C:N Ratio Calculator

Calculate C:N ratio for soil biology, decomposition rate, and nutrient cycling.

Please check your inputs and try again.

Why 25:1 Is the Threshold

Decomposer microbes have body tissue around 8:1 and use roughly two thirds of the carbon they consume for respiration rather than growth. That combination sets the balance point at approximately 25:1 in the substrate.

Substrate C:NMicrobial nitrogen balance
< 20:1Mineralisation — surplus N released to soil
20–30:1Approximately balanced
> 30:1Immobilisation — N drawn from soil

Above roughly 30:1 there is not enough nitrogen in the residue to build microbial biomass, so decomposers take it from surrounding soil. Nitrogen immobilisation temporarily starves plants, which is why incorporating fresh sawdust or straw causes visible nitrogen deficiency in a crop.

The effect reverses as decomposition proceeds. Carbon is lost as CO2 while nitrogen is retained, so the ratio falls and immobilised nitrogen is eventually released — but the lag can span a growing season.

MaterialC:N
Urea0.5:1
Poultry manure10:1
Grass clippings20:1
Finished compost15–20:1
Fresh leaves20–60:1
Straw80:1
Paper170:1
Sawdust400–500:1

Composting Practice

The target for active composting is 25–30:1, and the failure modes at each end are distinct. Too much nitrogen and excess ammonia volatilises, producing the characteristic smell and losing the nutrient. Too much carbon and the pile simply fails to heat, since microbial growth is nitrogen-limited.

A common confusion: the ratio is calculated by mass of carbon and nitrogen, not by volume of material. Bulky low-density inputs like leaves contribute far less mass than their volume suggests, so volume-based recipes systematically overestimate the carbon fraction.

Finished compost stabilises around 15–20:1, close to soil organic matter, which is why it releases nitrogen slowly rather than immobilising it.

Worked Examples

Example 1: Fresh leaves: C=45%, N=2%
C:N=22.5:1
Result: Moderate — will decompose reasonably
Add high-N material for faster composting
Example 2: Wood chips: C=50%, N=0.1%
C:N=500:1
Result: Extremely high — very slow decomposition
Will immobilize soil N — add N source
Example 3: Nitrogen immobilisation in practice
Fresh sawdust at 400:1 dug into soil
Result: Crop shows nitrogen deficiency
Decomposers draw nitrogen from the soil to process the carbon. Plants compete poorly against microbes for it, so deficiency appears within weeks.
Example 4: Why the pile will not heat
Compost mix at 60:1
Result: Nitrogen-limited
Microbial growth cannot proceed fast enough to generate heat. Adding a nitrogen source such as manure or grass brings it toward the 25–30:1 target.

Common Mistakes

⚠️
Applying volume ratios instead of mass ratios

C:N is a mass ratio. Bulky materials such as leaves contribute far less mass than their volume implies, so volume-based recipes overestimate carbon.

⚠️
Incorporating high C:N material before planting

Sawdust and straw immobilise soil nitrogen for months. Either compost first or add supplementary nitrogen alongside.

⚠️
Assuming immobilised nitrogen is lost

It is temporarily held in microbial biomass. As carbon is respired away the ratio falls and nitrogen is released — but the lag can exceed a growing season.

⚠️
Treating C:N as the only factor

Lignin content matters as much. Woody material resists decomposition regardless of ratio, because lignin is chemically recalcitrant.

Frequently Asked Questions

N immobilization?
High C:N material: soil microbes need N to decompose C. They outcompete plants for soil N → N immobilization. Temporary N deficiency in crops after straw incorporation. Wait 2-4 weeks or add N fertilizer.
Compost optimization?
Mix high-N (greens) with high-C (browns) to get 25-30:1. Greens: grass, food scraps (C:N ~15-20). Browns: straw, cardboard (C:N ~60-80). Layer or mix. Keep moist. Turn to aerate. Temperature 55-65°C for pathogen kill.
Why is 25:1 the critical ratio?
Decomposer tissue is around 8:1 and they respire roughly two thirds of consumed carbon. That combination means substrate above about 25:1 leaves them nitrogen-limited.
What is nitrogen immobilisation?
Microbes drawing nitrogen from the soil to decompose high-carbon material, temporarily making it unavailable to plants.
What C:N ratio should compost be?
Around 25–30:1. Lower produces ammonia loss and odour; higher fails to heat because microbial growth is nitrogen-limited.
Is C:N calculated by volume or mass?
By mass. Volume-based recipes overestimate the carbon contribution of bulky low-density materials such as dry leaves.