Why Do Cannabis Nutrient Deficiencies Occur?

Nutrient deficiencies occur in cannabis plants when they are not given enough nutrients. Deficiencies can also happen when they are given too much of one nutrient and it causes a nutrient lockout.

Cannabis nutrients are needed in gradually larger concentrations as a plant grows. Young plants prefer a gentle, low nitrogen fertilizer for root development. As your crop matures, it will require more nitrogen with base nutrient formulations. Use a pH stable base nutrient like the Holland Secret Micro, Grow and Bloom along with a feed chart, to help avoid these issues and save you a lot of headaches in the long run.

Importance of pH in Nutrient Absorption

A common reason for deficiencies is from growing with a medium or hydroponic solution that is too alkaline or acidic. For nutrients to be absorbed by the roots the pH must be in a plant-friendly range.
A non-plant friendly range would be if the pH is too high (above 7), or too low (below 5). Future Harvest recommends an optimal pH range for growing cannabis between 5.8-6.3. This is accomplished using pH adjusters, to either raise or lower the pH into the optimal nutrient availability range. 

If a cannabis-friendly pH  is not maintained (optimal range for cannabis is 5.8-6.2), your crop’s roots will be unable to access the nutrients in your soil or hydroponic solution. This leads to nutrient deficiencies. Nutrient availability charts are very useful in understanding the concept of nutrient availability.
The concentration of nutrients is also highly important, and is monitored by the use of TDS (ppm) or EC meters.  A new grower should be cautious not to use too much fertilizer. Follow the recommended ppm (or EC) ranges that are provided by feeding charts. 

The optimal ppm ranges provided in the Future Harvest feed schedule correspond to each week of growth. Your plant can show signs of nutrient toxicity if given too much of one nutrient, or an excess of a variety of nutrients. 

What Is Nutrient Burn?

Healthy Cannabis Leaf
Nutrient burn refers to damaged plant tissues as a result of too much fertilizer. The burnt-looking tips of leaves are usually the first symptoms of nutrient burn. When an excess of nutrients is given, salt accumulates in the plant. When cannabis is unable to use these excess nutrients, it causes damage to the leaves and roots.⠀
Here we will go over the most common nutrient deficiencies and toxicities and how to detect them. Most deficiencies and excesses will overlap, proving it hard to pinpoint the exact cause. This is why it's always important to follow a strict feeding schedule when feeding fertilizer to cannabis plants.

Click HERE for Future Harvest’s feed schedules. 

Photo of a Nitrogen Deficient Cannabis Leaf

Nitrogen Deficiencies, Toxicities, and Treatments

Nitrogen is highly mobile, associated with leaves producing chlorophyll (green pigment cells) for photosynthesis. 

Deficiency symptoms

 Yellowing (chlorosis) of older leaves, beginning from the base of the canopy and progressing upward. If left untreated, leaves lose their photosynthetic function turning yellow and will eventually die (necrosis), leading to diminished yield and stunted growth. The dead and dying leaves will fall to the ground.

Yellowing is a natural and healthy occurrence from about  week 6 of flower until harvest, so do not confuse this with a deficiency. Flowering plants require less nitrogen than vegetative plants. Keep an eye out for yellow leaves ascending your plant in the vegetative stage as this is a sign of nitrogen deficiency.

Nitrogen Deficiency Treatment

The best solution for nitrogen deficiency when growing hydroponically is to use a formula that provides all the necessary nitrogen for plants.

Using Future Harvest's Holland Secret Grow, Micro and Bloom as directed by our feeding schedule, one should not see a nitrogen deficiency. Using Future Harvest's Calnesium, will also help increase nitrogen uptake and can be diluted as a foliar spray for immediate relief. 

For soil and coco growers, using Easy Plus Grow powdered nutrients, verify the EC and pH of your medium and water. All Future Harvest products are designed to avoid deficiencies if you are using as directed by our feeding schedule. 

Chelated nutrients are taken up by the plant at a slower rate than in a hydroponic solution. Increasing soil nutrients beyond what feeding schedules suggest could lead to nitrogen toxicity or lockout as well. 
Photo of a Cannabis Leaf showing a Nitrogen Toxicity

Nitrogen Toxicity Symptoms

Leaves are dark green and curl in a claw-like fashion, weak stems, stunted growth. It’s common for growers to give their plant too much nitrogen in the flowering stage. Nitrogen toxicity can result in your plant producing smaller buds.

