils: application of the Henin-Dupuis model.
Saby, Nicolas; Bertouy, Benoit; Boulonne, Line; Bispo, Antonio; Ratié, Céline; Jolivet, Claudy. 2019. Summary statistics from RMQS on agronomic data and trace elements of French soils from 0 to 50 cm. https://entrepot.recherche.data.gouv.fr/dataset.xhtml?persistentId=doi:10.15454/BNCXYB
Levavasseur F., Mary B., Christensen B. T., Duparque A., Ferchaud F., Kätterer T., ... & Houot S. 2020. The simple AMG model accurately simulates organic carbon storage in soils after repeated application of exogenous organic matter. Nutrient Cycling in Agroecosystems, 117, 215-229.
Justes E., Mary B. & Nicolardot, B. 2009. Quantifying and modelling C and N mineralization kinetics of catch crop residues in soil: parameterization of the residue decomposition module of STICS model for mature and non mature residues. Plant and soil, 325(1), 171-185.