LECA 8/16mm - (Lightweight Expanded Clay Aggregate) Lightweight Expanded Clay Aggregate (L.E.C.A.) a type of clay growing medium which is super-fired to create a porous texture. Grow rocks are a non-degradable, sterile growing medium that holds moisture, has a neutral pH, and will wick up nutrient solution to the root systems of your plants. The lightweight roundish clay balls of various sizes are great for hydroponic growing substrates. Water flows between the pebbles quickly, supplying the roots with nutrients, yet allowing the roots to stay oxygenated. Roots and plants develop healthy and vibrant. LECA has high permeability, high durability and excellent sound and thermal insulating properties.