Vermicompost is at its best in a mix with other things. It supplies food and microbes, while cocopeat, soil or sand supply the lightness and drainage. The question is how much.
Shares of vermicompost in a mix, by volume
| Source | Share |
|---|---|
| NC State Extension | 20% (one part to four) |
| Oregon State University Extension | Up to 25% |
| University of Maryland Extension | No more than a quarter, used sparingly |
| The Composta, seedlings and nursery bags | 10 to 20%: one part to four to nine parts soil or cocopeat |
| Blouin and others (2019), yield gains | Best at 30 to 50% of soil volume |
Why the share is capped
The University of Maryland warns that pure castings can contain a lot of soluble salts, which can harm seedlings. In one trial with peat, the best growth came at 25 to 50 percent vermicompost, but that vermicompost had a pH of 8.8, and it needed the acidity of the peat to balance it. A mix that suits one vermicompost may not suit another.
A mix for seed trays
- Fill a bucket with cocopeat, or cocopeat and fine soil, that has been wetted and squeezed.
- Mix in vermicompost at 10 to 20 percent of the volume, so that it is spread evenly and not in lumps.
- Fill the trays, sow, and keep them evenly damp and out of harsh sun.
- Try one tray first before you fill a whole batch, and compare it with a tray of plain mix.
Cornell's extension team found that vermicompost can replace synthetic fertiliser in potting media for vegetable transplants, and that one facility's product protected cucumber seedlings from a Pythium disease. That was a property of that product and not of vermicompost in general.
Common questions
What percentage of vermicompost should I put in potting mix?
Extension services suggest 20 to 25 percent by volume. For seed trays and nursery bags, 10 to 20 percent is the usual share.
Can I start seeds in pure vermicompost?
It is not advised. Pure castings can be high in soluble salts. Mix it with cocopeat or soil and keep the share modest.
What is the ratio of cocopeat to vermicompost?
For seedlings, one part vermicompost to four to nine parts cocopeat or soil. For larger pots, up to about one part in four.
Does vermicompost need sieving for seed trays?
Fine, even material mixes better and the Fertiliser (Control) Order expects 90 percent of a pack to pass a 4 mm sieve. A finer sieve helps for the smallest seeds.
Can vermicompost burn seedlings?
It is much milder than raw manure, but too much can raise the salt level around young roots. Keep to the shares above.
Terms in this guide
Sources
- NC State Extension: vermicomposting in childcare center gardens
- Oregon State University Extension: using compost
- University of Maryland Extension: worm composting
- Blouin and others (2019): vermicompost effects on plant yield, a meta-analysis
- Cornell Waste Management Institute: vermicompost
- Belda and others (2013): vermicompost in peat-based growing media
- Fertiliser (Control) Order, 1985, Schedule IV (consolidated copy)