What it is
The plant doesn't have enough nitrogen (n), the nutrient it uses the most. It uses it to make chlorophyll (what makes it green) and proteins for growth.
How to tell
The lower leaves turn light green, then entirely yellow and fall off. The yellow keeps climbing from bottom to top. The plant grows slowly and stays pale.
It starts at the bottom because nitrogen is "mobile": when it's short, the plant pulls it from the old leaves to give it to the new ones.
What to do
Give fertilizer with nitrogen (the growth one). You see it in a few days: new leaves come out green (the ones that already turned yellow don't recover). First, it is worth checking the pH: if it is off, the fertilizer is useless.
Watch out: it isn't always a problem
In the last weeks of flowering it is normal for the lower leaves to turn yellow: the plant is eating its reserves to finish the buds. Don't panic or load it with fertilizer.
🎮 Practice it in UnderGrow
It shows up when the nutrient bar hits 0 %, or when the pH is out of range and the root can't take up what is there. It is fixed with a dose (or two) of fertilizer and, if it is the pH, by watering with adjusted pH.
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