Carbon To Nitrogen Ratio: Crucial Factor For Soil Productivity

Carbon to Nitrogen ratio or C:N ratio is the major factor that determines the productivity of any kind of soil to greater extent. Soil is a non-renewable natural resource that serves human till their death. It is a pre-requisite factor for the agriculture. However, in recent days soilless culture have been developed but it has several limitations.

After the end of second world-war, some of the primitive agricultural society have transformed to modern industrialist society. Despite this, agriculture is a source of livelihood for majority of world population in present context. In recent years several technological advancement have been made in agriculture sector that lead to several sophisticated production technologies. Apart from technological advancement, the excess use of chemical amendments has risen significantly. Studies were conducted to analyze the consequences of overuse of chemical amendments on soil physical and chemical parameter. The result of these studies suggest the addition of organic manure, incorporation of residue on soil is the ultimate way to the sustainable agriculture. To make this possible, we have to understand one of the basic and most important concept in Agriculture called C:N ratio. The C: N ratio of added manure and residue have significant impact on nutrient removal/absorption from them. Now, let’s talk about the concept of Carbon to Nitrogen ratio i.e C:N ratio.

Concept Of Carbon To Nitrogen Ratio/C: N ratio

The ratio of soil organic carbon to total nitrogen is called as Carbon to Nitrogen (C:N) ratio. It is the relative quantity of carbon and nitrogen in crop residue, other organic materials, soil organic matter and soil micro-organism. Generally, the ratio of carbon to nitrogen in a substance is C: N ratio. The carbon and nitrogen content of the soil are the main source of plant nutrient and are also the prime factor that determines the soil fertility. The carbon to nitrogen ratio of soil depends upon relative quantity of carbon and nitrogen in the soil which is determined by the input of carbon and nitrogen into the soil. The carbon and nitrogen of soil also play important role in carbon and nitrogen cycle.


The Carbon to Nitrogen ratio is the indication of mineralization of organic matter and this result in the mineralization or immobilization of soil nitrogen. Before we go on understanding how Carbon to Nitrogen ratio affect the mineralization and immobilization, It is important to know, what these terms means:-

Mineralization:- Mineralization is the process by the organic nutrients are converted into inorganic or ionic or readily available form. In short, Conversion from Organic to Inorganic form.


Immobilization:- Immobilization is opposite of mineralization i.e the process of conversation of inorganic or readily available nutrients into organic or unavailable form. Immobilization temporarily locks the availability of nutrients to the plants.


This two process i.e Mineralization and Immobilization are governed by the Carbon to Nitrogen ratio of the substances or materials we used in our land. Thus example will clarity all the concept of C:N ratio in relation to Mineralization i.e Nutrient Availability and Immobilization i.e Nutrient Unavailability.


Example:- We have to understand that, Most of the soil decomposing microbes has C:N ratio as 25:1. This means, For every 25 units carbon they consume, 1 unit nitrogen is required to balance the C:N ratio. Now, If we add organic material having C:N ratio greater than 25:1 say 40:1. Now, Microbes start to consume that material, When they eat 25 units carbon from 40 units of that material, they consume available 1 unit nitrogen from material to maintain the ratio of 25:1. Now, In material there remains 15:0 carbon to nitrogen ratio. Now, When microbes eat remaining 15 part carbon, they need some amount of nitrogen less than 1, That nitrogen is unavailable in material they consume, So – They takes up the nitrogen from the soil. That cause the unavailability of nutrient to the plant i.e Immobilization. So – When we put materials having high C:N ratio, they cause Immobilization. This Immobilization is temporary lock up of nutrients, when organic residues becomes less, then microbes will die ultimately releasing the nitrogen to the soil.


However, When we add materials having C:N ratio less then 25:1 say 10:1, Then, when microbes consume 10 unit of carbon, they don’t require all 1 unit nitrogen. So – The remaining nitrogen adds in the soil and plants can utilise that nutrient.i.e Mineralization. This way, C:N ratio affects the availability of nutrients in the soil.

High carbon to nitrogen (C:N) ratio of any material or subsidy signifies that there is low soil organic matter decomposition and mineralization whereas a low carbon to nitrogen (C:N) ratio indicates that soil organic matter are readily decomposed.

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A substance has Carbon to Nitrogen ratio ratio of 80:1 (rice straw) indicates that there is 80 units of carbon for each 1 unit of nitrogen in the given material. It can also be stated as the substance has 80 times more carbon than nitrogen. Basically it is the relative quantity of carbon and nitrogen in a substance and the carbon to nitrogen ratio differ from substance to substances. Carbon content is always higher than nitrogen content in a organic matter. The C: N ratio of a substance has a significant effect on crop residue decomposition, nutrient cycling especially nitrogen and play crucial role in soil fertility determination. Hence, it is good idea to know about the C: N ratio of different substances before incorporating them into the soil. The C: N ratio of some substances is mentioned in given table.

C:N Ratio List For Different Materials

MaterialC:N Ratio
Soil Microorganisms8:1
Soil organic Matter
Rye Straw82:1
Wheat Straw
Legume hay17:1
Corn Roots48:1
Corn Stalks33:1
Corn Leaves32:1
Saw dust400:1

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Marine and terrestrial sources have different C: N ratio. Marine sources have C: N ratio of 4-10:1 whereas terrestrial sources have slightly higher Carbon to Nitrogen ratio as compared to marine and is usually above 20. This is because of lack of cellulose and higher content of protein in marine sources. Many studies reveal that forest land like Tundra, natural wetlands and boreal forest has wider range of carbon to nitrogen ratio as compared to crop land. However the C: N ratio of crop land is influenced by land management practices.


