Looking - pre-flowering NUTRIENT Knowledge

I am at 8wks and in another three I am sending my babys into pre-flowering. My worries are that my plants are over 10.5in but the biggest is 17in, and are 8wks in??? Is that ok? I have a 4ft long, 4ft tall, 14in wide grow rig pumping 15,000 of lumens in with dual fans intake and out, they're in Miracle Grow soil, 24qt pots, tap watered every 3-6 days, club soda misted daily. I feel as though they should be taller at this stage. I was wondering what the method of upping the phosphorus in the soil during flowering would be without chemical burning it? Thanx for spreading the word
Banksybuds

I am thinking you light output is kinda low. If your thinking of adding your own individual fertilizer in MGrow, I would flush the soil just to make sure that there will be no over fertilizing check your soil PH leave for 2 to 3 days and see what they do. Depending on your feeling and relationship with your plants you can add 1/4 Strength Mono-potassium Phosphate 0.52.34. and watch for a few more days. All in All check your reading on your meters a amend the soil accordingly.

Rasta Fari Love