C / N (carbon / nitrogen) ratio in soil

Currently, we give vital importance to the contribution of fertilizers , most of them of inorganic origin, to achieve the desired productive results. We are going to study its relationship with the C / N relationship.

In most cases, this contribution far exceeds the needs of the plants and, however, many times it seems that we fall short.

What is happening is that we are not taking advantage of all the possibilities that a suitable soil in good condition could give us.

The response or performance of inorganic fertilizers (ammonium nitrate, calcium nitrate, potassium sulfate, etc.) is greatly influenced by the positive or negative conditions we have in the soil.

Said soil, as an entity, is a set of variables of which we have a great opportunity to modify and improve them at our whim.

Therefore, today we are going to talk about something as important as the Carbon Nitrogen ratio or the C / N ratio of the soil. Something that gives us a lot of information and that we can see as data in any soil analysis.


As its name indicates, the C / N ratio of the soil indicates the ratio between the carbon content and the nitrogen content that a soil may have.

It is not something measurable or a quantitative value. It is a dimensionless concept like any other ratio, for example, economic or stock.

However, based on the average content that is considered average or acceptable, we can find a solution to our possible problem. Having a lot of nitrogen content compared to carbon in the soil or vice versa reduces the potential that we can obtain from our crops and it is something easily correctable.

Later we will study some concrete cases.


At the end of the day, what this value offers us is to predict the development that the plant may have according to the availability of nitrogen or carbon that the soil may have.

If we have a high C / N ratio or carbon nitrogen ratio, we are saying that there is a prevalence of the carbon content (carbohydrates) over the nitrogen content.

As we all know, this element, nitrogen, is what guarantees the vegetative growth of the plant, since from it the plant is capable of transforming it into amino acids and these into proteins (more complex structures formed by a chain or group of amino acids ).


We are going to remember the previous Gardenprue article where we mentioned the average values ​​in soil analysis , and that it is not even painted for this part.

As we said, it was essential to know what values ​​we are facing or what is the objective we have to achieve in order to have a good quality floor.

Although it varies depending on the texture of the soil and the crop that houses it, they are quite approximate and general values.

Regarding the  C / N ratio , we see that the average value is between 8.5 and 11.5 . As we said, it is a dimensionless value that evaluates the concentration of carbon versus nitrogen.

Now we are also going to see an example of what happens if the values ​​of the carbon-nitrogen or C / N ratio skyrocket or fall short.

C / N ratio <8.5: lack of energy. High release of mineral nitrogen.

C / N ratio between 8.5 and 11.5: balanced soil. Control in the release of mineral nitrogen and the carbon content of the soil.

C / N ratio> 11.5: soil with excess carbon and excess energy.

The useful microbiology of the soil lives at the expense of using carbon and nitrogen in the transformation of organic matter or in the contribution of substances that can be assimilated by the roots, such as organic acids, enzymes, substances of hormonal interest, etc.

The C / N ratio controls the development of said microbiology as well as the mineralization process of organic matter.

We are not looking for an organic product that releases all its “energy” or raw material in a short period of time (nitrification of organic nitrogen to NH4 + and this to NO2- and NO3-).

Depending on the physiological stage in which we find ourselves (and this often happens in the early stages of development), the release of nitrogen can be very high and of little yield, since the nitrogenous substances do not come into contact with the estate.

Consequently, we have a massive waste of nitrogen, a high percolation or leaching and a great contamination of the subsoil or the aquifer.


Something different happens in the C / N ratio of organic matter, which usually has significantly higher average values ​​and undergoes a different process of decomposition during its process.

The rates of formation of plant or animal remains in organic matter undergoes a much faster decomposition process than in natural soil, hence the values ​​change.


In this article we explain how the C / N ratio works in compost and mean values. In this way we can control the process of its formation, based on knowing what products or vegetable, kitchen or animal remains to contribute.

The ideal is to have a C / N ratio between 25 and 40 in organic matter. A value much higher than the 8.5-11.5 of a soil.


However. The C / N ratio, as we have seen, can occur in organic matter (with a single value), in the soil (with another range of values) and in the plant.

We can evaluate a multitude of data and evidence to know if the nutrition of our plant is balanced with the C / N ratio.

We know that there are carbon (carbohydrates) formed during the photosynthesis process, and we also know that there is nitrogen that is absorbed through foliar applications, in fertigation or with the help of organisms that fix atmospheric nitrogen in certain types of plants (legumes and others).

Therefore, with carbon and nitrogen circulating in the sap, we can develop a comparative table to see which C / N value is adequate, although it cannot be measured.


In the early stages of a plant’s development , there is dizzying growth rate, stem formation, and leaf production. What happens is that the plant demands a lot of nitrogen to transform it into proteins. That is, there is a very low C / N (carbohydrates versus protein) ratio.

In short, all the energy is used in stem growth. Of flowers and fruits it is not necessary to speak yet because it does not exist in the crop.

Once the plant acquires a considerable size and its adaptation to the environment is complete, it begins to demand other nutrients instead of nitrogen. Physiologically what happens?

Not so many leaves or stems form, nor is root growth that fast . Remember that a large part of these carbohydrates (50% or more) goes to the roots, seeking their growth. Therefore, carbohydrates accumulate in reserve and the C / N ratio increases . That is, there is a higher ratio of carbohydrates than nitrogen.

We could speak of an excess of stored energy. In animal terms, the plant “gets fat.”

When there is an excess of energy or a positive balance in energy, we think about reproduction options, since we are in a comfortable situation.

The plant, at this moment, begins to use these carbohydrates to produce flowers, set fruits and, in the future, fatten and ripen them. It is in a reproductive phase. 

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