Never use soil during any aspect of hydroponics. If you ever move a plant from a soil-based situation to hydroponics, remove all traces of soil or potting mix from the roots. Soil holds lots of microbes and other organisms and materials that love to grow in and contaminate your hydroponic system. Some of these will actually parasitize your plant and slow its growth. This is another advantage of hydroponic growing: The plant can get on with growing without having to support a myriad of other organisms as happens in conventional soil growing.