A Carbon to Nitrogen (C/N) ratio is a measurement that compares the amount of carbon to nitrogen in a specific material or living organism. It’s expressed as a ratio, showing how many parts of carbon exist for every one part of nitrogen. C:N is important because it helps us understand how quickly something organic will break down in the soil.

To calculate the Carbon to Nitrogen (C/N) ratio you divide the percentage of carbon by the percentage of nitrogen. For instance, if a material contains 40% carbon and 2% nitrogen, the C/N ratio would be 20:1. If another material has 35% carbon and 5% nitrogen, the C/N ratio would be 7:1.

As soon as you add organic material to the soil, decay starts right away due to the activity of microorganisms. How quickly and efficiently this material breaks down depends on two main factors:

(A) The carbon-to-nitrogen (C:N) ratio of the organic material.
(B) The C:N needs of the microbes that break it down.

Microorganisms in the soil, including bacteria and fungi, generally require an 8:1 carbon-to-nitrogen (C:N) ratio for their nutritional needs. On top of this, they also need additional carbon as an energy source for digestion and other physiological functions.
Research indicates that a 24:1 C:N ratio is optimal for soil microbes. Out of 24 carbon units consumed, 16 are used for energy, and the remaining 8 are stored in the microbes’ bodies as microbial biomass.

During microbial decomposition of organic matter, about two-thirds of its carbon content is released back into the atmosphere as CO2*. This general rule is more applicable to above-ground plant parts like leaves, stems, and branches, as well as animal waste that is deposited on the surface. The carbon from decomposing roots and particularly root exudates is different; it is more likely to undergo humification and become stored, or “sequestered,” in the soil.

From a practical standpoint, the soil doesn’t usually receive nutrients in the theoretically ideal 24:1 carbon-to-nitrogen (C:N) ratio. Whether natural or synthetic, the nutrient sources you add to garden soil come in different amounts and with varying C:N ratios. In spite of this, the soil has a way of self-adjusting to achieve a C:N balance of around 11-12:1. It does this by mixing various sources of carbon and nitrogen found in different parts of the soil profile.

The most common ratios referred to in the biological approach to gardening are for:

Soil 11-12:1,
Humic substances 10:1 
Compost 15-20:1
Manures 7:1 
Beans/seeds 7:1
Plant biomass (legume 11-25:1, non-legume 80:1
Woody materials +400:1
Combined microbial 
(bacteria+fungi)  biomass 8:1

If a material has a high carbon-to-nitrogen (C:N) ratio, greater than 80:1, such as bark or wood chippings (which are proportionally low in nitrogen) it will decompose slowly. Microorganisms will need to take an additional 10 units of nitrogen from the surrounding soil to maintain their own optimal 8:1 C:N ratio. On the other hand, if the material has a low C:N ratio, like fresh legume clippings with a ratio of 11-25:1 (proportionally higher in nitrogen) , the decomposition will happen much faster. In this case, the soil microorganisms will pull carbon from the soil to maintain their ideal 24:1 ratio.

If you add material to the soil with a carbon-to-nitrogen (C:N) ratio greater than 24:1, you’ll temporarily reduce the available nitrogen in the soil, a process known as “immobilization.” On the other hand, materials with a C:N ratio less than 24:1 will lead to a temporary increase in available nitrogen, referred to as “mineralization.” In natural conditions, the soil balances these changes quite effectively. However, issues arise when highly concentrated synthetic nitrogen fertilizers, which contain little to no carbon, are used. The absence of carbon in these synthetic fertilizers and their continued use is a major drawback in chemically based horticulture.

* this carbon is not necessarily lost to the atmosphere if you have living plants,  as they will actively reabsorb the CO2 as they photosynthesize. 

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