New grow cabinet and Plant deficiency help

Just started growing, first and only plant I have going now under a 150w HPS 15,000 Lumens and 3 23w 1,600 Lumens. About 20,000 Lumens total. I've got the HPS lamp about 10 inches away and the 3 CFL's are 2-3 inches away from the plant. Its about 3-4 weeks from germination, transplanted from a Party cup to a 10"x10"x12" square pot 2 days ago and now I'm experiencing drooping and a little discoloration, I believe it may have been a combination of shock and over watering because I watered 2 days in a row.
So here's the setup:

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6 inch 240 cfm exhaust fan and intake fan (Havent been leaving the intake fan on because of tempurature problems, it's Winter almost)
I'm also having low humidity problems, been using damp perlite in a separate bowl to attempt to raise it, which doesn't seem to be working too well.
The full dimensions of this cabinet is 3 1/2' x 2' x 6' with a middle divider for 2 sides making each side 1 3/4' x 2' x 6' about 21 square feet each side, its made out of ply wood and flat white paint, I believe the plywood might be posing a problem with being too cold, the soil may be too cold for optimal root growth.
Here's the plant, temperature and humidity:
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Basically what I'm looking for here is a little advice, this is my first grow and I built this cabinet from scratch. My main problem right now is keeping an adequate temperature and humidity due to the cold and dry winter air, its about 30 degrees outside now and I've only got the exhaust fan running, pulling cold air from the intake fan which is not running because it gets way too cold, it settles at about 64 degrees with both running, 77 with just the exhaust and 85 with neither running, just the wind blowing cold air in occasionally because the it is vented only about 2 feet from the window. With the fans running, my humidity dramatically drops to about 20 percent, sometimes less, I know the optimum humidity is about 50 percent for vegging. I've got a bowl of perlite, 1/2 inch of water and 1 inch of perlite to try to bring humidity back to normal, and this worked for a few days, maybe the perlite is no good anymore? Also, I'm using MiracleGro gardening mix, should have used potting mix, but I mixed about a 4:1 ratio of soil and perlite, then I put about a half inch of perlite over the top of the soil. I was unaware at first that this mix feeds plants for up to 3 months and I've been using a B-12 calcium fertilizer about every 3 watering cycles, this could also be a cause to my drooping problem. The main stem of the plant is also turning red, I know for sure that's a nutrient deficiency but can that be caused from too much nutrients or what? Because I'm almost sure it's getting too much than too little, which can also pose my problem.

Any advice I can receive on this will be well appreciated, really don't want to kill this plant but I lack the experience to know exactly what is causing my problems. I'm thinking of getting a heating mat for the bottom of the pot to heat the roots better and also a water bottle humidifier because the perlite doesn't seem to emit enough, I've been misting a few times a day to try to correct that. Also if my thermometer is too close to the plant, I may not be getting accurate reading but I want to know how much heat is actually around the plant itself, instead of an overall temperature because if I move the thermometer up, the temperature drops to about 65 towards the middle of the grow room, I know the plant itself isn't at all feeling the 65 degree temperature because of the lights around it raise it about 10-12 degrees, any thoughts? I plan to stop watering until the soil in the bottom drainage holes of the pot is dry for now, is that the best way to tell or do you measure from the moisture on the top soil?
 
So, I backed away the lights a little bit and put a plastic vented cover over, the temperature inside the dome now sits at about 81 degrees with about 50 percent humidity.
Here's what I did:
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hopesluck

Member
small seedlings can take the heat and they definitely need the humidity. my seedlings did great under low humidity but they slowing began to degrade while maturing and i didnt notice it until one day one was practically dead. but yea just keep the plastic on and the light away bc high temp + high humidity + too much light and they will fry.
 
So I shut the 3 CFLs off because I figured it was sort of overkill. Now I've just got the HPS 150w lamp running about 10"-11" away from the plant. Its been running pretty smoothly since I put vented saran wrap over the top, haven't watered yet because I'm still waiting for the bottom to dry out. It's been running between 45%-55% humidity and 77-82 degrees Fahrenheit. Over the last 24 hours I've had a lot of growth, the top leaves are almost the same size as the bottom and the plant is looking a lot healthier. Still have a brownish-red stem but it looks like its finally fading back to green. Really happy with my results right now, basically answered my own questions via research, this site has a lot of good information to offer. I'll be giving updates
 
Just a little update, this plant is growing like a monster now! Check it out:
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As you can see, there's been some tremendous growth, great improvement. Still haven't watered, I plan on waiting until the bottom drainage holes are dry, unless I should water it sooner?
 
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