Nitrogen Toxicity Treatment

If you are experiencing toxicity, the remedy is to flush with fresh water. Remix a nutrient solution using calibrated pH & TDS (ppm/EC) meters. This will verify nutrient availability. Keep a watchful eye as your plant recovers from toxicity.

Organic growers can amend their soil using organic compost, blood meal, worm casting and fish emulsions.

a Photo of a Cannabis Leaf with a Phosphorus Deficiency

Phosphorus Deficiencies, Toxicities, and Treatments

Phosphorous deficiencies diminish a plant's ability to release stored carbohydrates for energy and hinder photosynthetic potential. A phosphorous deficiency can lead to low yields, less resin production during flowering and can delay flowering.

Deficiency Symptoms

Phosphorus is mobile so older growth will display symptoms first. Phosphorus deficiencies turn leaves a bluish-purple tint. Black and dead spots on the leaves then appear before the affected leaves eventually die and fall from the plant. 

Toxicity Symptoms

Toxicity is not likely to occur at a correct pH level. It is quite uncommon and takes weeks to show symptoms. Excess phosphorus can potentially lock out other nutrients like iron, zinc, magnesium, calcium and copper. This lockout will produce symptoms of micronutrient deficiency. 

Phosphorous Deficiency Treatment

Growers should flush with pH adjusted fresh water. Increase temperatures of the growing environment and decrease frequency of watering as a preventative measure. Use soil amendments like perlite increase aeration to ensure proper drainage prior to potting.

Phosphorous deficiencies can occur naturally in cold weather, compacted soils, overwatered media, as well as infestation and disease. Plants should bounce back in about a week's time after treatment, though affected foliage may not recover. 

a Photo of a Cannabis Leaf showing a Potassium Deficiency

Potassium Deficiencies, Toxicities, and Treatments

Potassium is a macronutrient responsible for transporting water and nutrients through your plant. Potassium is integral in water/nutrient uptake, transpiration, root production and cell division. A potassium deficiency increases the internal temperature inside the plant which causes excessive transpiration rates to cool a  cannabis plant to cool down.
Potassium deficient plants will be much more susceptible to disease and pest infestation. Potassium deficiencies can be caused by too much calcium and magnesium, which can also block nitrogen from being absorbed. Reverse osmosis water can cause calcium deficiency because it does not contain calcium, unlike regular tap water. 

Deficiency Symptoms

Distorted or curled leaves starting from the oldest foliage at the bottom of the canopy. Deep dull-green older leaves with burned tips, brown spots and interveinal yellowing. Symptoms will progress up the canopy and result in weak plants with drastically diminished yields. 

Toxicity Symptoms

Excess potassium does not have any known toxic effects, but it can affect the availability of other nutrients (nitrogen, calcium, magnesium).


Flush the media with pH adjusted water and suggested concentrations. Ensure Calnesium or any other calmag product, is not being used to excess by referencing a feeding chart. Calibrate all necessary meters and probes & remix your nutrient solution. 

a Photo of a Cannabis Leaf showing a Calcium Deficiency

Calcium Deficiencies, Toxicities, and Treatments

Calcium is an immobile secondary nutrient that aids in the plant's stability and helps with resistance to environmental stresses such as excessive heat.

Deficiency Symptoms 

Will be present in new growth first. The foliage may have small dark spots on leaves as well as yellowing that begins from the veins of the leaves and spreads outward. The new leaves may grow distorted and curled.

Yellowing does not occur in every symptomatic plant but is a secondary symptom to watch for. With time, the spotting or blotches will appear dark brown or rustic in colour. Calcium deficient plants are much more susceptible to diseases like root rot

Toxicity Symptoms

Calcium is not harmful to plants, but too much calcium can cause high pH levels. This makes it difficult for other essential nutrients to be absorbed. Reverse osmosis water can cause calcium deficiency because it does not contain calcium, unlike regular tap water. Calcium is found in abundance in soil, so soil growers need not worry unless growing in extremely acidic soil. 

Calcium Deficiency Treatment

Add a calmag product like Calnesium to your regular feeding schedule in conjunction with your base nutrients. Verify optimal pH and ppm ranges of your reservoir and stop feeding with RO water if possible. If you're growing organic you can use an organic calmag product like Organical Magic.

