A visual key to cannabis nutrient problems

Most symptom charts ask you to scroll until a photo looks close enough. That works until two nutrients share a look, which is most of the time. This page sorts the other way round: where the symptom is, then what it looks like there, so you are choosing between two or three real candidates before you have read a single description. A match here is a strong lead, not a confirmed diagnosis — every branch below ends in a specific test, and a specific page, not just a name.

Two checks before you trust any pattern

Before you follow any branch on this page, take two readings. A false deficiency from root-zone pH lockout and a false toxicity from salt build-up in the medium can each produce almost any pattern below, and neither is fixed by feeding more of the nutrient the leaf seems to be short of.

Before you follow any branch

Root-zone pH read at the drain or in runoff, not just in the mixed reservoirsoilless media: roughly pH 5.5–6.5; soil: roughly pH 6.0–7.0
Runoff or drainage EC compared with the EC you fedrunoff far above the input points to salt build-up; runoff far below it points to poor uptake or an underfed recipe

If either reading is off, start with checking pH and EC before you add anything and pH lockout vs deficiency. Only once those look normal does the leaf pattern itself become the more useful clue.

Check leaf age before you check the pattern

Cannabis moves some nutrients internally and cannot move others. When a mobile nutrient runs short, the plant strips it from its oldest leaves and reroutes it to new growth, so the symptom shows at the bottom of the plant first. Immobile nutrients cannot be relocated once they are built into leaf tissue, so a shortage shows where the plant is building new tissue right now: the newest leaves and the growing tip ^[1]. That single fact, more than the exact shade of yellow, is what does most of the sorting.

Fig. 1Location comes first. Cannabis moves some nutrients to new growth and leaves others exactly where they are, so where a symptom shows already narrows the field.Horus

If the pattern ignores leaf age altogether, follows an irrigation zone rather than a leaf's age, or shows up as distortion at the growing tip rather than a colour change, go straight to the third branch below.

Walk the key

The tool below runs the same logic the rest of this page explains in words: pick where the problem shows, then its pattern, and you land on the specific page for that nutrient, or on a look-alike page when telling two nutrients apart needs more than one photo can show.

Diagnose a leaf problem

Where does the problem show first?Check the whole plant, then find where it's worst.

Fig. 2Every branch of the key ends at one specific page and a confirming test, not just a name.Horus

Old leaves: five patterns, and the test that confirms each

The confirming test is the column that matters: it rules each candidate in or out without waiting for a correction to grow out.

Pattern on the oldest leaves	Most likely cause	Confirming test
Whole leaf yellows evenly, veins included	Nitrogen deficiency or late-flower fade	New growth stays mid-green if the cause is diet, not age; tissue nitrogen below about 3.3% of dry weight confirms it ^[2]
Yellow between the veins, veins stay green	Magnesium deficiency	Yellowing starts at the leaf margin and moves inward; the veins hold their colour longest
Brown, scorched margins and leaf tips	Potassium deficiency	Scorch starts at the tip and edge, not between the veins; tissue potassium below about 1.8% confirms it ^[2]
Leaf darkens or turns purple-red; petioles and stems too	Phosphorus deficiency	Purpling from cold night temperatures clears within days of warming the room; phosphorus purpling doesn't
Small, dark, almost black spots on otherwise normal-looking leaves	Manganese toxicity	Check root-zone pH first: most cases sit below pH 5.5 in soilless media, where manganese becomes too available rather than too scarce

Four of these five are shortages of a mobile nutrient, which is exactly why they land on old leaves. The fifth, manganese toxicity, shows on old leaves too but for the opposite reason: excess, not shortage, and the fix is to raise pH, not to add anything.

New leaves and the growing tip: four patterns, and one split a photo can't make

Pattern on the newest growth	Most likely cause	Confirming test
Yellow between the veins, veins stay green	Iron, manganese or zinc deficiency	Fine speckling with normal internode length points to manganese; a cleaner interveinal pattern with no speckling points to iron; short internodes alongside it point to zinc. See iron, manganese or zinc: yellow new growth
Whole new leaf pale, almost white; veins pale too	Sulfur deficiency	Unlike nitrogen, sulfur symptoms sit on new growth: it moves through the plant far more slowly
Growing tip twists, hooks over or dies back	Boron deficiency, or a pest	Check the tip at roughly ×30–60 magnification for broad mites or thrips before treating it as a nutrient problem
Internodes shorten; new leaves come in small and crowded	Zinc deficiency	Tissue zinc below about 33 ppm confirms it ^[2]; leaf size and internode length recover within one to two weeks of correction

The iron/manganese/zinc split is the one place on this page where the leaf alone genuinely isn't enough: all three move poorly once built into a leaf, all three land on new growth, and all three give interveinal chlorosis. The look-alike page walks through speckling, internode length and which one responds first to a foliar correction.

