Direct from the IWS site
QUOTE
We have recently run tests of the best growing medium to use and found expanded clay pebbles the best with the benefit of many feeds through out the light cycle. 60% clay / 40% coco was a close runner up with less feeds but using the benefits of coco fibre as a buffer against root diseases.
When using only expanded clay pebbles, always wash pebbles and, where possible, leave to soak in ph balancedwater. When feeding use a hydro based feed and set your timer to feed every 5-6 hours on 18 hours and every 4-5 hours on 12 hours. Never let the root zone dry out or be over saturated as this could damage it.This is only a guide line and we would suggest that you feed your
plants when your medium dries out to about 20% moisture.
When using coco/pebble mix, add a good layer of straight pebbles to the bottom of each inner pot (the ones with loads of holes in the bottom) and then top up the pots with your mixed growing medium. When feeding, saturate your growing medium and allow to almost dry out until feeding again.
When feeding, use a coco based nutrient and as a rule feed once or twice a day on 18 hours and up to 3 times a day on 12 hours. Never let your growing medium dry out as this could cause damage to the root zone. Check with your local retail outlet for instructions on your chosen growing medium.