Worm Castings for Indoor Plants Avoid: Worm castings = worm poop. But "casting" is a more polite way to describe the digested waste of your garden-grade earthworm, so we'll stick with it! Worm castings, unlike animal manure, provide plants and plants with rich nutrients and healthy bacteria, and are the secret superfood to help your indoor plants thrive. Five benefits of worm castings for indoor plants and how to make your own worm compost. Nutritious plant foods Worm castings contain a mixture of bacteria, enzymes, remains of plant material, and broken down animal manure (and other existing compost). These castings are rich in plant nutrients and contain 50% more humus (broken leaves and plant material) than regular topsoil. Castors also contain plant-essential minerals such as nitrates, magnesium, phosphorus, calcium and potassium. They contain manganese, copper, cobalt, borax, zinc, iron, nitrogen and carbon. Safer than chemical fertilizer and animal manur