White Widow

Hi mates, The week was pretty cool, trichomes production is going on ! I am quite surprised by the fastness of the bloom, which in 4 weeks is already having its pistils turning brown. The yields won't be huge but the best is to come... However, I have 2/3 leaves at the bottom that turned yellow quickly before necrosing (evolution on the pictures over 4 days). I removed them. I guess it's a Nitrogen deficiency because I didn't use a growth fertilizer. Or in Calcium/Magnesium (according to the seller's advice). I started the application on 04/17, including Fish Mix sprayed at night. The problem has stopped spreading but there are still visible signs. ??? Can someone advise me plz... ??? Peace, U.G.