NPK Problems in Cannabis: Nitrogen, Phosphorus and Potassium Deficiencies and Excesses

Cannabis plants show nutrient problems on their leaves: a nitrogen shortage yellows the lower leaves first, a phosphorus shortage turns leaves bluish-green with purple leaf stems, and a potassium shortage burns the leaf tips and edges. Excesses are just as damaging, often because they block the uptake of other nutrients. Just like people, weed plants need a healthy, balanced diet to survive and thrive, and when macronutrients run short or pile up, the leaves get spotty and turn yellow.

To figure out whether your plant is missing something or getting too much, look at the leaves. Spotted leaves, dry leaves and yellow cannabis leaves can point to a potassium deficiency during flowering or too much nitrogen.

A deficiency and excess chart can be a handy tool for understanding what cannabis needs. Overdo the fertilizer and you can damage the roots and stunt the plant. So feed in the right amounts and pay attention to the signals your plant is sending.

Nitrogen (N)

Nitrogen is a mobile macronutrient (the plant can move it from old leaves to new growth) that plays a big role in photosynthesis and in building essential plant proteins.

Signs of nitrogen deficiency:

- Lower leaves turn yellow and lose their shine, 

- The yellowing works its way up the plant, 

- Leaves start dropping, 

- Plants stay shorter, with smaller leaves, 

- Flowering starts early and yields are low. 

The fix: most organic fertilizers carry enough nitrogen to correct this, like fish meal, manure, alfalfa meal or feather meal. It's also important to adjust your pH accordingly, use compost tea as a foliar spray for a quick fix, and boost the nitrogen in your compost with food scraps, fresh pruning trimmings and grass clippings.

Signs of nitrogen excess (toxicity): 

- Lower leaves turn dark green, and that dark color moves upward, 

- Foliage and stems get weak, 

- The plant's water and fluid transport system weakens, 

- The harvest tastes grassy and green

To prevent it, keep your pH in the sweet spot (6.0-6.5), use a nutrient-rich soil mix and fertilize so the medium stays well stocked over time.

Phosphorus (P)

Phosphorus is a macronutrient for cannabis and, since it's mobile, the plant can send it wherever it's needed most. It's essential for photosynthesis and protein synthesis, and it's a building block of DNA. 

Signs of phosphorus deficiency: 

- Bluish-green leaves, 

- Petioles (where the leaf joins the stem) turning purple, 

- Vertical and side growth slowing down, 

- Dead spots on the lower leaves, from dark copper or purple to black, 

- Affected leaves taking on a dark, metallic, bronze-purple color, then curling, twisting, wilting and dropping

- A weak plant that's more vulnerable to disease and pests

How to fix it: nudge the pH toward the upper end of the ideal range, since the plant takes up phosphorus more easily there; work worm castings and fish meal into the soil; and use an organic, phosphate-rich fertilizer. Only water when the top 3 cm (about 1 in) of soil is dry, so the medium doesn't get  compacted, and move your plants somewhere warmer or cover them with a tarp to trap heat, since cold slows phosphorus uptake. 

Signs of phosphorus excess:

The plant will show symptoms of zinc and iron (micronutrient) deficiencies on one hand, and calcium and magnesium deficiencies on the other. 

Zinc and iron lockout looks like this: 

- The newest leaves develop interveinal chlorosis (yellowing between the veins)

- Leaves grow thin

- Leaf tips and edges burn

- Shorter internodes (the spacing between branch points), which means a smaller harvest.

Calcium and magnesium lockout looks like this: 

- Lower leaves curl

- Spots develop

- Root tips turn black

- Dried buds have a chemical taste.

To head off all of these problems, follow these tips: 

- Use soil rich in organic matter.

- Boost uptake with well-aerated soil.

- Add mycorrhizal fungi to the soil to improve phosphorus uptake. 

- Add more manure to your compost.

Potassium (K)

Potassium is the third and last of the macronutrients. It helps regulate CO₂ uptake and plays an important role in photosynthesis. It also pitches in on ATP production (ATP is the cell's energy currency).

Signs of potassium deficiency:

- Older leaves turn pale and yellow (chlorosis)

- Leaf edges and tips turn rusty and burn

- Stems can get weak, spindly and sometimes brittle

- Stem branching may increase

- Flowering is delayed and reduced

To fix it, flush the growing medium, measure and adjust the pH to undo any nutrient lockout, work chicken manure into the soil and use organic seaweed as a foliar spray.

Too much potassium causes calcium, magnesium, zinc and iron deficiencies:

- The newest leaves develop interveinal chlorosis

- Leaves grow thin

- Leaf tips and edges burn

- Internodes get shorter

- Lower leaves curl and develop spots

- Root tips turn black.

Prevention is the best cure here. Go easy on the fertilizer: feed too often and salts can build up and interfere with potassium uptake. Also beef up your compost with wood ash and kelp meal, and don't overwater.

Deficiencies and excesses can be prevented. It's all about learning to read your plant, staying ahead of deficiencies, keeping the right tools  on hand to fix whatever comes up, and reading up on the subject as much as you can. 

Why are my cannabis leaves turning yellow?

Cannabis leaves can turn yellow for several reasons, but one of the main ones is a nutrient deficiency, especially nitrogen, potassium or magnesium. It can also come from overwatering, disease or pests. So it's important to pin down the underlying cause and act to fix it.

How do you tell an excess from a deficiency?

Telling an excess from a deficiency on cannabis leaves can be tricky, but a few signs help you catch the problem. If the leaves are yellow, check whether it's just the tips that are yellowing or whether leaves are dropping in your indoor grow.

Yellow cannabis leaves can be a sign of a shortage of essential nutrients like nitrogen, potassium or magnesium, but also of too many nutrients. Yellow spots can point to nutritional problems or to disease and pests. Leaves that are dry and crispy at the tips can also signal too much fertilizer or air that's too dry.

To fix yellow leaves on a weed plant, figure out the underlying cause and correct it. If it's a potassium deficiency, adjust your feeding or watering and add the right nutrients to correct it. If nitrogen is the problem, add a nitrogen-rich fertilizer.

As for excesses, cut back on fertilizer as soon as you spot one. Also make sure your plants are getting enough water and airflow, since a damp, stuffy environment can cause leaf problems of its own.

What happens if you overfertilize cannabis?

Overfertilizing can seriously damage cannabis plants. It causes salts to build up in the growing medium, which makes it harder for the plant to take up nutrients and water. The result: slower growth, yellow and drooping leaves, and fewer buds. Too many nutrients can also hurt the flavor and aroma of your weed. So follow the dosing instructions on your fertilizers and don't overdo it, or you'll end up hurting the plants you're trying to help.

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