Vermiculite

Vermiculite is a natural mineral that is heated until it opens up into light, golden-brown flakes that soak up water and release it slowly. Mixed into potting soil, it keeps the mix moist for longer while still letting air reach the roots, and it stops heavy soil from packing down. That makes it especially good for seedlings and young plants, which need steady moisture, and for plants that like evenly damp soil. Sprinkle a thin layer over newly sown seeds to keep them moist while they sprout, or mix it with coco peat for a gentle seed-starting mix. It weighs very little, so a small pack goes a long way. Store it dry.