Calcium Deficiency? (Rusty Spots & Weak Growth)
Calcium issues show as rusty spots and weak new growth. pH and VPD often drive it, not just lack of Ca in solution.
Calcium deficiency in cannabis? Rusty spots and weak tips—see causes (pH, VPD, competition) and analyze your plant fast.
Common Causes
- Low pH or lockout in coco/soilless
- High potassium/magnesium competing
- Low transpiration (high humidity)
- Under-dosed Ca in RO water feeds
How to Tell Which One You Have
Look for small rust spots on mid/new leaves, often near edges, plus twisted new tips. Older leaves stay mostly green.
FAQ
Is this magnesium instead?
Mg shows interveinal yellowing; Ca shows spots and twisted new growth.
Do I just add Cal-Mag?
Check pH and VPD first. If using RO, add Cal-Mag but avoid overshooting EC.
Can high humidity cause it?
Yes. Low transpiration limits Ca movement to new tissue.