To keep the dust down, lightly moisten the ingredients before blending them thoroughly in a dishpan or wheelbarrow.
This mix strikes a balance between moisture retention and drainage, both of which are necessary for seedlings. “Regulating the moisture is key,” Johnson says. “It’s easy for the soil to stay too wet, and that can lead to damping-off.” Damping-off is a fungal disease that causes newly germinated seedlings to topple over and die. Some flower seedlings—Johnson mentions pansies, snapdragons, ‘Gem’ marigolds, and lisianthus—tend to be more sensitive to too much moisture. For those, she makes a special batch of the mix, using less compost and replacing coir with peat moss. Sphagnum peat moss and perlite tend to lighten the mix and allow it to drain more quickly. Compost, vermiculite, and coir increase moisture retention.
The compost in Johnson’s mix is made mostly from shredded leaves and other garden debris—but she avoids any organic materials that might introduce weed seeds to the compost. Having compost in the mix means that seedlings rarely need to be fertilized until they are moved outdoors to the garden; the compost provides a constant mild feeding. Compost also counters the natural acidity of peat moss. In mixes that don’t include compost, add 1/4 teaspoon of lime for every gallon of mix.