Identifying Nutrient Deficiency and Overdose in Cannabis

When the lower leaves turn yellow or the shoot tip curls, many growers suspect a nutrient deficiency and reach straight for the fertiliser bottle. Nutrients are the minerals the plant takes up from fertiliser and water. You narrow down the cause faster if you first read where the damage sits, what pattern it forms and how it progresses. This article explains that way of reading a plant and describes the patterns of six micronutrients, which the plant needs only in tiny amounts. It closes with salt damage and with patterns that merely resemble a deficiency.

How do I read a plant with discoloured leaves?

Start by asking where the damage sits, then look at the pattern it forms and the way it spreads. Where a deficiency starts depends on how mobile the nutrient is inside the plant, and the full assignment for each element is in the article on nutrient function and mobility. Leaf type is part of the location as well. Fan leaves are the large, fingered leaves on the stem and branches, each finger being a leaflet, and sugar leaves are the small leaves in the buds. Under a screen the lowest leaves are hard to see, so judge the older leaves on the main branches instead.

Abiotic damage, meaning damage with no pathogen behind it, comes for instance from nutrients or climate. It hits many plants in the same pattern and at the same leaf age, and it develops quickly, whereas diseases spread irregularly and build up slowly. In the deficiency trials, the first sign took five to nine weeks to appear in vegetative growth and three to four weeks in flowering. Young, healthy roots are clear to translucent, and light brown or brown roots may be damaged. Older roots also turn brownish naturally, however, because they become woody.

Pattern	What it looks like
Chlorosis	The term for yellowing: the whole leaf pales evenly from light green to yellow.
Interveinal chlorosis	Yellow between green veins, either as a fine net, as broad bands or as a green skeleton.
Margin yellowing	A yellow margin, later turning brown, moves inwards. With potassium deficiency it starts at the serrations.
Spots	Pale dots, olive-green water-soaked spots or brown spots with a yellow halo.
Speckles	Bronze or purplish-black dots, often on older leaves.
Necrosis	The term for dead, brown tissue.
Deformation	Young leaves grow twisted, narrow, rolled or cupped, and shoot tips die.

Note the location: bottom, middle or shoot tip, fan leaf or sugar leaf.
Look at young root tips, not the old roots at the stem.
Find the pattern in the table.
Count how many plants are affected and whether the same leaf age is hit.
Take a photo and compare the same leaf a few days later.
Write off heavily yellowed or brown leaves, and judge each change by the new growth.

Is it too little, too much or blocked?

In cannabis, an excess more often shows up as a deficiency of another nutrient than as a pattern of its own. Too much potassium, for instance, appears as a magnesium or calcium deficiency. Patterns of their own have been described for eight cases: nitrogen, ammonium, magnesium, zinc, copper, boron, manganese and too much salt. Most of them start at the tips and margins of the older leaves, where salts and boron accumulate as they travel with the water flow. Ammonium damage, by contrast, begins at the shoot tips. If the nutrient is in the fertiliser and the plant still lacks it, a lockout is often behind the problem. Besides the colour, the shape and surface of the leaf give further clues, and the table below sums up what they point to.

Shape and leaf feel	Points to
Dark green and glossy, leaves curled downwards	Overfeeding
Young leaves curled down and cupped, pale along the midrib	EC too high. The midrib is the main vein of the leaflet.
Curled shoot tips with brown margins, plus paling	Too much ammonium
Young leaves thick, brittle and twisted	Boron deficiency
Margins of older leaves dry, brown and crumbly	Late stage of high EC, although potassium deficiency and drought cause this too.
Tips and margins dry out	In other plants, excess chloride or boron
Soft, succulent new shoots and weak stems	A lot of nitrogen, as described in field crops
Thick, leathery leaves	Ammonium toxicity, as described in pot chrysanthemums

Compare the pattern with the article on nitrogen, also if you suspect ammonium.
Compare the pattern with the article on phosphorus.
Compare the pattern with the article on potassium.
Compare the pattern with the article on calcium.
Compare the pattern with the article on magnesium.
Compare the pattern with the article on sulphur.
Compare the pattern with the article on iron.
Compare the pattern with the article on boron.
With a deficiency despite feeding, find the cause with the article on nutrient lockout.
Before feeding more, measure pH and EC at the root, as described in the article on nutrient diagnosis.

How do manganese, zinc and copper show?

A deficiency of manganese, zinc or copper hits the younger leaves first, while an excess mostly shows on the older ones. None of the three moves on once the plant has built it into a leaf. Manganese deficiency resembles magnesium deficiency, but the latter starts on the old leaves. Leaf values in mg/kg give milligrams per kilogram of dried leaf from the lab analysis. Feed values in mg/L give milligrams per litre of feed solution, and ppm means the same thing.

