A question from a fellow grower:
I have 5 white widows growing in a burpee seed starting kit.
They came up in 2 days after germination.
The first week was good. I spray them twice daily.
The 2nd week the 2nd set of leaves are curling and the tips are yellow.
The,re under cool white bulbs 24 hrs.
What to do…I,m new
Be sure not to over water or over fertilize. The yellowing could be caused by either. Get a ph meter and a tds/ppm meter. You need these to know your ph of your soil and ph of your fertilizer mix and the strength of your fertilizer mix or the strength of the nutrients in the soil.
Other thoughts… Too much light too soon could cause these symptoms as well. Are the lights maybe too close?
Also if you could add the answers to the following support ticket questions, it would be of great help.
Size of space?
Soil or Hydro? Medium used?
PH?
Type of nutrients used? EC/TDS ppm levels?
Temperature?
Humidity %?
Light system/watts?
If they are still only in the burpee plugs then we do need to know the PH and concentration of nutrients in the plugs/growing medium.
Welcome to ILGM 
We will be ahppy to help you out. Fill in some info from the “support ticket” That McG posted, and we will dig right in. Peace