Reading Cannabis Leaves: Deficiency and Toxicity Cheat Sheet

First rule of leaf diagnosis: check pH before you add a single nutrient. Most "deficiencies" are actually pH lockout — the nutrient is in the medium, the roots just can't absorb it. MOBILE VS IMMOBILE — WHERE THE SYMPTOM SHOWS - Mobile nutrients (N, P, K, Mg): the plant moves them to new growth, so deficiencies show on the OLDER, lower leaves first. - Immobile nutrients (Ca, Fe, etc.): can't be relocated, so symptoms show on NEW growth at the top first. That one distinction narrows the diagnosis immediately. NITROGEN - Deficiency: lower leaves pale green → yellow → die off evenly. Normal in late flower (senescence); a problem in veg. - Toxicity (N-tox): leaves go dark glossy green and tips claw downward hard. This is the single most common problem in new grows — too much Grow/Bloom feed or hot soil. Back off the feed; don't add more. PHOSPHORUS - Deficiency: dark, bluish-green leaves, sometimes purpling stems, slow growth, occasionally bronze spotting on lower leaves. Often a cold-root-zone symptom too — cold wet soil locks out P even when it's there. POTASSIUM - Deficiency: edges of leaves yellow then brown and crisp (marginal burn), usually on older growth first. CALCIUM / MAGNESIUM ("calmag") - Calcium deficiency: rust-brown spots on new growth, distorted leaf tips. - Magnesium deficiency: interveinal yellowing on older leaves — veins stay green, tissue between them pales. - Both are common under LED lighting, which drives faster growth than old HPS setups. If you run LEDs and RO water, calmag is almost mandatory. pH LOCKOUT RANGES - Soil: feed/water at 6.0–7.0. - Coco & hydro: 5.8–6.2. Drift outside those and uptake collapses in a specific order — which is why a "mystery deficiency" that ignores supplements is usually a pH problem, not a nutrient shortage. FIX PROTOCOL 1. Test runoff pH (water through, measure what comes out). 2. If pH is off, correct it and give plain pH'd water for a watering or two. 3. Only then adjust nutrients — and go up in half-strength steps, not full doses.