As we move forward and work towards increasing soil health, microbial activity, and organic matter content, we must understand how the Carbon to Nitrogen ratio works. The C:N ratio is the mass of carbon to the mass of nitrogen in a particular substance. For example, if we have a C:N ratio of 24:1, this means we have 24 units of carbon to 1 unit of nitrogen. The C:N ratio is important because due to the fact that it has a direct impact on residue decomposition and also nitrogen cycling in our soils.
Our optimum C:N ratio is 24:1 for desired decomposition of our crop residue. The ratio 24:1 is the proper carbon and nitrogen that a microorganism must have in order sustain its health. A microorganism living in the soil has a C:N ratio of about 8:1; this is what they must maintain in their bodies. For optimum health the microbe requires approximately 16 parts of carbon for energy and then 8 parts for maintenance. This is where the ratio of 24:1 comes from.
When selecting our crop and cover crop rotation it is important that we take into consideration the C:N ratio of certain plants that we may want to include in our rotation. This is especially true if the amount of residue on the surface is a concern when planting time comes around. As a rule of thumb, the higher the ratio, the longer it takes for the material to decompose. Likewise, the smaller the ratio is, the more rapidly the plant material will decompose. This also has a direct relationship with the amount of nitrogen that is tied up in the soil that will be available to the next growing plant.
The reason it takes longer for the higher C:N ratio residue to decompose is due to the fact that the further away form 24:1 we get, the more nitrogen the microbe must find from other sources in the soil. The microbes will pull all available nitrogen from the soil that is necessary to reach its optimum ratio of 24:1. This will lead to the excess nitrogen in the soil to be tied up (immobilization) and may lead to a deficit until some microbes die and release the nitrogen (immobilization) or other sources of nitrogen are provided. Now just because a certain crop has a high C:N ratio, it does not mean it’s a bad thing. It simply means that we must be aware of what is taking place in our soils and be aware that some adjustments may need to be made in our operation to account for the additional nitrogen tie up.
A crop that has a lower C:N ratio, like legumes, allows the decomposition takes place rather quickly and the excess nitrogen becomes available in the soil for other growing plants. This is what allows us to begin to consider reducing our nitrogen rates, once we have a well-established, properly managed system, and understand how to utilize that nitrogen.
As farm managers we can adjust our C:N ratios by blending certain types of crops with others. For example, a fully matured cereal rye plant can have a ratio of up 82:1. But we can blend our cereal rye with a crop such as hairy vetch that has a ratio of 11:1, in order to help pull our ratio closer to the optimum 24:1 ratio.
Keep in mind the ratios change as the plant grows, so the C:N ratio in a young plant will be much less as that compared to the plant as it fully matures. Also as we increase our soil health and have in place an established system or cycle, our microbial activity will increase. Meaning that even our higher C:N ratio plants will begin to break down quicker. Many long term no-tillers and avid cover croppers have noticed that when they come in in the spring that their soils are almost bare, with the exception of the living cover crops they have yet to destroy. This means we are doing are job as farm managers and have got our bio-tillage (microorganisms) working and they are healthy.
C:N Ratio Chart
Source: Carbon to Nitrogen Ratios in Cropping Systems