im not sure if flushing the mg soil will help, but it seems like it would/should. give it a really good flushing(lots of water) it would be just like flushing if you had over fertilized right? then if you were going to fertilize, for veg. use something high in n and lower in p&k. for flowering use something high in p and lower in n&k. my understanding is that n is for leaf/plant growth, p is for flowers/fruit growth, and k is for root growth. btw i found a place that has the best cheapest soil around. ACE hardware has awesome potting soil and compost, and its only around 3$ for a 40 lb bag. and its organic.