OK, the yellow tips don't seem to be the problem anymore, but now the lower leaves started yellowing, drying and dying. I already removed first four leaves on one of the plants. Please look at the pics and advise:







Except those leaves, the plants actually look quite fine:





I added nutrients 4 days ago for the first time (for flowering plants N:P:K = 4:5:6), 1/2 of dosage. I usually water every plant with 1/2 liter (2 US cups) of water. All the other info about my grow are in the first post. Also, if there's a moderator reading this, please change the subject of original post to "Leaves yellowing and dying". Thanks.