Buy Organical Magic or Calnesium today


Magnesium Deficiencies, Toxicities, and Treatments

Magnesium is a mobile secondary nutrient, the core of chlorophyll molecules (green pigment) and serves many essential enzymatic functions. Magnesium deficiency symptoms present themselves on the oldest growth first, at the bottom of the canopy. 

a Photo of a Cannabis leaf showing signs of Magnesium Deficiency

Deficiency Symptoms

Interveinal yellowing progressing up the canopy, yellowing occurs from the center of the leaf out toward the margins. The affected leaves may also curl in an upward direction. Magnesium can also be flushed from the media due to overwatering. Using bloom fertilizers in excess can cause high levels of phosphorus and potassium that can lead to magnesium lockout. 
Iron deficiency can occur in alkaline soils, where too much calcium or bicarbonate can prevent the iron from being absorbed.

Toxicity Symptoms

Toxicity is very rare, though it can occur by interfering with calcium uptake. A magnesium toxicity will display the same symptoms as a calcium deficiency. (see above)


Flush the medium with pH-adjusted fresh water. Add Future Harvest Calnesium (Organical Magic for soil and organic growers) to your regular feeding schedule in conjunction with Holland Secret Grow/Micro/Bloom.

Verify optimal pH and ppm ranges of your reservoir. Cut back on bloom formulation concentrations in your reservoir. Verify humidity and temperature in your growing environment are within optimal ranges. 

a photo of a cannabis plant showing signs of Iron Deficiency

Iron Deficiencies, Toxicities, and Treatments

Iron plays roles in leaf pigment, respiration and is involved in enzyme production within a plant. Iron is most readily available in ferric form, though it can be absorbed organically as well. Iron deficiencies are usually the result of pH fluctuation.

A pH above 7.0 will lead to iron deficiency through lockout.  High levels of salt content in the media, too much moisture, and excessively high or low temperatures will also affect iron uptake. 

Deficiency symptoms

Symptoms appear on new growth first. Young leaves turn yellow while the veins remain a darker shade of green. Similar symptoms to magnesium deficiency (see above), but occurring on young foliage. New growth may be distorted or lack leaflets.

Iron deficiency is rare in soil but can occur due to an abundance of calcium and/or bicarbonate causing iron lockout in an overly alkaline soil. 

Toxicity symptoms

Are the same as deficiency symptoms for various other macronutrients because excess iron causes lockout of other nutrients. 


As a preventative measure, make sure the medium has proper drainage prior to potting. If deficiency or toxicity symptoms appear, flush media with fresh water, verify optimal pH and ppm ranges of your reservoir. Recalibrate environmental controls. -(humidity, temp, etc.).

a photo of a Cannabis Leaf showing signs of a Sulphur Deficiency

Sulphur Deficiencies and Treatments

Sulphur aids in chlorophyll production, protein production, enzymatic functions and the synthesis of essential resins and oils. 

Sulfur Deficiency Symptoms 

Yellowing similar to that of Nitrogen but occurring in new growth as Sulphur predominantly mobile. The yellowing is unique however, in the sense that it occurs from the base of the leaf outward. Sulfur deficiencies can also cause discoloration on the underside of the leaf with pigments of pink, red or orange.
Cannabis plants that are deficient in sulfur during flowering may see their flowers die if left untreated. Sulfur deficiency is rare however, as plants can derive sulfur from the atmosphere through the stomata. Future Harvest formulations provide sulfur in carefully formulated concentrations to avoid toxicity and deficiency with proper use.


If deficiency or toxicity symptoms appear, flush media with Future Harvest Plant Flush, verify optimal pH and ppm ranges of your reservoir. Recalibrate environmental controls (humidity, temp, etc).

Those that work in commercial facilities are accustomed to sulfur burns to provide sulfur in the atmosphere and help aid in powdery mildew control. These applications are much better suited to the commercial market. Sulfur burns require a 24-hour period to elapse before re-entry of cultivation personnel until the greenhouse growing environment is considered safe. 

Grow Doc Deficiency Chart showing all of the most common DeficienciesGrow Doc Deficiency Poster

Final Thoughts 

Diagnosing cannabis deficiencies can be notoriously difficult. It is best to have a photographic aid as guidance, along with a detailed description of the toxicity or deficiency symptoms. 

We recommend the groundbreaking Growdoc app for smart devices. Growdoc allows growers to use AI technology and scientific research to identify cannabis nutrient deficiencies, pests and diseases with  pictures of their own plants!

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