Using vermicompost in nursery potting mixes

A nursery is less forgiving than a field

In a field, a lump of unscreened material is an inconvenience. In a seedling tray it is a failed cell. Root volume is tiny, and anything uneven in the medium shows up immediately as patchy germination and variable growth.

That is why screening, not nutrient content, is usually the first thing a nursery should ask a compost supplier about.

Start conservative on the ratio

Vermicompost normally forms part of the organic fraction of a mix rather than the whole medium. Too high a proportion can hold more water than a young root system can cope with. Start conservative, trial the blend on a small batch, and adjust from what you see rather than from a table.

• Trial any new blend on one batch before committing a season's trays
• Watch drainage as closely as growth in the first week
• Keep the mix consistent once it works, variability is the real enemy
• Record the ratio that worked, and the supplier and grade it worked with

Maturity is non-negotiable

Immature compost continues to decompose in the tray and can tie up nitrogen exactly when a seedling needs it. In a field this is a setback; in a nursery it is a loss. Only fully matured, screened material belongs in a potting mix.

Consistency is what you are buying

A nursery blend that works is worth protecting. Once you have it, the supplier's job is to deliver the same grade next time. That is exactly what a controlled production line and a screening step exist to do.