Cannabis Nutrient Deficiencies and How to Fix Them

Cannabis plants are pretty vocal about what they need. When something is off nutritionally, the leaves change color, curl, spot, or die in specific patterns that tell you exactly which nutrient is lacking. Learning to read these signals early saves you from losing weeks of growth to problems that are usually easy to fix.

Before You Diagnose: Check pH First

About 80% of apparent nutrient deficiency problems are actually pH problems.

If your soil pH drifts outside the 6.0 to 7.0 range (or 5.5 to 6.5 in coco/hydro), your plant cannot absorb certain nutrients even if they are present in the medium. This is called nutrient lockout and it mimics deficiency symptoms exactly.

Before adding more nutrients, test and adjust your pH. Get a pH pen (Apera PH20 at about $50 is reliable) and test your runoff water. If pH is off, flush with properly pH-adjusted water and retest.

Correcting pH alone often resolves what looks like a deficiency within a few days.

Nitrogen Deficiency

Nitrogen is the nutrient cannabis uses most during vegetative growth. A deficiency shows up as yellowing of the lower, older leaves first. The yellowing starts at the tips and spreads inward. Eventually the affected leaves turn completely yellow and fall off. The plant will look pale green overall and growth slows noticeably.

How to fix it: Increase nitrogen in your feeding.

If you are using a bottled nutrient line, increase the grow/veg component slightly. For organic growers, top-dress with blood meal, fish meal, or a compost tea heavy on nitrogen. You should see improvement within 5 to 7 days. Note that some yellowing of lower leaves during late flowering is normal as the plant redirects nitrogen to the buds. Do not chase nitrogen deficiency in the last two weeks of flower.

Phosphorus Deficiency

Phosphorus is critical during flowering for bud development.

Deficiency shows as dark green or blue-green leaves that develop purple or reddish stems. Affected lower leaves may develop dark spots and eventually turn brown and crispy. Growth slows and buds stay small.

How to fix it: Increase phosphorus. For bottled nutrients, bump up the bloom component. Organically, bone meal or bat guano (the high-phosphorus variety) work well. Make sure soil pH is in range, because phosphorus locks out below pH 6.0 particularly fast.

Potassium Deficiency

Potassium helps with water regulation, disease resistance, and overall plant vigor. Deficiency shows as brown, crispy edges on leaves (leaf margin burn), starting with older leaves.

The leaves may also curl upward. Stems can become weak and the plant may look overall unhealthy even if color is okay.

How to fix it: Increase potassium. Bottled bloom nutrients usually contain plenty of potassium, so this deficiency is less common when using a complete nutrient line. Organically, kelp meal, wood ash (use sparingly as it raises pH), or langbeinite provide potassium.

Calcium and Magnesium Deficiency

These two are grouped together because they commonly occur together, especially in coco coir (which naturally binds calcium) and when using filtered or reverse osmosis water.

Calcium deficiency shows as brown spots on newer growth, crinkled or distorted new leaves, and overall weak structure.

Magnesium deficiency shows as interveinal chlorosis on middle-aged leaves. The veins stay green while the tissue between them yellows, creating a striped pattern.

How to fix it: Cal-Mag supplements are the standard solution. General Hydroponics CaliMagic or Botanicare Cal-Mag Plus at 2 to 5 ml per gallon resolves most cases. If you are growing in coco, add Cal-Mag to every watering as a preventive measure.

In soil, dolomite lime mixed into the soil before planting provides slow-release calcium and magnesium.

Iron Deficiency

Iron deficiency looks similar to magnesium deficiency but affects the newest growth at the top of the plant instead of middle-aged leaves. New leaves emerge yellow or pale with green veins. This is almost always caused by high pH (above 7.0) locking out iron rather than an actual lack of iron in the soil.

How to fix it: Lower your pH. Flushing with water at pH 6.0 to 6.3 usually resolves iron lockout quickly. Chelated iron supplements can help in acute cases, but fixing pH is the real solution.

Prevention Is Easier Than Treatment

Most nutrient problems are preventable with three practices: maintaining proper pH (6.0 to 7.0 in soil), using a complete and balanced nutrient line at the recommended strength, and not overwatering (which reduces oxygen at the roots and impairs nutrient uptake). Start nutrients at half the recommended strength and increase gradually. Overfeeding causes just as many problems as underfeeding, and the symptoms can be confusingly similar. When in doubt, less is more.