Wiggle Worm Soil Builder PURE Worm Castings are an organic fertilizer that, because they are pure, when a small handful is added during transplant you could see plants that are 3 times larger and more fruitful. This makes it extremely cost effective. Wiggle Worm Soil Builder has been on the market for 4 decades. It's OMRI and CDFA certified and a registered organic fertilizer.