Hey bud,sorry for the delay but it's the holidays & the wife is keeping me busy as hell,you know the drill.
Anyhow looking at the pics you posted im not seeing anything dangerous to the point of the plant going to die soon,whatever is going on is very minimual,what i see looks more along the lines of a ph problem or something else,before i go into ph issues let me ask you this,is there a possibility that another person in the home is farting around with the plant,trying to help without telling you ?
The reason i ask is because the leaf's do not look like they are lacking nutrients to me,the damaged leaves appear to have had water with fertilizer in it spilled on the leaf surface,then left on the leaf to bake under the lights,is there a possibility that the wife is trying to help ?If there is that possibility she needs to stop now,only one grower at a time.
Other than the bad spots the plants look pretty good with nice tight node spacing,that shows they are not stretching at all from the light being too high.
Ok with that being said i highly doubt that adding fertilizer to the soil is the answer to correcting the damage,unless the soil your using is completely depleted & barren of all nutrients then the nutes within the soil will easily carry a 3 week old plant,being that your chosen soil mix has what appears to be wood chips floating around in it there is a possibility that something is going on in the soil,i stay away from woody soil mixes for just that reason.
There are a few things you can do without adding ferilizer to the soil.
Unless you've found proper mixing ratios for the sensi bloom fertilizer then forget the product for now,fertilizer is fertilizer & there is little or no difference in expensive botique (hydro shop) nutes & off the shelf nutes like Miracle grow or other brands.
Go to home depot & get a jug of Miracle Grow All purpose 24-8-16 plant food,this fertilizer contains everything a plant needs including all the micro nutrients that most hydro liquid fertilizers are lacking,while your at home depot pick up a spray bottle to mist the plants with,do not use a empty household cleaner bottle as ive seen many guys use.
Once you get the fertilizer & spray bottle mix the fertilizer at a 25% ratio & let stand overnight,the next day spray the entire plant heavily until all leaf surfaces & all areas of the plant are ringing wet & leaves have water run off,turn the lights off until the plant dries 100%,drying should be complete within 2 to 3 hours & will not hurt the plant,do this every other day for 3 applications,on the days inbetween foliar feeding with the 25% ratio mist the plants heavily with plain room temp water,this will rinse off any salts that the plants did not absorb,if there is a nutrient problem foliar feeding at a 25% ratio will correct the problem much faster than soil feeding.
At the same time your going to need to start checking the ph of your tap water & the soil,this is where my money lies on being the problem,is your tap water hard water or well water ?,if so stop using it & get bottled water,you'll need a basic ph test kit from wallmart,petsmart or a pool supply along with one bottle ph up & one bottle ph down,stay away from quick fix ph things like vinegar,baking soda & lemon jiuce & buy the proper chemicals as the whole shebang will only cost $20 or so.
Start by testing the water the plant is being fed,in a perfect world you want a ph range of 6.5 to 7.0,if the water is not within this range then adjust it with the corresponding lower/raise product until its within the acceptable range.
Now water the plant with the properly ph'ed water until the runoff is extremely excessive,not just a few drops but until the water flows heavily from the bottom of the planter,this will flush any concentrated salt build up from the soil as well as take you to yout next step which is testing the ph of the soil,when the flow from the pot has slowed but not stopped take a cup & put it under a drain hole,tilt the pot to allow more runoff to reach the single hole,now measure the ph of the water runoff.
Doing this will tell you roughly what the ph of your soil is because you know what the ph of the water going in was,if the ph was way off then the soil can only act as a buffer so long before problems arise,watering with properly ph'ed water from this point on will correct any soil ph problems.
Try foliar feeding as i outlined above as well as testing the water & correcting any ph issues as outlined & you'll find the culprit.
If i was you i'd also loose that soil mix,just looking at the wood chips floating all over the soil tells me that keeping a constant ph level in the soil will be very hard,wood will collect & hold nutrient salts,then release them as water is applied to the soil mix causing an unknown variable..
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