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Here’s Why the Carbon-Nitrogen Ratio Matters

By Gil Gullickson   

Ever had a finely tuned vehicle or implement that just purrs as it goes down the road or field?

That’s akin to microbes purring in soils with a carbon-to-nitrogen (C:N) ratio of 24:1. (24 parts carbon to 1 part nitrogen). Under these optimum conditions, soil microbes can spur release of nutrients like N, phosphorous and zinc to crops. Meanwhile, this ratio influences the amount of soil-protecting residue cover that remains on the soil. The 24:1 ratio strikes a balance between the two. Alter it, and matters can go awry. 

The NRCS has a good pdf explaining the importance of maintaining this carbon-nitrogen ratio. 

To find the pdf, go to http://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/health. Hit Soil Health literature on the left-hand corner under “Dig Deeper, Learn More. This will again take you to the “Dig Deeper, Learn More” heading on the page’s left-hand corner. Go to Carbon to Nitrogen Ratios in Cropping Systems for the complete information. 

In the meantime, the following is a summary of the pdf.

What is the C:N Ratio? 

Well, it’s pretty much what it says. It’s the ratio of the mass of carbon-to-nitrogen in a substance (in this case, soil). A C:N ratio of 

10:1 means there are ten units of carbon (C) for each unit of nitrogen (N) in the soil. This ratio can significantly impact:

  • Crop residue decomposition, particularly residue-cover on the soil.
  • Crop nutrient cycling (predominantly N).

It’s important to understand these ratios when planning cash crops rotations and cover crops in rotations.  To stay alive, microbes need a C:N ratio near 24:1: Around 16 parts of carbon are used for energy and eight parts for maintenance.

Residues differ as to their C:N ratio. Mature alfalfa hay has nearly the perfect balance that soil microorganisms love with a 25:1 C:N ratio. Microbes consume it quickly and leave little excess C or N.

There are C:N ratio extremes for feedstuffs. On one side, wheat straw has a C:N ratio of 80:1. On the other extreme is hairy vetch with a C:N ratio of 11:1. 

How C:N Ratio Impacts Soil Cover
The quicker microbes consume residue, the less time soil is covered. That lessens the time residue protects against soil-shattering raindrops. It also lessens the time residue shades soils from moisture-sucking sunshine and scorching winds. Ditto for the habitat that residue provides for the arthropods?that shred crop residue and eat weed seeds. 

On the other kind, these same residues need to eventually decompose to release plant nutrients and build soil organic matter. That’s why it’s important to maintain a balance to maintain soil cover that ultimately breaks down. 

It’s akin to humans eating a balanced diet. To a microbe, eating surface-covering wheat residue with its high C:N ratio of 80:1 mimics you when you chow down on celery. It’s good for you, but takes a while to eat.  Although the wheat covers the ground, it also creates a temporary deficit of N. 

Since it provides a higher proportion of C to N, microbes have to find additional N to balance out the excess C as they consume the wheat straw. This has to come from excessive N in the soil. This could temporarily create an N deficit (immobilization). This could continue until some microbes die and release N contained in their bodies (mineralization). 

On the other hand, microbes rapidly devour a low C:N ratio crop like hairy vetch (11:1 C:N ratio) similar to the way most humans would quickly devour chocolate cake. Unfortunately for us, chocolate cake has little nutritional benefit. It’s that same way with hairy vetch residue cover. Since microbes quickly devour residue, little hairy vetch cover remains on the soil surface. 

One the plus side, since hairy vetch contains less C to N than the optimum 24:1 ratio, the microbes will devour the vetch and leave the excess N for growing plants.  These microbes could also chow down other residues with a C:N ratio greater than 24:1. 

Strike a Balance
What’s needed for your soil is a microbe’s version of a well-balanced meal for you: dairy, fruits, vegetables, grains, and meats and beans. That’s what you can get by adding cover crops to a cash crop rotation. 

For example, a low C:N ratio cover crop containing legumes (pea, lentil, cowpea, soybean, sunn hemp, or clovers) and/or brassicas (turnip, radish, canola, rape, or mustard) can follow a high C:N ratio crop like corn or wheat. This can help residues decompose while enhancing nutrient availability for the next crop. 

Similarly, a high C:N ratio cover crop might include sunflowers or millet that can provide soil cover after a low-residue, low C:N ratio cash crop like peas or soybeans. Still, they can decompose during the next growing season to make nutrients available to the following crop.

Here’s a list of C:N ratios of crop residues and other organic materials. These can aid you in approaching the optimum 24:1 C:N ratio.

Rye Straw 82:1 

Wheat Straw: 80:1 

Oat Straw: 70:1

Corn Stover: 57:1

Rye Cover Crop (Flowering) 37:1

Pea Straw: 29:1

Rye Cover Crop (Vegetative) 26:1

Mature Alfalfa Hay 25:1

Ideal Microbial Diet: 24:1

Rotted Barnyard Manure: 20:1

Legume Hay 17:1

Beef Manure 17:1

Young Alfalfa Hay: 13:1

Hairy Vetch Cover Crop: 11:1

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