I don't know if I would go so far as it is "needed" for soil but it does help. Silica benefits the plant in any style of grow. For a cheap and easy way to add silica to your soil, use rice hulls. The also provide excellent drainage. Just be sure to get the par boiled ones. If you use just regular rice hulls, you can end up with lots of little rice seedling popping up in your soil, plus they can transmit pests and diseases.