What to do Between Vegetative Growth and Flowering

Good growers know that when plants are in different phases of their life cycle (seedlings, vegetative growth and flowering) they have different nutrient requirements. 
Read my post on how to improve flowering
If you just read the above post, you know you want to reduce nitrogen, but not eliminate it when your plants are in flowering phase.  All living things need nitrogen, and plants are no acceptation.  Many fertilizers labeled “For Improved Flowering” have no nitrogen, and I think this is a mistake.  Nitrogen is used by plants in all phases of the life cycle.  While nitrogen promotes vegetative growth in large quantities eliminating it will slow growth, and lower yields.  Everything in Biology follows the Goldilocks principle, not too much, not too little, just the right amount.  When my plants are flowering, I use a fertilizer that has a low number for nitrogen, like Big Swellwhich is 2-5-7.    
If plants are green, healthy looking and over all doing well when you alter lights for flowering) just use something like Big Swell2-5-7.  If you have brown leaf tips, stretching nodes, and reduced root growth you may have a lot of nitrogen and you should use something like Big Bud which contains no nitrogen after the plants dry out from  thoroughly  being watered with pure water.  After this, I'd go with a now nitrogen fertilizer for the rest of the flowering cycle.  Hydroponics like soil growers should always provide some nitrogen to their plants.  It is normal that yellowing should start to occur in flowering, this is a sign the plants are focusing on flowering, not making new leaves.  Be sure to give the plant a bit of nitrogen to keep it growing well through the flowering phase, which for most of us, is the most important part of plant growth.
Good Growing,
Dr. E.R. Myers