Cannabis Nutrient Deficiencies: How to Spot the Most Common Ones

The most common cannabis nutrient deficiencies are nitrogen, phosphorus, magnesium and potassium, and you spot them by reading the leaves: what color they turn, how they curl, and whether the damage starts on old or new growth. Nutrient mistakes are about the most common thing that goes wrong in a grow, since every plant is different and has its own needs, so excesses and deficiencies are bound to pop up.

Most problems you'll run into while growing come down to three key factors: environment, nutrition and watering. Any swing or inconsistency in those can have a big impact on your plants' health. Deficiencies show up on cannabis leaves and on the plant in different ways depending on what's going on.

Common symptoms include:

• Yellow leaves: could be a sign of almost any deficiency, or of mild burn
• Leaves curling up: could point to a potassium deficiency or overwatering
• Brown leaves: could point to a phosphorus, potassium, magnesium, sulfur or manganese deficiency. It could also be nutrient burn (more solid brown) or heat stress (brown around the edges)
• Droopy leaves: could be a sign of overwatering
• Leaves curling down: could point to a potassium or phosphorus deficiency, or overwatering
• Red or pink coloring on the leaves
• Burnt leaf tips
• White powdery spots
• Wilting plants

How to tell a nutrient excess from a deficiency

Spotting deficiencies and excesses on cannabis leaves takes a lot of close observation, because your plants will tell you what's going on nutritionally.

To properly identify an excess or a deficiency, you need to know what kinds of nutrients your plants need and how they work. Nutrients fall into these groups based on how much of each the plant uses:

1) Primary nutrients: Nitrogen (N), phosphorus (P) and potassium (K). The most important, since they're what the plant uses the most of.

2) Secondary nutrients: Calcium (Ca), magnesium (Mg), sulfur (S). Second in importance, and they also help the plant take up the primary nutrients.

3) Micronutrients or trace elements: boron (B), manganese (Mn), zinc (Zn),  iron (Fe),  chlorine (Cl), copper (Cu), silicon (Si) and a few more. Essential for optimal growth and top-quality results.

There's another classification worth knowing: the mobility of nutrients, since some move easily through the plant and others don't. Knowing this helps you pin down the deficiency or excess just by checking whether the changes show up on the top or the bottom of the plant.

1) Mobile nutrients (symptoms start on older, lower leaves): Nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), zinc (Zn).

2) Immobile nutrients (symptoms start on new, upper growth): Calcium (Ca), boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), silicon (Si), sulfur (S).

Pale, yellowing leaves? Spotting nitrogen deficiency

This is one of the most common deficiencies among rookie growers. It's easy to spot and you can fix it as soon as it shows, although in the early stages it looks a lot like other deficiencies.

So why do cannabis leaves turn yellow? In the early stages of a nitrogen deficiency, the leaves look too pale, almost yellowish green, compared to the deep green you want on healthy leaves.

With this deficiency in particular, the veins usually look a bit darker than the very pale leaf tissue around them, though the contrast isn't as sharp as with other deficiencies.

What does nitrogen toxicity look like?

On the flip side, too much nitrogen gives you leaves with a very dark green tone that can look almost black in extreme cases.

Prevention

• Keep pH in the sweet spot (6.0 to 6.5).
• Your soil mix should be rich in nutrients.
• Regular feeding keeps your medium nutrient-rich down the road.
• Mycorrhizae will help boost your plant's access to this nutrient.

Fixes

• Organic fertilizers have enough nitrogen to correct this deficiency.
• Adjust your pH accordingly.
• Compost tea makes a perfect foliar spray for a quick fix.
• Bump up the nitrogen in your compost with food scraps, fresh trimmings and grass clippings.

Phosphorus deficiency in cannabis

Phosphorus deficiency isn't very common. When it hits during the vegetative stage, you can see slower growth, small leaves and weak roots.

Early on, a phosphorus deficiency tends to make leaves look dark but dull, with very dark veins and slightly lighter leaf tissue.

As it progresses, the leaves start to change: purplish discoloration appears across the main part of the leaf, and the edges start turning brown and curling down.

The petioles (the little stems that connect each leaf to the main stem) can also start turning purple. In the final stages, large patches of the leaves turn purplish brown and die, while the rest of the leaf fills with yellowish-green blotches.

Prevention

• Make sure your soil is rich in organic matter.
• Well-aerated soil improves uptake.
• Adding mycorrhizal fungi to the soil helps improve phosphorus uptake.
• Add more manure to your compost.

Fix

• Nudge your pH toward the high end of the ideal range.
• Worm castings and fish meal worked into the soil are perfect.
• Use an organic fertilizer that's high in phosphate.
• Don't overwater: only water when the top 3 cm (about 1 inch) of soil is dry, so it doesn't get compacted
• Move your plants somewhere warmer, or put down a tarp to trap heat, since cold makes phosphorus harder to absorb.

Magnesium deficiency in cannabis

Magnesium deficiency can look a lot like potassium deficiency. In the early stages, leaf edges start turning yellow while the veins stay dark. As it progresses, the edges dry out and curl inward. The upper leaves get hit too, until eventually even new growth comes in pale and chlorotic (lacking chlorophyll). Brown spots can also show up on the lower leaves before they drop off and die.

Prevention

• Add dolomite lime to your growing medium.
• Use a manure-rich compost.
• Keep your pH well balanced.

Fixes

• If your pH is off, flush your medium with water at pH 6.0.
• Epsom salt is another source of magnesium. Mix 1 to 2 teaspoons of Epsom salt into about 4 liters (1 gallon) of water and use it until the symptoms clear up.

Potassium deficiency in flowering cannabis

Potassium deficiencies usually show up when there's too much nitrogen or too much calcium in the medium.

In the early stages, spots appear on the leaves and they start turning pale yellow while the veins stay green. Leaf tips and edges can dry out, turn brown and start curling up or down.

As the deficiency progresses, brown necrotic (dead) spots start appearing all over the leaves, and the edges look burnt and crispy. New leaves can twist and deform, and you'll see droopy cannabis leaves.

Prevention:

• Go easy on the fertilizer.
• Beef up your compost with wood ash and kelp meal.
• Don't water too often.

Fixes:

• Flush your growing medium.
• Measure and adjust pH to fix any possible nutrient lockout.
• Chicken manure in the soil can help.
• Organic seaweed as a foliar spray could help.

When it comes to nutrient deficiencies and excesses, the key is catching them early. The sooner you notice a problem in your grow, the better your chances of fixing it and bringing your plants back in time.