How much worm castings should go into a potting mix, and what do they actually do?

Ten to twenty per cent by volume is the useful range for most container mixes. Below that the effect is marginal; above roughly a quarter the mix starts to hold too much water and compact, because castings are fine-textured and behave more like a clay fraction than like compost.

What they contribute is less about raw nutrition than the marketing suggests. The NPK content of castings is low and slow — typically around 1-0.5-0.5 — so they will not feed a hungry crop on their own. What they do supply is a broad range of micronutrients in plant-available form, a large and diverse microbial population, and humic substances that improve cation exchange capacity, meaning the mix holds onto nutrients rather than letting them flush straight through.

There is reasonable evidence that castings also improve germination rates and reduce damping-off in seedlings, probably through microbial competition with the pathogens involved.

Practical use: mix them through rather than layering, because a discrete layer creates a moisture discontinuity that roots struggle to cross. Add them at potting time rather than as a top dressing, where much of the benefit stays on the surface. For seed-starting mixes use the lower end, around ten per cent, since seedlings need little nutrition and good drainage matters more.

Buy or make them fresh. Castings that have dried into hard pellets have lost most of their microbial value and are essentially a mild mineral amendment. Good castings are dark, crumbly, uniformly textured and smell like woodland soil.

For hungry plants — tomatoes, heavy-feeding annuals — treat castings as a soil conditioner and still feed separately during the season.