Earth Science Earthworm Castings are 100% vermicompost and never contain fillers of any kind. Worm castings are a natural fertilizer full of organic matter that feeds the beneficial microbes in your soil. Worm castings improve your soil structure so it can retain more water and nutrients, leading to healthier plants. Earthworm Castings can be worked into the soil at the time of planting or can be used as a top dressing for shrubs, flowers, raised gardens or potted plants. Earth Science Earthworm Castings can also be used to make compost tea or be added to compost piles.