Health Library

Nitrogen deficiency is the most common nutrient issue you'll run into. Because nitrogen is mobile, the plant pulls it from older lower leaves to feed new growth — so you'll always see symptoms starting at the bottom and working their way up. Affected leaves fade to a uniform pale yellow (tips first, spreading inward across the whole blade). Unlike magnesium deficiency, the yellowing is even across the leaf — veins and tissue all turn yellow together. It's most common during rapid veg growth when demand spikes, or when plants are root-bound or overwatered. Some yellowing of lower leaves during late flowering is completely normal as the plant redirects energy to buds — don't overcorrect it.

Phosphorus deficiency looks different from most other mobile nutrient issues — instead of yellowing, you'll see leaves getting darker. The plant redirects P from older lower leaves to new growth, so symptoms always start at the bottom. Leaves turn an unusually dark green or blue-green, often with purple stems and petioles. This is caused by anthocyanin pigment building up in starving tissue. Cold root zones (below 60°F/15°C), pH issues, or overwatering are the usual triggers. During flowering, P demand surges and deficiency can be devastating — leaves closest to buds and directly under lights often show the worst discoloration.

Potassium deficiency is one of the easier problems to identify — look for brown, crispy leaf edges on the lower leaves while the veins stay green. It's a mobile nutrient, so the plant robs older leaves first. Yellowing starts at the leaf margins and tips, then works inward toward the midrib with a blotchy, patchy pattern (not the smooth uniform fade you'd see with nitrogen). Stems and petioles may turn reddish. Plants often show increased stretching between nodes early on, then stunted growth as it worsens. K deficiency tanks disease resistance and makes plants vulnerable to drought. Common causes: pH above 8.0 in soil (7.5 in coco, 7.1 in hydro), excess calcium or nitrogen locking out K, overwatering, salt buildup, or cold root zones. Demand surges hard during flowering weeks 3–7.

Magnesium deficiency is one of the most common issues, especially in coco coir or under LED lights (LEDs produce less infrared heat, reducing transpiration and passive Mg uptake). As a mobile nutrient, the plant pulls Mg from older leaves to feed new growth — so symptoms always start at the bottom. The telltale sign is interveinal chlorosis: the tissue between veins turns yellow while the veins themselves stay dark green, creating a distinctive green-vein network overlaid on yellow tissue. The yellowing has a soft, cloudy quality and spreads inward from the leaf margins toward the central vein. It often worsens during flowering as developing buds act like a nutrient sink, pulling magnesium from leaves. Cold root zones below 64°F (18°C) also reduce Mg absorption significantly.

Molybdenum deficiency is rare but sneaky — the plant has plenty of nitrogen available, yet can't use it. Without Mo, the enzyme that converts nitrate to usable ammonium doesn't work, so symptoms mimic N deficiency even with adequate feeding. Unlike other mobile nutrients that show symptoms at the very bottom, Mo uniquely starts at mid-canopy leaves and spreads upward — this is the key way to tell it apart from nitrogen deficiency. Look for yellowing at the leaf margins (not between veins like Mg) with distinctive orange, pink, or red tints at the edges. Usually caused by acidic pH — a root zone drifting below its target (6.3 in soil, 5.8 in coco, 5.5 in hydro) — or cold root zone temperatures.

Chlorine deficiency is arguably the least likely nutrient problem you'll ever encounter. Plants need only trace amounts of chloride and naturally accumulate 10–100x more than their minimum requirement from tap water, atmospheric deposition, soil reserves, and fertilizer residues alone. The only conceivable scenario would be using pure RO or distilled water with chlorine-free nutrients in a completely sealed system — something virtually no grower does. In reality, chlorine toxicity from heavily chlorinated municipal water is the far more common concern. Because chlorine is a mobile nutrient, deficiency would theoretically appear on older leaves first, though this has essentially never been documented in cannabis cultivation.