Perlite also known as sponge rock is a very popular ingredient for bonsai soil mixes. The structure of the sponge rock helps create space for root growth, and aeration. The unique properties of perlite also allows it to absorb water and retain it for your bonsai tree. Most commonly paired with peat moss to create a base soil mix. 

Sifted Perlite Sponge Rock information:

• Amounts: 3 Dry Qt (Fills 8"-10" pot), 6 Dry Qt (Fills 12"-14" pot), and 12 Dry Qt (Fills 20" + pot)
• Ingredients: Small Perlite Sponge Rock
• Excellent for water adsorption
• Promotes aeration and space for root growth