Soil Amendment Calculator: How Much to Mix In
Mixing compost or grit into existing soil is the classic way to improve clay or sandy ground. The question is how much: a light sprinkling does little, while too much can create a sinking, over-rich bed.
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What this calculator does
Enter your area, the depth you will dig and the share you want the amendment to make up. The calculator tells you how thick a layer to spread, the volume and the bags to buy.
How to use the soil amendment calculator
- Enter the length and width of the area.
- Set how deep you will dig or till the amendment in.
- Choose the share of amendment you want in the final mix.
- Enter your bag size.
- Spread the layer evenly, then dig or till it in to the full depth.
How the calculation works
layer to spread = dig depth × amendment share volume = area × layer bags = ceil(volume ÷ bag size)
If the amendment should make up 25% of the top 20 cm, spread a 5 cm layer and mix it into the full 20 cm.
Worked example
To make the top 20 cm of a 5 × 2 m bed one-quarter compost, spread 5 cm of compost (0.5 m³) and dig it in: 10 bags of 50 L.
Assumptions and limitations
- Simple volume blending; it ignores soil fluff and settling after digging.
- For clay soils, organic matter helps more than sand, which can make clay set harder.
- Have soil tested before adding lime, sulfur or fertiliser; this tool covers bulk amendments only.
Frequently asked questions
How much compost should I mix into my soil?
For new beds, mixing in a layer of 5–8 cm (2–3 in) of compost is common, roughly 25–30% of the top 20–25 cm.
Should I add sand to clay soil?
Small amounts of sand can make clay worse. Organic matter such as compost is the safer way to improve structure.
How deep should amendments be dug in?
The top 15–25 cm (6–10 in) holds most roots of vegetables and annuals.
Can I add amendments without digging?
Yes. Spreading compost on top every year lets worms mix it in; this is the no-dig approach.