Source: USDA. National Resources Conservation Services
the microbial diet
carbon:nitrogen ratio
Carbon to Nitrogen ratio is the mass of carbon to the mass of nitrogen in a substance. For example, a C:N ratio of 10:1 tells us that a substance has 10 parts carbon and 1 part nitrogen. This ratio directly impacts residue decomposition & nutrient cycling and is the foundation of a biologically active soil. Source: USDA. National Resources Conservation Services
In order for soil microbes to stay alive (energy + body maintenance ) they need a C:N ratio near 24:1. (around 16 parts of carbon are used for energy and 8 parts for body maintenance)
This 24:1 ratio rules the soil. Under these optimum conditions, microbes control your soil’s ability to:
Cycle Nutrients
Manage Soil-Protecting Residue
The National Resource Conservation Services of the USDA have a great educational PDF explaining further. Click here.
low c:N Ratio
A C:N ratio lower than 24:1, will break down (decompose) rapidly.
Low C:N Ratio. (Approximately 15:1)
The quicker microbes consume residue, the less time soil is covered. This results in less residue to protect from wind & water erosion and keeps the soil temperature too hot for a functioning soil microbial community - the same environment in which arthropods need to shred residue and saprophytic fungi to decompose it.
high c:N Ratio
A C:N ratio higher than 24:1, will break down (decompose) slowly.
High C:N Ratio (Approximately 70:1)
When soil microbes are consuming a higher C:N residue (corn residue for example) the nitrogen they are using to do this is being "tied up". This is also known as Immobilization & can result in an early nitrogen deficiency for a subsequent crop. This is why balance is important.
Important to note: It is very difficult to obtain a “healthy soil” without providing the ideal environment these microbes need to function and carry out their evolutionary tasks to support plant growth. The good news? We have the ability to do that by selecting the proper plant species in our rotations to balance that ratio.
C:N ratio
A balanced C:N Ratio
Intercrop Environment: Vegetative legumes with high carbon cereals provide a balanced diet for microbial populations. This balanced diet promotes an environment in which soil micro-organisms (specifically Mycorrhizal Fungi) can thrive and build soil aggregates. These soil aggregates allow the soil to infiltrate and hold water.
Image: Corn & Hairy Vetch Intercrop. Balanced C:N Ratio
C:N Ratio
simplified approach to C:N
If you were microbe, which food source would you prefer or think would be more easily digested?
A great resource/podcast would be Gary Zimmer on The Regenerative Agriculture podcast discussing residue breakdown and C:N ratio.
Image: Simplified Approach To C:N Ratio
C:N ratio
Farmer experience
See below for a farmer sharing his story around C:N and how his cover crop selection was a lesson learned.
Corn after: High C:N Cover Crop Mix Sown In Fall. (Cover crop was mostly cereals)
Corn after: Low C:N Cover Crop Mix Sown In Fall. (Cover crop was mostly legumes
"This corn has received no synthetic nitrogen yet. Here is a great example of the importance of selecting the proper cover crop mix to suit your next cash crop. The left picture was a high C:N ratio cover crop mix predominate in cereals. The right picture is a low C:N ratio cover crop predominate in legumes. With these poorly functioning soils, cover crop selection is important. Great example of nutrient tie up caused by poor soil health and biological imbalances. This is all from the same field and planted same day. The drill ran out of seed when we seeded it last fall. So part of field got the wrong cover crop mix. Mistake. Costly! Experience isn’t free! #covercrops #soilhealth
-Farmer Unknown
C:N ratio
water infiltration & holding capacity
How important is water infiltration and water holding capacity to your farm? Remember that, 24:1 C:N ratio rules the soil. This balanced diet promotes an environment in which soil micro-organisms (specifically Mycorrhizal Fungi) can thrive and build soil aggregates. These soil aggregates allow the soil to infiltrate and hold water.
This is best demonstrated by comparing water infiltration rates done between mono-crop corn (high C:N) & and a corn/vetch intercrop (balanced C:N)
Corn/Vetch Intercrop. A balanced C:N Ratio
*Infiltration tests done the same day, same soil type.
Monocrop Corn. A high C:N Ratio
*Infiltration tests done the same day, same soil type.
Steve Groff
highly recommended watch
Listen to Steve Groff, The Cover Crop Coach, better explain the C:N Ratio.