From our friends at Greenhill Worm Farm, worm castings are an all-natural soil amendment made exclusively by composting worms. Unlike other worms that burrow deep in the ground, composting worms stay near the surface, consuming dead or decaying organic material. As they feed, worms leave behind castings packed with nutrients and beneficial organisms plants love. Worm castings look like fine, dark particles of soil, and have a rich, earthy smell.