Cannabis Nutrient Deficiencies: A Visual Diagnostic Guide

When cannabis leaves start yellowing, curling, or spotting, the instinct is to dump nutrients at the problem. That is often the worst move, because the majority of what look like deficiencies are actually pH lockout, where the nutrient is present in the medium but unavailable because the pH is out of range. Reading deficiencies correctly requires understanding one fundamental distinction: mobile nutrients move within the plant and show symptoms on older, lower leaves first, while immobile nutrients get stuck where they are first used and show up on new, upper growth. Once you grasp that pattern and confirm your pH, diagnosing most problems becomes a matter of pattern recognition. This guide walks through the major deficiencies, how to tell them apart, and how to fix them without making things worse.

Mobile vs immobile nutrients

Before anything else, identify where the symptom appears. This single observation cuts the diagnostic list roughly in half.

Mobile nutrients include nitrogen, phosphorus, potassium, chlorine, molybdenum, and magnesium. When the plant runs short, it pulls these from older lower leaves to feed new growth, so symptoms appear first at the bottom of the plant and climb upward.

Immobile nutrients include calcium, sulfur, iron, boron, copper, and manganese. Once deposited in tissue, the plant cannot move them, so deficiency signs show up in fresh top growth, the newest leaves and shoot tips.

The role of pH in nutrient availability

Cannabis absorbs nutrients only within a specific pH window. In soil, that window is roughly 6.0 to 7.0. In soilless and hydroponic media like coco coir or rockwool, it is narrower, around 5.5 to 6.5. Outside these ranges, key nutrients become chemically locked out even when they are abundant in the medium.

• In soil above pH 7.0, iron, manganese, boron, copper, and zinc become unavailable, producing interveinal chlorosis in new growth.
• Below pH 6.0 in soil, calcium, magnesium, and phosphorus lock out, producing classic cal-mag symptoms.

Always check pH and electrical conductivity (EC) of the runoff before adjusting nutrients. The fix for a lockout is a pH correction and a plain-water flush, not more fertilizer.

The diagnostic table

| Nutrient | Mobility | Where it shows | Classic symptom | Likely cause | |---|---|---|---|---| | Nitrogen (N) | Mobile | Older lower leaves | Uniform yellowing, early leaf drop | Late flower, underfeeding | | Phosphorus (P) | Mobile | Older leaves, stems | Purple stems, dark bluish leaves, bronze spots | Cold medium, pH lockout | | Potassium (K) | Mobile | Leaf margins | Burnt, yellow-brown edges moving inward | Heavy flower feed, salt buildup | | Magnesium (Mg) | Mobile | Older leaves | Yellowing between green veins | Coco or RO water lacking Mg | | Calcium (Ca) | Immobile | New growth, shoot tips | Twisted new leaves, brown tips, weak stems | Low Ca water, pH below 6.0 | | Sulfur (S) | Immobile | Newer leaves | Uniform yellowing of entire young leaf | Rare; usually underfeeding | | Iron (Fe) | Immobile | Newest top growth | Sharp yellow between green veins | pH above 7.0, overwatering |

Nitrogen deficiency

Nitrogen drives vegetative growth, and a shortage shows up as a uniform yellowing, or chlorosis, of the oldest fan leaves, starting at the bottom and climbing. As it worsens, leaves pale to almost white and drop off. This is entirely normal in the final weeks of flower as the plant cannibalizes its own nitrogen, but it should not appear during active vegetative growth. The fix is straightforward: increase the nitrogen component of your feed or top-dress with a high-nitrogen amendment.

Phosphorus deficiency

Phosphorus fuels root development and flowering, and shortages often appear as dark, bluish-green leaves with purple stems and petioles, sometimes progressing to bronze or purple spotting. Cool root-zone temperatures, below 60 degrees Fahrenheit, are a common trigger because cold roots cannot absorb phosphorus even when it is present. Before assuming true deficiency, check medium temperature and pH, and remember that some strains naturally display purple coloration in cold conditions.

Potassium deficiency

Potassium regulates water movement, enzyme activation, and bud swelling. The telltale symptom is a burnt, chlorotic margin on older leaves, a yellow or brown edge that spreads inward while the center of the leaf stays green. It frequently appears in mid-to-late flowering when plants draw heavily on potassium for bud production. Salt buildup in the medium from overfeeding can also cause this, so an EC check of the runoff helps distinguish true deficiency from a lockout caused by excess salts.

Calcium and magnesium deficiencies

Calcium builds cell walls, and a deficiency shows up in fast-growing new tissue as twisted, hooked, or deformed young leaves with brown tips and brittle stems. Magnesium, the central atom of chlorophyll, shows as interveinal chlorosis on older leaves, where the tissue between the veins yellows while the veins remain green.

Both are extremely common in coco coir and hydroponics because those media lack the natural calcium and magnesium found in soil. They are also triggered by reverse-osmosis water, which strips these minerals out. A quality cal-mag supplement added at 1 to 2 milliliters per liter is the standard fix. Confirm the medium pH is in range first, because both lock out below 6.0.

Sulfur and iron deficiencies

Sulfur is unusual because it looks like nitrogen deficiency but appears in newer growth rather than older leaves. The entire young leaf, including veins, turns a uniform pale yellow. It is uncommon in soil fed with organic amendments but can appear in synthetically fed systems.

Iron deficiency is a classic symptom of high pH. It shows as sharp interveinal chlorosis on the youngest top growth, with veins staying noticeably green against yellow tissue. The first response is always to lower pH into the proper range; adding iron to a medium that is too alkaline will not help because it will immediately lock out again.

Fixes and a diagnostic routine

When symptoms appear, follow this order rather than guessing:

1. Check pH of the input and the runoff. Out-of-range pH is the most common cause.
2. Check EC of the runoff to detect salt buildup, which mimics several deficiencies.
3. Identify whether the symptom is on old or new growth to classify it as mobile or immobile.
4. Match the visual pattern to the diagnostic table.
5. Apply a targeted fix at half strength, and observe for several days before escalating.

Key takeaways

• Mobile nutrient deficiencies appear on older lower leaves; immobile deficiencies appear on new upper growth.
• Most deficiencies are actually pH lockout, so always check pH and EC of the runoff before adding nutrients.
• Nitrogen shows as uniform yellowing from the bottom up; phosphorus shows as purple stems and dark leaves; potassium shows as burnt leaf edges.
• Calcium and magnesium are common in coco and hydroponics, fixed with a cal-mag supplement after confirming pH.
• Iron deficiency almost always means the pH is too high, and the cure is lowering pH, not adding iron.

Frequently asked questions