Significance of Carbon To Nitrogen ratio

The importance of carbon to Nitrogen ratio are so many in Agriculture that it is hard to include in a single sub-topic. However – Here, We have included the most important significance of C:N ratio.

1. Crop residue decomposition

Generally, crop residue are used to serve as soil cover to prevent the soil from heavy rainfall, conserving the moisture of soil, prevent the leaching loss and to make good soil aggregation. Also, the same residue decompose to release plant nutrients and add up to the soil organic matter. Therefore it is important to pay attention to crop residue C: N ratio to utilize it as soil cover and allow it to the decomposition as a source of soil organic matter.

If the C: N ratio of residue is higher than 25:1 it decomposes slowly and serves as soil cover for longer duration of time. In the same time if the C: N ratio of residue is less than 25:1, it readily decompose and surplus nitrogen is available for the plants. The addition of residue with low carbon to nitrogen ratio does not add up to soil organic matter as it gets readily decomposed.

Note: As FYM has high C:N ratio, It takes time to release the nutrients from them. Therefore, we should add FYM in field a month earlier than sowing date.

Hence, carbon to nitrogen ratio of residue should be taken into account before incorporating them in the soil.


2. Composting

C:N ratio plays important role in decomposition process. There are two component that determines the success of composting. They are carbon and nitrogen. The carbon serves as source of energy and nitrogen serves as the source of protein, enzyme etc. So, there should be balanced amount of carbon and nitrogen in a compost mixture. Green vegetation has higher nitrogen content as compared to brown vegetation like dried leaves. The thermophilic temperature range for composting is > 500C indicating the quick establishment of microbial activity associated with respiratory metabolism. A research study revealed that lower the C: N ratio higher the maximum temperature attained and longer duration of thermophilic phase. Among the materials with carbon to nitrogen ratio 60, 52, 40 and 29, the maximum degradation of dry matter was observed in material with C: N ratio 29 which was 59%. This suggest that lower the C: N ratio (Higher amount of Nitrogen) higher the rate of composting. The recommended rate of C: N ratio at the start of composting is 30:1 i.e. 30 parts carbon to 1 parts nitrogen by weight.

3. Mineralization of Organic Matter

The C: N ratio determines the decomposition rate of organic matter. It is also a major controlling factor for release of Nitrogen in the soil. Many studies have revealed that the demarcation line of mineralization occurs at the C: N ratio of 25. Below this there is higher mineralization and beyond this there is slow mineralization. When the C: N ratio is of organic substrate is between 1 to15, rapid mineralization occurs and consequently nitrogen present in it is released and is available for crops. Thus lower the C: N ratio more rapidly nitrogen will be released and is available for immediate crop use. There is equilibrium between mineralization and immobilization between the C: N ratio of 20-30. The organic substrate with higher C: N ratio slows down the mineralization process and even leads to temporary immobilization of nitrogen. Thus the soil with wider C: N ratio have more soil organic matter content as compared to the soil with narrow range of carbon to nitrogen ratio.

4. Soil nutrient cycling

Microorganism play important role in soil nutrient cycling. However, C: N ratio has also a major impact in this process. Basically, soil organism utilizes carbon of residue as a source of energy and nitrogen for building cell structure. Microorganism generally require more carbon than nitrogen. The addition of residue having excess carbon creates a problem. To make the efficient use of available carbon of the added residue, microorganism utilizes soil available nitrogen and leads temporary immobilization of available soil nitrogen. The process is known as robbing the soil nitrogen and delays availability of added nitrogen for current season. If the carbon content of added residue is less than that required for converting available nitrogen into protein, they make the full use of available carbon and remove excess nitrogen as ammonia. Thus released nitrogen in the form of ammonia is subjected to loss. Thus, C: N ratio has a crucial role in nutrient cycling and the ratio of 25:1 is considered good for residue incorporation.


Factors affecting C: N ratio of soil

1. Climatic condition
2. Soil properties
3. Land management practices


Carbon To Nitrogen ratio of microorganism

Microorganism also differ in Carbon to Nitrogen ratio. Generally, fungi show higher range of C: N ratio than bacteria. The C: N ratio of fungi and bacteria is 41: 6 and 28: 7 respectively. Therefore more amount of nitrogen and less carbon is required to form bacterial cell as compared to fungal cell. With all other things remaining constant, the soil with lower carbon to nitrogen ratio has less fungal organism as compared to the soil with higher C: N ratio. The decrease in the soil carbon to nitrogen ratio is associated with the loss of biodiversity of soil.


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Impact of addition of crop residue in soil

After the addition of crop residue in soil, the microbes start to consume it. They feed on the crop residue which temporarily locks up the nitrogen in soil and this cause reduced nitrogen in soil and increase microbes which in turn increase CO2 (Microbes Respiration). When the residue finishes, the microbes do not get any feeding material and dies. After death, the nitrogen and carbon is finally mixed into crop. Therefore, we should add crop residue about 1 month prior to sowing of seed as this entire process takes some time. This occurs all for the balance of C:N ratio. The relationship between CO2 produced by microbes and NO3^- is shown in graph below:-

C:N ratio, Carbon to Nitrogen ratio
C:N ratio / Carbon to Nitrogen ratio


Final Verdict

Thus, Carbon to Nitrogen ratio or C:N ratio plays great role in overall soil Productivity. As different materials has different C:N ratio, we must choose our manure source properly to get the most out of them.



Mr.Govind Raj Joshi (Student)

Agriculture And Forestry University

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