C:N ratio (C:N ratio)

C:N ratio is the standard shorthand notation for Carbon to Nitrogen Ratio, expressing the proportion of total carbon to total nitrogen in an organic feedstock or fertiliser product as a dimensionless ratio (for example 25:1, written as C:N = 25:1 or simply C:N 25). The notation appears across anaerobic digestion, composting, soil science, and fertiliser regulation in India because it captures in a single number whether a substrate has the right elemental balance to support stable microbial activity. The optimal range for anaerobic digestion is 20:1 to 30:1.

Measurement methodology for C:N determination:

• Elemental analyser (CHN/CHNS): combusts sample at 950-1,050 degC and quantifies CO2, H2O, NO2 by thermal conductivity or IR detection; accuracy +/- 0.5%; analysis time about 8 minutes per sample.
• Kjeldahl method for nitrogen + combustion or wet oxidation for carbon: traditional approach; +/- 2-5% accuracy; widely used in Indian fertiliser testing labs.
• Walkley-Black method for organic carbon: oxidation with potassium dichromate; common in soil science; recovers 76% of carbon, applying a correction factor.

For Indian CBG plant operators, C:N is checked at several points:

• Incoming feedstock acceptance: monthly composite samples per feedstock type to confirm specifications.
• Mixed feed before digester inlet: weekly to verify blending ratios deliver target C:N within the 20-30:1 window.
• Digestate output: monthly; should fall to 12-18:1 indicating proper mineralisation.
• Finished FOM product: regulatory requirement under FCO — below 20:1 for compliance.

The shorthand has subtle pitfalls in practice. Total C:N includes all carbon and nitrogen in the sample, while biodegradable C:N excludes lignin-locked carbon and protein-locked nitrogen unavailable to microbes — the latter is more predictive of actual digester behaviour. Paddy straw with 35% lignin has total C:N of about 70:1 but biodegradable C:N closer to 40-50:1 because the lignin carbon is largely inaccessible. Sophisticated operators measure both, while screening-level plants rely on total C:N corrected by experience-based factors.

The trade-off in C:N optimisation is information value versus testing cost. NABL-accredited C:N analysis runs 800-2,500 INR per sample with 3-7 day turnaround; routine in-house Kjeldahl plus Walkley-Black analysis costs 200-500 INR per sample but requires lab equipment and trained chemist. Indian plants serving SATAT contracts almost universally maintain an in-house lab for daily monitoring with NABL backup for compliance reporting and disputed batches.