Flushing question

I'm starting some girls in 3 gal containers using 600 watt hps with Pro-mix and 1/2 cup of lime.
Using FoxFarm schedule which states I should flush 3 times throughout grow? Is that necessary with lime and Pro-Mix or is that more for hydroponic?
Any help please!!

promix doesn't have any nutrients in it normally(there is a type i think that does, but normally it doesnt)...so yes follow the fox farm instructions