i am not too sure there is a way to fertilize for root growth! from my short growing experience, good root growth comes from not over watering early on, let the plant show you it needs to be watered, this will cause the roots to grow and search for water till it gets fatigued, then when you water it, it absorbs much more and you will see a nice growth spurt. there are natural bacteria that you can put in your soils that help make your soil root friendly, but i think correct watering habits will give you the best root growth! i still fertilize but i do it for foliage or bloom growth. this has just been my experience and i could be compltely wrong by thought i would share what i have noticed.