Compost Recipe and Application Calculator
Compost fails for two reasons — the wrong carbon-to-nitrogen ratio, or a rate applied by shovel-load instead of by tonne.
Compost is the cheapest fertility and structure money can buy on a banana block — but only when the recipe balances and the rate is deliberate. This calculator takes two materials (a nitrogen-rich "green" and a carbon-rich "brown"), works the C:N of the mix, tells you how much of the richer material to add to hit your target, then converts an application rate into tonnes, depth and the soil organic matter gain that follows. Batch sizing pairs with the fertiliser calculator for what compost cannot supply.
Compost recipe calculator
Two materials → C:N verdict, batch size, application tonnes and SOM gain.
Manure, fresh crop residue, kitchen waste, legume prunings
Cattle manure ~1.6–2 · poultry 1.6–3 · legume leaves ~2.5–4
Straw, dry leaves, sawdust, coconut husk
25–35 is the working band for hot composting
Application
5–15 t/ha typical; 10 t/ha ≈ a 5 mm organic layer
Plough or mulch layer — 150 mm typical
The formulas#
Total C = (green kg × green C%) + (brown kg × brown C%)
Total N = (green kg × green N%) + (brown kg × brown N%)
C:N = Total C ÷ Total N
Adjust green to hit target: add = (target × N − C) ÷ (greenC − target × greenN)
(positive → add green · negative → add brown or a carbon bulking agent)
Finished compost ≈ 50% of input mass (shrink from CO₂ and water loss)
Tonnes needed = rate (t/ha) × area (ha)
Depth (mm) = rate × 1000 ÷ bulk density (kg/m³) ÷ 10 ← 600 kg/m³ assumed
SOM gain (%) = rate × 0.4 (OM) × 0.5 (humification) ÷ soil mass of layer × 100Worked example#
400 kg green (4% N, 40% C) + 1,600 kg brown (0.6% N, 45% C):
Total C = 400×0.40 + 1600×0.45 = 160 + 720 = 880 kg
Total N = 400×0.04 + 1600×0.006 = 16 + 9.6 = 25.6 kg
C:N = 880 ÷ 25.6 = 34.4 (slightly carbon-rich)
Add green to reach 30: (30×25.6 − 880) ÷ (0.40 − 30×0.04) = −112 ÷ −0.8 = 140 kg
Apply 10 t/ha on 3 ha:
Tonnes needed = 10 × 3 = 30 t
Depth = 10,000 ÷ 600 ÷ 10 ≈ 1.67 mm (per application)
Finished compost from the 2 t batch ≈ 1,000 kgReading the results#
| Output | Why it matters |
|---|---|
| C:N of the mix | Below ~15: ammonia loss and stench; above ~40: the pile sits and sulks |
| Green to add | One number that fixes an unbalanced recipe — no guesswork |
| Finished compost | What the batch actually yields after shrink — plan inputs on this |
| Tonnes to deliver | What to source or spread — not "a few wheelbarrows" |
| Depth / SOM gain | Whether the rate moves soil health at all, or just dresses the surface |
Next steps#
- The system around compost: composting and organic matter guide.
- What compost cannot supply: fertiliser calculator and nutrition reference.
- Soil organic matter context: soil and land preparation.
- Organic certification limits: organic banana farming.