In excess, manganese, zinc and copper all suppress iron uptake, and an iron deficiency in the new growth then joins in. In field crops zinc deficiency occurs most often alongside excess phosphorus, but for cannabis this is not settled. The researchers of the flowering trial interpret part of its zinc symptoms as a result of high leaf phosphorus. For flowering, the NC State group gives 0.5 to 0.88 ppm for manganese, 0.05 to 0.2 ppm for zinc and 0.1 to 0.18 ppm for copper. In a trial series with graded zinc rates, 0.35 mg/L was optimal and bud mass fell at 0.05 mg/L, so the zinc guide value lies below this optimum.

In geraniums, applying manganese below pH 5.5 easily leads to toxicity. A chelate keeps a metal in solution, and iron chelate eases manganese toxicity because the iron displaces the manganese. In hydroponics, where the roots stand in nutrient solution without any soil, the copper damage threshold for lettuce and tomato was about a hundred times lower than in peat.

Disorder and stage	Appearance
Manganese deficiency, first sign	Upper and middle leaves turn bright yellow in a net between the veins, starting at the midrib.
Manganese deficiency, advanced	The net spreads to the margin, while veins and margin stay a darker green.
Manganese deficiency, late	Small light brown spots appear between the veins, like white or bronze pinpricks.
Manganese deficiency, flowering	No sign and no loss of bud mass in the flowering trial.
Manganese deficiency, leaf value	At the first sign 7.56 mg/kg instead of 29.40 mg/kg.
Too much manganese	Lower leaves yellow from tip and margin inwards. Then the margins die and the leaves drop.
Too much manganese, leaf value	Ten times the rate raised it to 47.88 mg/kg, which still lies within the usual range of 41 to 93 mg/kg.
Too much manganese, other crops	Blackish-brown or red dots on older leaves.
Zinc deficiency, first sign	Newer leaves yellow at the margin along the serrations. In the hemp cultivar Finola, pale to white bands appeared.
Zinc deficiency, advanced	Irregular light brown spots appear, and new leaves stay thin, narrow and strap-like.
Zinc deficiency, late	New leaves can be completely white, and the plant stays shorter with a thinner stem.
Zinc deficiency, flowering	Young leaves yellow in a mottled way. Bud mass falls, while the roots stay unchanged.
Zinc deficiency, leaf value	At the first sign 10.7 mg/kg instead of 25.33 mg/kg.
Too much zinc	At 4 mg/L the leaf tips died. Roots and yield stayed unchanged, but the cannabinoids fell.
Too much zinc, other crops	Smaller dark green leaves with yellowing.
Copper deficiency, first sign	Very late, after nine weeks in the trial. New leaves are distorted, with a narrow and slightly yellow base.
Copper deficiency in Finola	Young leaves turn dark bluish green and wilt.
Copper deficiency, advanced	A fine yellowing between the veins starts from the margin, and the margin bends inwards and down.
Copper deficiency, late	New growth goes limp and droops, and fibre hemp can fall over.
Copper deficiency, leaf value	At the first sign 1.41 mg/kg instead of 4.65 mg/kg.
Too much copper	Young hemp yellowed at 20 µmol/L copper, against 0.2 µmol/L in the control solution. The shoot grew 58 % less.
Too much copper, tomato	In hydroponics, short brown roots formed at just 8 µmol/L.

Measure the pH, which shows how acidic a solution or the substrate is, meaning the material in the pot.
With zinc deficiency, check the phosphorus in the fertiliser, and with copper deficiency also the zinc.
Only then give the missing element, preferably as a chelate.
Do not dose zinc at the lower limit of 0.05 ppm.
In acidic substrate, dose manganese carefully.
With manganese excess, raise the pH, flush and give iron chelate.
With zinc or copper excess, check fertiliser and irrigation water and lower the rate.
Check the new growth to see whether the new leaves grow back without signs.

How do molybdenum, chlorine and nickel show?

Cannabis has hardly any described patterns for molybdenum, chlorine and nickel, and most of what exists comes from the hemp cultivar Finola or from other plants. Without molybdenum, cannabis showed no sign for nine weeks in the vegetative trial, although its leaf content fell by 96 %, from 1.46 to 0.06 mg/kg. Poinsettias tolerated up to 60 ppm molybdenum without damage, whereas some crops react above 0.5 ppm.

The plant takes up chlorine as chloride, and in lady’s mantle with 240 ppm chloride in the irrigation water the margins of older leaves were brown and crispy. For ornamentals the risk rises from about 110 ppm chloride in the irrigation water.

The plant needs nickel to break down urea, so without it urea accumulates and kills the leaf tips. For nickel in cannabis there is one trial, which used a mix of lead, cobalt, cadmium and nickel. The leaves yellowed there, including those at the buds, but growth did not suffer, and how much of this was due to nickel cannot be separated. In other plants, the root reacts more sensitively to nickel than the shoot.

