The carbon-to-nitrogen ratio of a mix is its total carbon divided by its total nitrogen, so bringing humanure at about 8:1 up to the target of 25-35:1 needs cover whose carbon is 2.3 to 3.7 times the waste’s own carbon when the cover is sawdust at 400:1, and 3.1 to 6.0 times when it is straw at 80:1. Weaker cover carries nitrogen of its own, so more of it is needed to reach the same ratio.
Part of Composting Toilet Systems
The carbon-to-nitrogen (C:N) ratio compares the carbon that microbes burn for energy with the nitrogen they need to build cells. Human waste is nitrogen-rich, so every deposit needs carbon-rich cover to bring the mix into the range where a compost pile heats.
Every Figure Here Comes From The Parent Page
This page adds arithmetic, not new data. The inputs are the parent page’s: humanure at a C:N of about 6-10:1 (8:1 used as the middle), a target of 25-35:1, sawdust at about 400:1, wood shavings about 300:1, straw about 80:1 and dry leaves about 50-80:1 (65:1 used as the middle). No outside tolerance is introduced. Ratios are by element, not by weight of the material.
The Mix Ratio Is A Weighted Sum
For waste with nitrogen N_w and carbon ratio W, and cover with nitrogen N_s and carbon ratio S, the mixed ratio R is:
R = (W × N_w + S × N_s) ÷ (N_w + N_s)
Solving for the cover nitrogen needed per unit of waste nitrogen:
N_s ÷ N_w = (R − W) ÷ (S − R)
Worked Example: Sawdust Onto Humanure
Inputs: W = 8, S = 400, target R = 30.
- N_s ÷ N_w = (30 − 8) ÷ (400 − 30) = 22 ÷ 370 = 0.0595. The sawdust adds only about 6% as much nitrogen as the waste.
- Carbon from the sawdust = S × N_s = 400 × 0.0595 N_w = 23.8 N_w.
- Carbon in the waste = 8 × N_w = 8 N_w.
- Cover carbon ÷ waste carbon = 23.8 ÷ 8 = 2.97, about 3.0.
Check: (8 + 23.8) ÷ (1 + 0.0595) = 31.8 ÷ 1.0595 = 30.0.
The Same Calculation Across The Parent Page’s Covers
Cover carbon as a multiple of the waste’s carbon, with W = 8:
| Cover (C:N) | R = 25 | R = 30 | R = 35 |
|---|---|---|---|
| Sawdust (400) | 2.3 | 3.0 | 3.7 |
| Wood shavings (300) | 2.3 | 3.1 | 3.8 |
| Straw (80) | 3.1 | 4.4 | 6.0 |
| Dry leaves (65, middle of 50-80) | 3.5 | 5.1 | 7.3 |
Weaker covers need a larger multiple because they bring their own nitrogen: at R = 30, straw’s N_s ÷ N_w is (30 − 8) ÷ (80 − 30) = 0.44, over seven times the sawdust figure. That is the arithmetic behind the parent page’s note that dry leaves “need more volume than sawdust”.
The Waste’s Own Ratio Moves The Answer
The parent page gives humanure as 6-10:1. For sawdust at R = 30:
- W = 6: N_s ÷ N_w = 24 ÷ 370 = 0.0649; cover carbon = 400 × 0.0649 ÷ 6 = 4.3 times the waste carbon.
- W = 10: N_s ÷ N_w = 20 ÷ 370 = 0.0541; cover carbon = 400 × 0.0541 ÷ 10 = 2.2 times the waste carbon.
So the required cover for sawdust spans roughly 2.2 to 4.3 times, a factor of two, from the waste’s own variation alone. Diet changes it.
What The Arithmetic Cannot Tell You
- Quantities of cover. Converting “3 times the carbon” to litres needs the waste’s nitrogen content, the sawdust’s moisture and its nitrogen content, none of which you can measure. Use the parent page’s practical rule, a covering of at least 2-3 cm over every deposit, and use the ratio to diagnose.
- Availability. A ratio counts all the carbon. Microbes cannot use woody carbon as fast as soft carbon, so a mix that adds up to 30:1 on paper can behave like a nitrogen-richer pile. I am recalling this general composting behaviour, not a figure from the corpus. Treat the table as a floor, not a target.
- Urine. Composting Basics lists urine at about 0.8:1, very nitrogen-rich. Urine in the mix pulls the ratio down quickly, which is one reason Urine-Diverting Systems help.
Reading The Pile Is The Real Check
- Ammonia smell, wet and slumped: too much nitrogen. Add dry cover.
- Dry, bulky and cold: too much carbon or too dry. The parent page says below 40°C suggests a carbon-heavy or dry pile; add nitrogen and moisture.
- Earthy, loose, heating within 1-3 weeks: in range.
Common Mistakes
- Treating the ratio as weight. It is carbon atoms to nitrogen atoms; the weights of the materials are different.
- Using one ratio for all sawdust. The parent page’s 400:1 is a typical figure; fresh, wet or sappy sawdust differs.
- Computing to three decimals. The inputs are good to about one significant figure; the table is a guide to proportion.
Safety
The mixture is human waste
Adjusting a mix means handling it. Wear gloves, work upwind of ammonia, never put hands into a bucket or bin without gloves, and wash hands with soap or ash and water afterwards.
What’s Next
- Learn next: Sawdust, Straw & Leaf Mold — choosing and storing the cover
- Combine with: Thermophilic Decomposition — the heat the right ratio produces