Yellow leaves

How to tell

Starts with lower/older leaves, yellows evenly, often progresses to the whole leaf before it drops

Fix

Increase nitrogen in the next feeding; this is most common in mid-to-late vegetative stage when the plant's appetite has grown past its current feeding schedule

How to tell

Yellowing paired with drooping, and soil that stays wet for days rather than drying between waterings

Fix

Let soil dry out more between waterings; check that pots have adequate drainage

How to tell

Yellowing that doesn't respond to more feeding, sometimes with other odd discoloration

Fix

Check soil/water pH (most soil grows want roughly 6.0–7.0) — nutrients can be present but unavailable to the plant outside that range