Disorder	Appearance
Molybdenum deficiency in Finola	Like nitrogen deficiency, first on older leaves and then on younger ones, plus margin burn and cupped or rolled leaves.
Molybdenum deficiency in Finola, later	Spots break through.
Molybdenum deficiency in poinsettias	The margins of just-matured leaves in the middle of the plant yellow, and later they turn white and burn.
Too much molybdenum	Not described in cannabis. Field crops stay small, with yellow-brown leaves.
Chlorine deficiency in Finola	Limp shoot tips with yellow and brown spots, plus cupped young leaves and bronzing.
Too much chloride	The very tip of the leaf dies first. The damage then moves back along the margin, and leaves drop early.
Nickel deficiency in other plants	The leaf tips die.
Too much nickel in other plants	Yellowing between the veins, often together with iron deficiency.
Too much nickel, roots	The main root stays short and the root system becomes compact, with brown root tips.

If molybdenum deficiency is suspected, keep the pH above 5.5.
Give molybdenum at 0.075 to 0.1 ppm, the guide value of the NC State group.
Confirm molybdenum deficiency with a leaf tissue analysis, not by the pattern alone.
With brown, crispy margins on older leaves, have the irrigation water tested for chloride.
With too much chloride, flush with about one and a half times the pot volume of clear water.
If nickel or other heavy metals are suspected, have fertiliser, additives and water analysed.

How do I recognise too much salt in the root zone?

Too much salt shows on the lower leaves and on the unfolding shoot tips at the same time. Electrical conductivity (EC) measures all dissolved salts together, including the nutrient salts of the fertiliser, and if it climbs too high the root takes up water less easily. With subirrigation, where the plant drinks from a tray below, the root damage sits in the upper third of the root ball. Two look-alikes behave differently: potassium deficiency moves more slowly from a yellow margin into brown tissue, and drought stress usually shows margin burn only after the next watering.

The trials state the salt dose in mmol/L, a unit that gives how much substance is dissolved in one litre. From 5 mmol/L sodium chloride, tips and margins yellowed after six weeks of flowering, and by calculation that is about 290 mg/L of table salt. In hydroponics, leaves and buds were completely dead by the end of the trial at 20 and 40 mmol/L. In young hemp, the leaf margins yellowed and died at 0.2 and 0.3 mol/L. In other crops, sodium damage starts at the outer margin of older leaves and moves inwards between the veins, whereas chloride damage starts at the very tip.

Stage	Appearance with EC too high
First sign	New leaves bend down and cup like a bowl, and the tissue pales along the midrib.
First sign, young leaves	They turn light green with a brown margin, although cucurbits look dark green at first.
Advanced	The leaves wilt, roll and deform strongly, and the tissue between the veins yellows.
Advanced, lower leaves	An even brown margin runs around the leaf and moves towards the midrib.
Late	Margins of older leaves turn limp, brittle and brown, until only a green strip along the midrib remains.
Very late	The leaflets die, and the shoot tip dies.
Roots	With top watering, the roots at the bottom of the pot suffer most, and a salt crust can form on the substrate.

Measure EC in the runoff, the water that drains from the bottom of the pot. Compare it with the target range for the medium.
Look at the roots at the bottom and top of the root ball.
Lower the fertiliser rate.
At very high EC, flush twice in a row with clear water.
Check the irrigation water for sodium, using the limits in the article on calculating a nutrient solution.
Judge on new growth whether the cause is fixed. The burnt margins remain.

What looks like a nutrient problem but is not one?

Pests and fungi produce patterns that resemble a deficiency, and so do water, climate and sprays. Constantly wet substrate even triggers real deficiencies, iron deficiency above all. Towards the end of flowering the oldest fan leaves yellow even when the plant is fully fed, and that is not a deficiency. The article on leaf senescence explains how to tell this ageing apart from a deficiency.

Cause	How to recognise it
Too wet	Soft yellow leaves drop, old and young alike. The pot is heavy, and the roots are mushy brown to black.
Too dry	Leaf margins dry first and older leaves drop. The roots are thin.
Heat	Leaves curl up and scorch.
High humidity	Water droplets at the leaf margins, burnt tips and leaf rot.
Spider mites	Many small yellow dots that later turn bronze. The mites sit on the underside of the leaf.
Thrips	White flecks or silvery scars with tiny dark specks of droppings. Young leaves become deformed.
Hemp russet mite	Margins roll in, and the tissue turns dull bronze to rusty. The mites measure 0.11 to 0.21 mm.
Hemp russet mite, confusion	Invisible to the naked eye, and its damage is explicitly confused with nutrient deficiency.
Broad mites	Young leaves and flower buds deform and harden. The mites are visible only with at least a tenfold hand lens.
Septoria	Brown spots up to about 6 mm with a yellow halo, first on damp inner leaves.
Septoria, later	The centres turn grey, with tiny black dots that are the fruiting bodies of the fungus.
Powdery mildew	White, powdery spots on the leaves.
Root rot	Brown roots from Fusarium fungi or from Pythium, a water mould. Iron deficiency can show at the top.
Root rot, plant	The plants stay stunted and yellow.
Herbicides	Products with growth-regulator action twist and roll leaves and stems even in tiny amounts.

Lift the pot before watering, and let substrate that is too wet dry down.
Search leaf undersides with a hand lens of at least tenfold magnification.
Pull the roots out of the pot and check their colour.
Measure temperature and humidity before changing the fertiliser.
Find out whether weed killers were sprayed nearby.