Compost Builder

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Overview

The Compost Builder helps you design a fungal-dominated compost heap using locally available waste streams. Enter the feedstocks you have access to, set a C:N ratio target, and the calculator will work out the volumes you need.

1. Choose a Compost Target

Select one of the four target cards at the top. Each card represents a different C:N ratio range suited to a specific composting goal:

• Bacterial (15–20 : 1) — annual crops, vegetables, fast nutrient cycling
• Balanced (20–25 : 1) — mixed systems, transitional orchards
• Fungal (25–30 : 1) — orchards, perennials, forest systems
• Highly Fungal (30–40 : 1) — mature orchards, woody mulch systems

The selected target sets the C:N range the calculator aims for. The gauge and status indicator update in real time as you add materials.

2. Set Heap Dimensions

Choose the heap shape: a windrow with straight sides, a windrow with a rounded top, or a round heap (cone). Enter the length measured on the ground from one end to the other, the base width (or base diameter for a round heap), and the height, all in metres. The calculator shows the heap volume automatically.

For straight sides and round heaps, the calculator also needs the angle of repose: the natural slope a material settles at when it is dropped from a loader bucket. The ends of a windrow slope at the same angle as the sides. If the height is more than the angle allows, the calculator works out the volume at the highest the heap can stand and tells you. If the height is less, the heap has a flat top.

The calculator suggests an angle from your feedstocks, weighted by their volume in the mix. You can accept it or type your own. Sourced angles: bagasse 45° (Engineering ToolBox); dry leaves 42° and layer manure 50° (Cornell Waste Management Institute); wood chips 47° and kitchen waste 41° (midpoints where the two sources differ). Other feedstocks have no sourced angle and are left out of the suggestion.

3. Add Feedstocks by Priority

Feedstocks are entered one at a time in order of priority — cheapest or most readily available first.

1. Select a material from the dropdown (or choose “Add new material…” to enter a custom feedstock with its own C:N ratio)
2. Enter the volume available in cubic metres
3. Click Add
4. The material appears in the priority list with a priority number. A new dropdown row opens for the next feedstock
5. Repeat until all your available feedstocks are entered

Materials already in the list are removed from the dropdown so you cannot add duplicates. Use the × button to remove a feedstock from the list.

4. Auto-Calculate

Press Auto-Calculate to let the tool work out the optimal volumes. The algorithm follows your priority order:

1. It uses all of the highest-priority feedstock first
2. Then adds as much of the next priority as needed
3. It continues down the list until the C:N ratio falls within your target range
4. When a partial volume of a feedstock brings the ratio into range, it calculates the exact amount needed

If the Heap Cap is on, the total will not exceed your heap volume. The algorithm may swap volumes between feedstocks to fit within the cap while still hitting the target ratio.

5. The C:N Gauge

The gauge shows your current C:N ratio in real time. The coloured zone represents your selected target range:

• Green zone — your mix is within the target range
• Left of green — too nitrogen-heavy (add more carbon materials)
• Right of green — too carbon-heavy (add more nitrogen materials)

The status text below the gauge tells you whether you are on target, and suggests which direction to adjust.

6. Editing Results

After auto-calculating, the Use Volume column in the results table is editable. Click or tap a value and it is selected, so you can type the new figure straight over it. The C:N ratio, gauge, and amendments update live as you type. Press Enter or click away to finish.

7. Amendments

The amendments section shows recommended additions based on your heap volume:

• Rock Phosphate — 2% of heap volume
• Agricultural Lime — 1% of heap volume
• Liquid IMO3 (LIMO) — 15 L per m³ of heap

These values adjust automatically when you change the heap dimensions or toggle the heap cap.

Tips

• Enter feedstocks in order from cheapest (free waste streams) to most expensive — the auto-calculator maximises use of your cheapest materials
• Use the heap cap when you have a fixed windrow size and want to fill it optimally
• Turn the heap cap off when you want to find out how much material you need regardless of heap size
• Custom feedstocks require a C:N ratio — if you do not know it, check a reference table or laboratory analysis
• Biochar is entered as a percentage of the total mix (up to 20%), not as a volume. You can change the percentage in the results table. It is counted for volume only: about 97% of its carbon is stable and is not available to the microbes in the heap, so it does not change the C:N ratio
• Press Reset All to clear everything and start a new calculation