C / N ratio

from Wikipedia, the free encyclopedia

The C / N ratio describes the weight proportions of carbon (C) and nitrogen (N) - for example in soils or parts of plants - and is a common indicator of nitrogen availability for plants and microorganisms.


The smaller the number, the closer the C / N ratio and the more nitrogen is available.

Furthermore, the degree of humification of the organic material can also be determined via this ratio. During the decomposition, CO 2 is also released and N is largely incorporated into the microbial biomass . This makes the C / N ratio tighter, which in turn suggests advanced humification. The C / N ratio is also different depending on the type of humus .

The C / N ratio of the materials to be rotted is also important during composting , as the activity of the degrading microorganisms is heavily dependent on it. In biogas plants , the carbon is converted into methane , so that the fermentation residue contains significantly more nitrogen than the starting material. This must be taken into account when composting.


The absolute amount of nitrogen is a fuzzy measure used to determine the availability of nitrogen for plants. Because microorganisms can absorb nitrogen better than plants because they mineralize organic parts , so sit directly at the source and have a much larger surface (i.e. also more absorption systems for nitrogen) than plants. Since microorganisms, unlike plants, which get carbon and energy from the air, depend on the carbon in the soil, their growth is restricted by too little carbon and the plants that are disadvantaged in terms of nitrogen uptake are better off.


The C / N ratio is determined as follows:

  • the sample is oxidized (burned) at 950 ° C → CO 2 + N 2 (after reduction by reducing agents such as copper or tungsten)
  • Conditioning (calibration) of the thermal conductivity detector (TCD) with two blank values ​​and two standard samples to determine the calibration factor
  • Measurement takes place in the WLD after entering the weight and sample numbers
  • Output of the results of the C and N contents in mg / g C tot and N tot
  • If necessary, the sample is divided in the case of samples containing carbonate:
  1. The first measurement is incinerated in a muffle furnace at 430 ° C and then the remaining C inorg is determined by means of elemental analysis
  2. second measurement C tot determined without pretreatment
  3. Difference C tot - C anorg = C org
  • Conversion and specification of the results in C org / N tot (in percent)
  • Estimated value by multiplying C by 1.72 for mineral soil or 2 for organic layers results in an approximate humus content in the soil

Size information

  • Green waste between 7 and 15 C / N, mildew at 20 C / N, raw humus between 25 and 40
  • Grassland around 11 C / N, arable land around 14 C / N, forest around 21 C / N
  • young grass between 10 and 15 C / N, corn at 55 C / N, grain between 60 and 100 C / N
  • Elm forest approx. 28 C / N, linden forest approx. 37 C / N, beech forest approx. 51 C / N


For agriculture in particular, meaningful conclusions can be drawn from the ratio of C and N regarding the maintenance of soil fertility . So should z. B. a cereal farmer after a harvest by leaving the carbon-rich straw on the field fertilize the field with nitrogen or grow legumes to push the shifted C / N ratio back in the direction that increases microbe activity.


  • Fritz Scheffer, Paul Schachtschabel: Textbook of soil science . 15th edition. Stuttgart 2002, ISBN 3-8274-1324-9 .
  • F. Werres: Determination of the concentration of dissolved nitrogen in natural water by means of thermal conductivity detection . In: water and soil . tape 52 , no. 3 . Berlin 2000, p. 36-40 .
  • H. Wild: Environmentally oriented soil science - an introduction . Heidelberg 1995.