"TL;DR: Smart soil is vermiculite, a widely used hydroponic growing medium. You can find it at most garden centers or plant nurseries, typically in the gardening section.
What grade to buy
Vermiculite comes in fine, medium, and coarse grades. For hydroponic systems and indoor herb gardens, choose fine or medium grade. Coarse grade is better suited to outdoor garden beds and will not perform as well in your system.
Why vermiculite works well for hydroponics
It holds moisture well while still allowing air to reach the roots. That balance of water retention and aeration is what makes it a reliable choice for seed starting and ongoing plant growth indoors."
Apr 29, 2026