SOIL NOTE · COMPOST
How much compost
does a greenhouse need?
There is no magic wheelbarrow number. Start with the bed, the crop and the condition of the growing medium. Percy’s answer begins with looking, not ordering.
First ask what the compost is doing
Compost may be part of a growing medium, a source of organic matter or a way to refresh a tired bed. Those are different jobs. The right amount depends on the material, the existing soil or medium, the crop and whether the bed is in the ground or a container.
Read the product instructions and the crop guidance before spreading anything. If the bed already drains well and the crop is growing happily, more is not automatically better.
Calculate the volume in litres
Measure the bed’s length and width in metres, then decide the depth of the layer you intend to add. Multiply length × width × depth in centimetres × 10 to get litres.
Example: a bed 2 m long and 1 m wide, with a 5 cm layer, needs 2 × 1 × 5 × 10 = 100 litres, or two 50-litre bags.
The 5 cm is an example for the calculation, not a prescription for every bed. These are example dimensions, not measurements of James’s greenhouse.
For filling a new bed, use the intended full depth in the same formula. A 2 m × 1 m bed filled to 30 cm holds 600 litres. That does not mean it should contain 600 litres of pure garden compost: choose a suitable soil and organic-matter mix for a soil-based bed, or a growing medium designed for containers. Allow for existing material, headroom and settling.
Garden compost and potting compost have different jobs
Garden compost is decomposed organic material used to improve soil. Peat-free potting compost is a prepared growing medium for containers. Check which material a recommendation means before using its volume.
Use the bed as the unit
Measure the area you are actually going to improve, rather than buying for the whole greenhouse by instinct. Work in a small section, mix or apply the material as directed and observe what happens. That gives you a repeatable record for the next bed.
- Identify the crop and its growing position.
- Check drainage and existing moisture before adding material.
- Use the product’s stated application guidance.
- Keep fresh compost away from stems unless the crop guidance says otherwise.
- Record the material, date and area so the result can be compared.
Do not confuse compost with a watering plan
A new layer of material does not remove the need to check moisture. Percy’s first instruction is still the same: finger in the growing medium first. The watering can can wait.
“We do not need to feed the whole village.”
The honest answer
For a small first season, buy and use only what the actual bed and crop need. Avoid a blanket promise such as “one bag per bed” unless the product, bed depth and crop make that calculation meaningful. A measured small improvement is easier to learn from than a heroic pile of compost with no record.
Further reading: RHS mulches and mulching. Product instructions, crop requirements and the actual condition of the bed take priority.