Soil first: what castings do that fertilizer does not

Synthetic fertilizer feeds the plant directly, which produces a fast visible result and does nothing for the ground underneath. Castings feed the biology in the soil, which then feeds the plant on the plant's own schedule.

The first approach is a loan. The second is a deposit.

A generous handful in each transplant hole in spring, and a half-inch top dressing on established beds. Water it in. That is genuinely the whole practice.

You cannot burn a plant with castings, which makes them one of the few garden inputs where being enthusiastic does no harm.