Whole plant and the patterns that ignore leaf age

Pattern	Most likely cause	Confirming test
Leaves very dark green and glossy; tips curl downward (clawing)	Nitrogen toxicity	Feed EC and recent nitrogen rate are the giveaway; back the feed off a step and new growth should come in lighter within a week
Older leaves scorch at the margin even though feed EC reads normal	Sodium and chloride salinity	Test the source water on its own, before it's mixed with anything: sodium and chloride rarely show on a standard nutrient EC meter
Dead spots on new growth, or inside dense, tight flower clusters	Calcium deficiency	Spots appear where airflow and calcium delivery are both weakest, which is why it often shows inside the cola before it shows on a fan leaf
Growing tip distorted or stunted, with no clear leaf-age pattern	A pest, before boron: see pest damage or deficiency spots	Check under magnification before you touch the feed: distortion with no consistent leaf-age pattern is more often broad mites, thrips or russet mites than a nutrient

Every nutrient the section covers, at a glance

This key routes the common cases. As the most-linked page in the section, it also works as an index: every nutrient's full profile, its deficiency page where one is written, and its toxicity page where excess is the more common problem in practice.

Old leaves, lower thirdShortages of mobile nutrients, plus two excesses usually first seen here

Nitrogen deficiencyveins yellow too
Magnesium deficiencyveins stay green
Potassium deficiencyedges and tips scorch
Phosphorus deficiencyor cold-induced purpling
Manganese toxicityusually low root-zone pH
Potassium excessshows as Mg or Ca shortage

New leaves, growing tipShortages of nutrients the plant cannot pull back out of old leaves

Iron deficiencynear-white, no speckling
Manganese deficiencyfine speckling
Zinc deficiencysmall leaves, short internodes
Sulfur deficiencyveins pale too
Boron deficiencyrule out mites first
Copper deficiencyUncommondark, twisted, limp
Molybdenum deficiencyUncommonedges cup upward

Whole plant, or otherExcess, water quality and calcium delivery: patterns leaf age does not explain

Nitrogen toxicitywhole canopy; tips claw down
Calcium deficiencynew growth, or inside dense flowers
Sodium and chloride salinityusually from source water
Iron toxicityUncommonfine bronze stippling
Boron toxicityolder leaf tips first
Phosphorus excessshows as Zn, Fe or Cu shortage
Ammonium toxicitydark curl; brown root tips

Flat colour: living tissue, green to yellow to cream; purple and bronze where named
Hatching: dead tissue
Dots: spots or speckling
Dashed frame: uncommon, shown for completeness
Arrow to inset: the shortage an excess can cause

Schematic patterns at moderate severity. A match narrows the field: check root-zone pH and runoff EC, then confirm with the test on this page.

Horus, composite index; individual patterns after Cockson et al. (2019) and Marschner’s Mineral Nutrition of Plants (4th ed., 2023).

Fig. 3Every pattern this section diagnoses, in one scannable index grouped by where it shows on the plant.Horus

Nutrient	Moves in the plant	Deficiency	Toxicity
Nitrogen (N)	Mobile	Nitrogen deficiency	Nitrogen toxicity
Phosphorus (P)	Mobile	Phosphorus deficiency	Phosphorus excess
Potassium (K)	Mobile	Potassium deficiency	Potassium excess
Calcium (Ca)	Immobile	Calcium deficiency	not typically diagnosed
Magnesium (Mg)	Mobile	Magnesium deficiency	not typically diagnosed
Sulfur (S)	Partly mobile	Sulfur deficiency	not typically diagnosed
Iron (Fe)	Immobile	Iron deficiency	Iron toxicity
Manganese (Mn)	Immobile	Manganese deficiency	Manganese toxicity
Zinc (Zn)	Partly mobile	Zinc deficiency	not typically diagnosed
Copper (Cu)	Partly mobile	Copper deficiency	not typically diagnosed
Boron (B)	Immobile	Boron deficiency	Boron toxicity
Molybdenum (Mo)	Partly mobile	Molybdenum deficiency	not typically diagnosed
Nickel (Ni)	Partly mobile	rarely diagnosed in practice	not typically diagnosed
Chloride (Cl)	Mobile	rarely limiting	Sodium and chloride salinity
Silicon (Si)	Immobile	helps the plant, not classed as required	not typically diagnosed
Sodium (Na)	Mobile	not a required nutrient	Sodium and chloride salinity

Confirm it with a tissue test, then judge the fix on new growth

A tissue test is the tie-breaker no photograph can be. Sample the most recently mature leaf, usually the third to fifth leaf down from the growing tip, pooled from ten to twenty plants, and compare it against published sufficiency ranges: nitrogen 3.3–5.0%, phosphorus 0.27–0.48% and potassium 1.8–2.7% of dry weight; iron 70–150 ppm and zinc 33–60 ppm ^[2].

Give any correction one full leaf cycle, roughly seven to ten days, before deciding it hasn't worked. Leaf tissue that has already yellowed or scorched will not turn green again, so judge the result on what the new growth looks like, not on the leaves that already show damage. If two rounds of a targeted fix produce no change on new growth, stop guessing from leaves and go to a tissue test or a source-water analysis instead.

For the ranked, setup-specific version of this same question, see lower leaves turning yellow and new growth turning yellow. Everything on this page assumes you are growing where cultivation is legal for you; plant counts, testing rules and home-grow limits vary by country and are covered in the law section.