Yes you need nitrogen even in flower. All the way up to week 5 or 6 depends on strain and how long they flower. Then u stop nitrogen. But with those pH issues u r having could be why plant isn't asorbing enough nitrogen. U need better soil cuz starting at 8 is way to high. I can recommend roots organic soil which I use. Or before you plant your seed work out the pH in soil. I would not water under 6 pH. And every time u put any nutrients or even only water needs to be ph. If not your plant isn't getting all the nutrients it needs to stay healthy.