Soil Organic Matter Calculator: Compost to Raise OM %
Organic matter drives soil life, structure and water holding, and soil tests report it as a percentage. Raising it by a few points takes far more compost than most people expect, because the top 20 cm of soil weighs over a quarter of a tonne per square metre.
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What this calculator does
Enter your area, depth, current and target organic matter and your compost's details. The calculator gives the compost volume, layer depth, weight and bags.
How to use the soil organic matter calculator
- Enter the area and the depth you will mix compost into.
- Enter your current organic matter from a soil test, and your target.
- Leave the soil and compost settings at typical values unless you know better.
- Read the compost needed and the layer to spread.
- Spread it over two or three seasons if the layer is very thick.
How the calculation works
soil weight = area × depth × bulk density dry compost = soil × (target − current) ÷ (compost OM − target) compost as delivered = dry ÷ (1 − moisture) volume = weight ÷ compost density; layer = volume ÷ area
This is a mass balance: the organic matter in the soil plus the compost, divided by their combined dry weight, equals the target.
Worked example
Raising 10 m² from 2% to 5% organic matter in the top 20 cm (2.6 t of soil) takes 173 kg of dry compost, 289 kg as delivered at 40% moisture: about 0.58 m³, a 5.8 cm layer, or 12 bags of 50 L.
Assumptions and limitations
- Organic matter decomposes; expect to top up with a few centimetres of compost each year.
- Compost organic matter and moisture vary; a supplier analysis improves the estimate.
- Above about 8–10% organic matter, extra gains are small for most crops.
Frequently asked questions
What is a good organic matter percentage for garden soil?
Around 3–6% is good for most vegetable gardens; many cultivated soils start at 1–3%.
How much compost does it take to raise organic matter by 1%?
For the top 15–20 cm, typically a layer of 1.5–2.5 cm (0.6–1 in) of compost, depending on compost quality and soil density.
How fast does soil organic matter break down?
In warm, tilled soils several percent of it can break down each year, which is why regular additions matter.
Is compost the only way to add organic matter?
No. Cover crops, mulches, leaf mould and manure all add organic matter over time.