High Makaveli,
This I can ensure you for 100% is a nitrogen deficency.
The symptoms for a nitrogen deficiency in a cannabis plant
in the early stages are that the leaves appear to be too light in colour almost yellowish-green compared to the desired deep green of healthy leaves.
The cause of this deficiency could be underfeeding (but I am sure that this is not the case because your other plants look perfect and she is still to little to be that hungry for nitrogen).
Or overfeeding and I do think that this is your problem here a nutrient lockout due overfeeding. Or the pH of your soil is the cause for this nutrient lockout.
You can fix both of these problems with a good flush of your soil and I will explain you how you can do that the best.
You can do this flushing by slowly pour water onto the top of the soil, allowing it to drain freely from the bottom of the pot. Pour slowly, so the water doesn't overflow from the top of the pot. Use approximately four times the pot's volume in water to flush the soil. For example, use 1 gallon of water for a 1 quart pot.
After the flushing you need to let the soil (almost) dry out completely.
From the moment that the soil is dry again you can start watering and build up your nutritions again. Start with a low dose EC 1.0 and if you do not have an EC meter around 25% of the amount that they recommend on the nutrition bottle. From here you can start up the nutes slowely again. But do not give more then EC 1.8 or 75% of the amount that they recommend on the nutrition bottle.
Happy Growing! 😋
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