my plant has a problem HELP PLEASE!!!!!!!!

i read all the faqs but still cant figure it out, i started my plant in normal soil DIDNT MIX WITH ANYTHING, I add MIRACLE GROW (liguid) on ocasion WITH THE WATER BUT NO OTHER NUTRIENTS I USE A 70 watt HPS LIGHT HUMITY IS 55% I WATER EVERY 3 DAYS THE LEAVES ARE TURNING YELLOW THEN BROWN AND SHRIVELING mainly starting at the bottom of the plant but if i look closely also in the middle its starting to happen the top of the plant is extremely healthy and has nothing wrong...WHY?????

SOME OF THE STEMS ARE PURPLE ON THE TOP ALSO COULD THIS BE A PROBLEM??? TEMP IS 85 ON 70 OFF

how old are your plants? are they flowering ? whats the npk of that miracle gro ?
Whats your ph?

It does look like a deficiency of P (Phosphorus)
to fix the problem make sure your ph is between 6 & 7 this is where phosphorus is absorbed the most.

the ph is right at 7 !!!!!!!!!!!!!!!!!!!! how do i add phos

also its not to flowering yet is about 1 month 10 days old

can you put up pics of the entire plant. Phosphorus absorbs best 6.2-6.5, id lower the ph to around 6.5. whats the npk of the miracle grow and how much of it are you using? at that age of growth a full fertilizer should have enough npk for the plant so id be guessing either over nute or too high ph.