Clay Pebbles, Clay Balls or Lightweight Expanded Clay Aggregate - The only thing you need for a simple semi-hydro set-up (except water of course). The small balls of clay wick water upwards, meaning a little pool of water at the bottom of a pot made for semi-hydro can hydrate all the way to the top! The balls also provide massive chambers of oxygen for roots to grow super strong which you will see the rewards of in your foliage. Conversely, you can add clay balls into soil for extra water retention in your mixture.