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Carbon and Nitrogen Ratio of The Soil

Carbon and Nitrogen are two of the most essential elemental components of healthy soil. In soil ecosystems, they interact intimately with each other and form an important component of the nutrient cycling process. Therefore, having a disbalance of either can significantly affect plants' health and soil fertility.

What are Soil Carbon and Nitrogen?

Before we get into the nitty-gritty of their interactions, let's briefly review what they are independently.

  • Carbon: Carbon is the elemental backbone of all biological life. It forms the scaffolding of all living organisms, from the smallest microbe to the largest trees. It forms the structure of such things as wood and skin. When living things die and decay, their carbon is released. In soil, it can exist in undecomposed forms (such as leaves and twigs) or in decomposed forms such as fine particles of organic matter called humus. Humus is what gives rich soils and compost a dark, spongy characteristic. Humus is vital for countless functions, particularly nutrient and water-holding capacity. In addition, soil organic carbon is rich in energy, and its origins can be traced back to the carbohydrates made during photosynthesis.

  • Nitrogen: Nitrogen makes up about 78% of the Earth's air. It is, however, often limited in the soil even though it is the most critical nutrient for plant growth. For plants, it is an essential nutrient for the formation of DNA, chlorophyll, amino acids, and many other fundamental components of a plant. Rich, natural nitrogen sources include green leafy materials, manure, and animal urine.

How do Nitrogen and Carbon Interact in the Soil?

Have you ever seen a steaming pile of leaves, grass clippings, or compost on a chilly morning? Perhaps you've even approached a pile like this and felt the warmth emanating from it. This warmth is created from the bustling activity of millions of microorganisms taking part in the decomposition process that breaks down carbon.

That's because when carbon is broken down, it releases energy. This same carbon-based energy is embedded within sugars and carbohydrates we utilize as food energy. Carbon energy is also released when we burn wood or fossil fuels. During decomposition, some of this is the energy used by microorganisms feeding on carbon; the rest is released as heat.

This is where Nitrogen comes into the story. Nitrogen is the "fuel" that microorganisms require to break down carbon. Without Nitrogen, bacteria are unable to feed on and decompose organic carbon. When Nitrogen is abundant, it voraciously breaks down carbon.

  • Too Much Nitrogen Is Bad For Soil Carbon

When you add rich sources of Nitrogen directly into your soil, be it in the form of un-composted green vegetation, manure or a synthetic nitrogen fertilizer, soil carbon is negatively affected. That's because this rich source of Nitrogen feeds the soil bacteria and fuels the decomposition of any soil carbon you may have. This can deplete your soil's organic matter, which vanishes into thin air as carbon dioxide. Along with this loss, you lose your soil's nutrient and water-holding capacity. Excess Nitrogen also affects healthy soil microbiology and can negatively affect plant health.

  • Too Much Carbon is Bad for Soil Nitrogen

This is the other side of the coin. If you generously mix undecomposed brown vegetation into your soil, whether it's wood chips, twigs, or other carbon-rich materials, your soil nitrogen will decline. That is because microorganisms in your soil will quickly consume your soil nitrogen to break down this new carbon source. This can lead to life-threatening and growth-diminishing nitrogen deficiency in your plants.

Finding the Balance with A Carbon-Nitrogen Ratio

A Carbon-Nitrogen (C:N) ratio is a term used to describe the relative quantities of carbon and Nitrogen in a medium. The term is most often used in the context of preparing compost, which is approximately at 20:1 or 25:1 C:N. Below is a list of common materials and their C:N ratios. When preparing most forms of compost, you want to mix your materials to achieve a ratio of about 20:1 or 25:1 to avoid excess Carbon or Nitrogen.

Carbon-Rich Materials C:N Ratios

Nitrogen-Rich Materials C:N Ratios

Corn Stalks


Grass Clippings


Evergreen Leaves


Kitchen Scraps


Hardwood Dust


Deciduous Leaves


Softwood Dust


Chicken Manure




Coffee Grounds




Sheep Manure


Achieving a Healthy C:N Ratio in Your Soil

To cut to the chase, the best way to nourish your soil with Carbon and Nitrogen is to add a well-decomposed compost. A well-decomposed compost is rich in humus and adds all of the soil benefits that we want from carbon without any risks of robbing nutrients from the soil. Humus found within composts is actually chock-full of Nitrogen that it absorbs during the composting process.

To maintain a healthy C:N ratio, it is also important to manage your soils properly. For example, avoiding any form of synthetic nitrogen fertilizer is crucial in preventing negatively impacting your soil carbon. It is also best to avoid using undecomposed manures and other nitrogen-rich materials directly into your soil.

What C:N Ratio In Soil Do Plants Like Best?

For most annual and garden crops, you want to achieve the 20:1 or 25:1 carbon-nitrogen ratio used for most composts. This is a ratio commonly achieved via hot-composting methods. These tend to be bacterially dominated composts best suited for short-lived annual crops.

Trees and other perennial plant species often enjoy a higher C:N ratio in their soil. This can be 40:1 or even greater, depending on the species. These C:N ratios can be achieved using "fungal composts" that are prepared using a higher C:N ratio. These composts usually take long periods to decompose and should not be "flipped, " unlike regular composts. Adding layers of wood chips and other carbon-rich mulch above the topsoil is another excellent way to promote a carbon-rich environment for plants.


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