Hydroponics - Rockwool as growing substrate

Don't know if you've seen this rockwool info.....might be worth a look through. I just am not sure about flushing the stuff.....it holds so damn much water.
Weedhound Reviewed by Weedhound on . Its Always Something... I think this is a phosphorus deficiency, but I am not totally sure. The edges are crispy and the new small leaves around the buds have yellow, crispy, turned down tips. The guide I read said a little P deficiency in flower is normal, but if P is what it is, I think I have more than a little. What should I do? PPM is 1150, pH is 5.6, double checked with liquid pH kit. They are starting Week 4 tomorrow. The last pic is what I would say is the worst looking one. Rating: 5