Earthworm castings contain abundant essential elements plants need for healthy growth, but all worm castings are not created equal! Castings rival chemical fertilizers in their nutrient composition, providing a concentrated source of calcium, magnesium, and nitrogen. Outdoors, line the bottom of your hole or furrow with three inches of castings. Indoors, use one part castings to three parts potting soil. Farmed right here in Minnesota, Paramount Green worm castings offer unsurpassed consistency and quality.