Calculator
Enter lab or field measurements. If you do not have a value, keep the default. The index combines stability, plant safety, and salinity indicators.
Example data table
Sample scenarios to show how maturity can vary by indicator balance.
| Scenario | ΔT (°C) | C/N | Resp | NH4/NO3 | GI (%) | EC | Days | Index | Class |
|---|---|---|---|---|---|---|---|---|---|
| Home pile, still active | 18 | 28 | 8.5 | 3.2 | 55 | 3.8 | 12 | 33 | Immature |
| Curing in bin | 9 | 22 | 5.2 | 1.4 | 72 | 2.9 | 20 | 52 | Curing |
| Finished compost | 3 | 17 | 2.6 | 0.6 | 88 | 2.2 | 35 | 75 | Mostly mature |
| Seed-start mix grade | 2 | 15 | 1.8 | 0.3 | 95 | 1.6 | 55 | 90 | Mature & stable |
| Salty manure compost | 4 | 14 | 2.9 | 0.7 | 84 | 6.8 | 45 | 61 | Mostly mature |
Formula used
This calculator converts each maturity signal into a 0–100 sub-score using practical thresholds (better stability and plant-safety equals higher scores). It then computes a weighted average.
Included indicators: temperature stability (|ΔT|), C/N ratio, respiration rate, NH4/NO3 balance, germination index, moisture, pH, electrical conductivity, and curing time.
How to use this calculator
- Measure compost core temperature and the surrounding ambient temperature.
- Enter lab values if available: C/N, respiration, NH4-N, NO3-N, pH, and EC.
- Add the germination index from a simple seed test, if possible.
- Input moisture and the number of curing days since active heating ended.
- Click Calculate maturity and review the index and weak indicators.
- Download CSV or PDF to save results with your batch notes.
Compost maturity guide
Use these notes to interpret the index and improve your curing results.
1) Temperature stability and curing stage
A mature pile stops reheating after turning and stays close to ambient temperature. In practice, a core-to-ambient difference (|ΔT|) under 5°C often indicates the active phase has ended. If |ΔT| stays above 15°C, microbial activity is still strong and curing should continue with regular aeration.
2) C/N ratio as a stabilization signal
Carbon-to-nitrogen ratio typically narrows during decomposition as carbon is lost as CO2. Many finished composts fall between 10 and 20. Ratios above 25 can increase nitrogen drawdown in soil, especially when applied fresh. Balance high C/N inputs with greener materials and extended curing.
3) Respiration rate and biological activity
Respiration (mg CO2-C/g VS/day) is a direct stability metric: lower values mean fewer readily degradable compounds. Values near 2–4 suggest a mostly stable product, while values above 6 often correlate with ongoing heat, odor risk, and unstable organic fractions. Turning and time reduce respiration.
4) Nitrogen form balance and seed safety
A declining NH4/NO3 ratio usually reflects nitrification during curing. High ammonium can irritate roots and suppress germination, while nitrate is generally less phytotoxic. Germination index (GI) combines emergence and root growth; GI ≥ 80% is commonly acceptable, and GI ≥ 90% is preferred for seedlings.
5) Salinity, pH, and practical use rates
Electrical conductivity (EC) indicates soluble salts. Compost made from manures or ash can exceed 4 dS/m, which may stress salt-sensitive crops. Aim for pH near 6.5–8.0. If EC is high, blend with low-salt materials and apply in thinner layers while monitoring plant response.
FAQs
1) What maturity index should I target for vegetables?
For most vegetable beds, aim for an index of 60–80 or higher. If you transplant seedlings, prefer 80+ and confirm stable temperature and a strong germination index.
2) I don’t have lab tests. Can I still use this?
Yes. Use temperature, curing days, moisture, pH, EC, and a simple germination test. Leave lab-only fields at default values, then focus on improving the lowest sub-scores.
3) How do I estimate the germination index?
Soak compost extract (1:10 compost:water), germinate fast seeds, and compare to water-only controls. GI reflects both percent germination and root length relative to the control.
4) Why is my EC high even when compost looks finished?
Salts can remain after decomposition, especially with manure, seaweed, or ash inputs. High EC may still stress plants. Blend with leaf mold or soil and apply lightly.
5) What moisture range is best during curing?
Keep moisture around 40–55%. Too wet limits oxygen and raises odors; too dry slows microbial finishing. If material feels dusty, add water while turning for even wetting.
6) Can I use compost with a low C/N ratio?
Often yes, but check salts, pH, and respiration. Very low C/N can occur in manure-rich composts, which may also have higher EC. Use the index to balance risks.
7) How often should I retest maturity?
Retest after 10–14 days of additional curing, or after any major change like re-wetting or adding fresh feedstock. Consistent scores across checks indicate true stability.