I had issues with PH in my setup. it has causing really bad yellowing of my leaves i assume from lockout. I just got the PH under control and my plants havnt had time to show they getting healthy again.
in saying that i am wondering if they ever will stop or if it is time to start naturally.
I have read that yellowing is actually natural when the plant gets close to harvest. When do they typically start yellowing naturally. I am in the fourth week of flowering. Should they naturally start soon.

the lower leafes should be yellowing and dying off by 4 weeks into flower.
100% normal with most strains.
yellowing of the lower leafes is also an indication of an M.G defficiency.
(best posting a picture).

LUDA.

yes if ur in 4th week flower it will loose its lower leaves. most people notice them dying off in that time frame and clip them. as the plant reaches closer to harvest it will loose more leaves working its way to the top. depending on strain and environment you can looses just a couple leaves or most of them

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check out my second grow
http://www.rollitup.org/members/heathaa-albums-2nd-grow.html

yellowing fan leaves

Here is a link to my pics last week. My light is off now so i cant take updated pics until tomorrow. I 95% sure this yellow is deficiency. If you look thru this post you can see i am having ph issues that i can not figure out. I thought i had it figured out but it was due to my pump being unplugged for a day. Everytime my pump comes on the PH sky rockets. I will save you from hearing the whole issue here. If you have time i would appreciate you taking a look at my other post and see if you have any thoughts.
Thanks for the info.
By the way i dont know my strain this is my second grow and wanted to figure out all of the details before i bought seeds from over seas. These are random bag seeds.

IMO, and i know Uncle Ben would agree. The leaves should remain as green as possible through the entire duration of the grow. Leaves produce sugar and energy in photosynthesis that the plant uses to produce flowers. More healthy leaves = more flowers. Like uncle ben says "stick to botony, rather than hype generated by fertilizer companies. If you have your pH right, add more nitrogen.

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Kush & Skywalker 400w grow

The PH is still out of control. Every time i feed the PH will jump from 5.2 to around 6.4 or so. I have tried lowering by base line PH from around 5.5 to 5.8 now i drop it to 5.2 in the morning and it still ends up above 6 after a feeding.