Earth Science Earthworm Castings, 4 lb, are made with 100% earthworm manure. We never cheapen our product with lower cost fillers of any kind. Professional growers have relied on earthworm castings for years to improve the soil. Regular use of earthworm castings increases the water and nutrient holding capacity of soils and encourages plant growth. Earth Science Earthworm Castings are carefully harvested and packaged to provide the best results. They are an excellent supplement for all types of plants, whether in the ground or in containers. This product provides a good source of Nitrogen. Nitrogen encourages plant growth and helps provide foliage with its green coloring. Healthy soil contains and requires this nutrient to thrive.