Brunnings Hydro Clay Pebbles are a baked clay balls made by baking and tumbling natural clay at high temperatures. As a result, they are super lightweight and porous with high air content. Hydro Clay Pebbles have a neutral pH value, are fungus free and will not rot. They also absorb moisture when soaked in water. This is what makes them an ideal growing medium for selected plants, a top soil dressing or a drainage device for plants growing in potting soils.