castings are rich in water-soluble plant nutrients, and contain more than 50% more humus than what is normally found in topsoil. growth, such as concentrated nitrates, phosphorus, magnesium, potassium and calcium. It also contains manganese, copper, zinc, cobalt, borax, iron, carbon and nitrogen.
Castings do contain 4% to 5% more nitrogen than your average garden soil, but in a slow release form due to the mucous the worms secrete as they digest. (SIC)
Yeah I'd top dress. Get a 20 liter bag or worm castings and top off each pot. Cheap. Like 12 bucks.