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The leaves are darkening and there are small brown spots on the leaves. The plant is at 34 days of flower. There are two other plants right beside this plant, and they show no problems. Nice green healthy leaves. Anyone good enough to figure this out from just a couple pics? I'm growing in FFOF soil and using FF nutes and molasses. You can see more pictures in my grow log in my signature link.
What kind of lighting and distance? How old are the bulbs?
What part of the plant is affected? (lower fans, perhaps?)
How much molasses are you using?
What's your ingoing ph, and if you have it, the runoff ph?
Did you follow the FF schedule in regards to week(s) 7 and 8 (weeks 3 and 4 of flower)...specifically the addition of Grow Big? This gives a little bump of nitrogen and micro's, delaying the need to scavenge from the lower fans late in flower. My belief is that it helps maintain proper balance for photosynthesis, too. But I have no proof of this, just a casual observation in my growroom. Speaking of the feeding schedule...do you follow the FF feeding schedule?
Now...we could play this 'I'll ask a couple questions and wait for an answer to ask more follow-up questions'...or you could speed the process by copying and pasting the troubleshooting form here, and fill it out to best of your ability...thus giving us all of the background info all the same time.
E-indoor or outdoor
E-soil, soilless, coco, aero, or hydroponic
E-specific medium - FFOF + 30% perlite
SCLR-Soil or slab runoff pH unknown
E-Water source tap
E-Source water pH 8, but after nutes it drops to 6.5ish, on non-nute days I haven't been adjusting
E-Age of plant 70 days
E-Type of fertilizer FF 3, following feeind schedule on their site. Just added Grow Big 2 days ago (first schedule feeding of grow big after start of flowering)
E-Rate of application Watering every 4-5 days when the soil is dry. 1 tablespoon grandmas blackstrap molasses/gallon when watering without nutes. Nutes with every other water.
E-Lighting source and distance from plant 4 foot, 6 lamp T5.
E-Air temperature (both day and night if you are running a dark period) 65-72
E-Air % Relative humidity 40%
E-Lighting schedule 12/12
E-Type of ventilation your room has ceiling vent sucking air, 2 fans inside blowing it around.... more than plenty
TR-Did you pre-soak your media in pH corrected solution? nope
To answer a couple of your other questions...
The blackening/browning is happening on the upper fan leaves. The ones coming right out of the bud.
The whole plant is a light green color, much lighter than the two plants right beside it.
Personally, I use 1 tsp molasses per gallon of properly ph'd water, once a week on non-feeding days. Likely you are overdoing it with the molasses. Fox Farms has most of what you need, just a dab of molasses goes a lonnnnng way.
Do you let the medium dry between waterings/feedings?
Most likely the problem is ph though. Any time you add a liquid to a plant you'll want to properly adjust the ph. Likely the buffers in the soil gave it their best, but are exhausted. Literally. I'd do two things...
First: get some phDown by General Hydroponics, or something similar. I have 7.9 to 8.0 wellwater, and 1 tsp of phDown brings a five gallon bucket of water to 6.8ish. A gallon of the stuff lasts me over 1 year.
Second: Until you get the phDown, carefully replant into fresh potting soil and adjust water to acceptable range with a citric acid or some sort of table vinegar...but this is only to be used temporarily, as it's not the best tool for the job and can be toxic if used too long.
Douobtful it will be long if your numbers are correct...till all the plants complain about your ph. This isn't something to procrastinate about. Ph must be in proper range.