leaves yellowing 3 1/2 weeks into flowering

funktions said:

Thought it was nitrogen deficiency, but I may be wrong ,can anyone give me any advice on what's going on

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I see this constantly in gardens in weeks 2-4 usually week 3 and this used to happen to me a lot till I figured out the problem. I found I was overfeeding in weeks 1-3 which caused a lockout in week 3-4 of cal-mag. I can't offer a suggestion to fix it, but I usually go lite on food at the beginning and gradually up my feed to week 3-4 when I feed the most. I found that plants use the most cal mag in 3-4 and after that I drop it down gradually. Since I started tweeking the feed this way I never have this happen anymore. Also if your using a synthetic line its good to achieve runnoff to avoid salt build up.

It's not pests. Your leaves are starting out with the distinct yellowing between the veins, then spreads throughout the leaf. It's called a magnesium deficiency.

Flush with ph 6.25 half a teaspoon Epson salts/gallon water wait a few days add 1 teaspoon blackstrap molasses/gallon and half strength bloom. should be back on track in about a week. Then a 3/4 bloom feed with 3ml food grade hydrogen peroxide@30% then back to normal ,and its feed,feed,water or feed, water, water depending on the strain some strains like lots of food , some don't . You have to learn what your babies like, most or all affected leaves will most likely die .
Good Luck !!

also any stalls you get at this stage or any stage of flower = lost yield as the plant does nothing but the weeks to finish are still the same.1 week stall per plant = a gram or so a day in lost yield x 7 = a quarter or more per plant per week of stalls and that's if you fix the problem in a week.

LOL, It's not Mg! It's not low N!

Bird turds = WAY to high in P! But you had a high P problem even before that..

That is the classic show of to much P way to early!!!
NO amount of N will fix that!

As some have said earlier,,,,,I too do not start a "bloom" food till around week 2-4 of flower....strain dependent.
Rule of thumb: No "bloom" food till week start of week 3.

Water it with Ca/Mg'ed water pH'ed to 6.5 and no feed for 7-10 days and then restart it slowly with the Bloom feed and no bird turds!

Skip the Molasses and Mg sulfate from above,,,,,and add a Kelp extract to help the soil bio's. If you feel so inclined....I like Age Old Kelp,,,,best one out there. Actually that's a good idea as the Humic/Fulvic's in the kelp will help regulate the up take of P and increase the use of the existing nutrients in the soil.
Don't "flush"!
So then, Ca/Mg and Kelp at 5ml each to 1 gallon of pure water. pH to 6.5 and water with that for 7-10 days and then add back the feed "bloom" till finish.

Doc

Shoot i run heavy nitrogen until week 6 and taper it off the last 4 weeks. If you aren't giving them any nitrogen and youre in a hydro setup you can bet that's a nitrogen def.

Right on and point well taken, thought it was a mag def.

On a side note if original poster doesn't mind: I run a powerful LED, the Apache AT600, and never ran into Cal def issues when I used HPS while in flower.

Can Dr. Who or anyone else chime in?