Guide
Mel’s Mix, and why it is measured by volume
The three parts, what each one does, what you can substitute, and how much your bed needs.
The recipe
One part compost, one part peat moss or coco coir, one part vermiculite or perlite: by volume. Use the same bucket for all three and it is hard to get wrong. A wheelbarrow of each, a bucket of each, it does not matter as long as the measure is the same.
| Part | Job | Substitutes |
|---|---|---|
| Compost | Feeds the plants and holds moisture. The only part with nutrition in it. | Any mature compost; two or three kinds is better than one. Never use fresh manure. |
| Peat moss or coir | Holds water and keeps the mix light and open. | Coco coir (re-wet it first), or leaf mould in part. Peat is acidic, which suits most vegetables. |
| Vermiculite or perlite | Drainage and aeration. Stops the mix compacting into a brick. | Either works; vermiculite holds more water, perlite drains faster. Coarse sand is a poor substitute. |
Why volume and not weight
Compost is wet and heavy, peat is light and dusty, vermiculite is almost weightless. Equal weights of the three would give you a mix that is mostly vermiculite by volume and would not hold water or feed anything. Volume is also how you will actually measure it in a garden, with a bucket.
How much for your bed
The calculator gives the litres of each part from your bed dimensions:
Open the raised bed calculator
Cost and feeding
“The vermiculite is expensive.”
It is, and it is still the part not to skip. A mix of compost and peat alone collapses into an airless mass within a season. Perlite is usually cheaper than vermiculite and works fine where drainage matters more than water retention.
“Does the mix need fertiliser?”
In the first season, no: the compost carries it. From the second season, a balanced organic fertiliser before planting and a mid-season top-up for heavy feeders like tomatoes and courgettes is worth it.