Potting mix eliminates poor soil conditions and disease. Water moves from the reservoir up through the dry potting mix by capillary action, in which the potting mix acts as a wick. As water adheres to small particles of the potting mix, it slowly works into the small spaces between these particles. As water adheres to additional particles, it is continuously drawn from the reservoir. When the potting mix is saturated with water, the plants use precisely as much water as they need. As a plant absorbs water from the potting mix, more water is pulled from the reservoir.