Bookmark this one for when your leaves start doing something weird at 2am.

First, the rule that saves you every time

Check your pH before you blame a deficiency. The most common "deficiency" is nutrient lockout: the nutrients are in your medium but the pH is wrong so the roots can't take them up. Soil wants roughly 6.0 to 7.0, coco and hydro want 5.5 to 6.5. If pH is off, fix that first and reassess in a few days, because adding more nutrients to a lockout just makes it worse.

Okay, the big five:

1. Nitrogen (N)

• Look: lower leaves turn pale, then yellow, then drop. Yellowing moves from the bottom of the plant upward.
  
• When: usually mid to late veg. Note that some yellowing in late flower is normal and means the plant is finishing, don't fight it there.
  

2. Calcium/Magnesium (CalMag)

• Look: rust colored spots and blotches, often on newer growth (calcium), or yellowing between the veins while the veins stay green (magnesium).
  
• When: constantly, if you're in coco or under LED lights. Coco growers basically run CalMag by default.
  

3. Phosphorus (P)

• Look: dark blue-green leaves, purple or red stems, sometimes purple blotches on leaves. Plant looks "dark and stunted."
  
• When: flower, when demand spikes. Also shows up in cold root zones even when P is present.
  

4. Potassium (K)

• Look: leaf edges and tips turn yellow then burnt brown, like the leaf got toasted from the outside in.
  
• When: flower. Easy to confuse with nutrient burn, but nute burn is mostly just tips, K deficiency creeps along the whole edge.
  

5. Iron (Fe)

• Look: NEW growth comes out yellow to almost white while veins stay green. The key is it hits the top of the plant first, opposite of nitrogen.
  
• When: almost always a pH problem in disguise. Iron locks out hard above 6.5.
  

Posting for help? Include these or we're all guessing:

• Photos in natural light (turn the grow light off, purple LED photos hide everything)
  
• Medium (soil/coco/hydro), pH going in and runoff if you have it
  
• What you're feeding and how often
  
• Week of veg or flower
  

Drop your problem plants below and let's figure them out.