Cannabis Nutrient Deficiencies Guide

It is not uncommon for cannabis plants to come across some type of deficiency during their life cycle. The good news is there is always a quick way to resolve most nutrient deficiencies, providing they are treated properly. This guide will help you diagnose any possible nutrient deficiencies in your cannabis plants.

A Healthy Cannabis Plant

Normally when we think about a cannabis plant we imagine big frosty buds surrounded by bushy, lush green leaves. A thriving marijuana plant does not require much other than sunlight, air, water and some nutrients. Any problems and the plant will let us know through different physical changes. These signs are typically spotted through the plant's leaves. 

A healthy cannabis plant has the natural green colour and appearance you would expect it to. It is quite easy to tell when there is a problem because the leaves will not look normal to you. Even if a cannabis plant is growing well, monitoring leaves daily means deficiencies can be detected early before they become too serious. 

Mobile And Immobile Nutrients

Understanding the difference between mobile and immobile nutrients makes it easier to diagnose a deficiency in a cannabis plant. These minerals can be distinguished by how they are able to move from their storage point to other areas of the plant.

In the event of a deficiency, mobile nutrients can be transferred from the leaves to other leaves that need it more. This means that the required mineral will be sent from the stores in the older fan leaves to support new growth. If you've ever noticed, this is why cannabis plants yellow from bottom to top towards the end of their life cycle as they use up their remaining mobile minerals.

Immobile nutrients are not readily available to plants like mobile nutrients are. Once stored in a cannabis plant, they cannot be redistributed and do not move. The problem with this is that plants begin to use fresh minerals being sent to the new foliage growth. Immobile nutrient deficiencies can stunt a plants development more severely because plants have to work harder to search and deliver the nutrition to the new leaves.

Why is my Cannabis Plant Deficient?

Cannabis plants can become deficient at any point in their life. If plants are receiving a good amount of sunlight and water, it is quite likely the problem is to do with the nutrient levels. However, there are a few other factors which can help explain why your cannabis plant might be suffering from a lack of nutrition.

pH

Whether you are growing in soil or hydro, the acidity or alkalinity of your grow medium is extremely important for proper nutrient absorption. The pH will determine how effectively certain minerals are delivered from the roots to the plant. If the pH level falls out the correct range the different nutrients will start to become locked out.

For soil cultivation, the ideal pH range is between 6 and 7, whereas for hydroponics cannabis grows best with a pH of around 5.8. This is due to the cation exchange and how roots uptake minerals in different mediums. In soil the minerals have medium to absorb into, whereas in hydro the exchange is happening directly between the water and the roots.

Electrical Conductivity (EC)

EC (electrical conductivity) and PPM (parts per million) measures the salt concentration of water or a nutrient solution. Measuring the EC or PPM won't tell you exactly what mineral your cannabis plants are deficient in, but it will give you a good indication as to the strength of your water and feeding solution.

Normally EC is checked to avoid giving plants a high concentration of minerals but it can also be useful to get an idea of how much plants are consuming. Store bought nutrient solutions for cannabis are often heavily concentrated meaning most growers dilute them to avoid over feeding (which can lead to nutrient lock outs).

Adding nutrients to your tap water increases its electrical conductivity so it might be that a deficiency occurs because the concentration is too high. Depending on what you buy, make sure the EC of your solution stays more or less between these levels to avoid deficiencies or toxicities:

• Seedlings - 0.7 - 1.2 mS/cm2
• Vegetative - 1.6 - 1.9 mS/cm2
• Flowering - 1.9 - 2.2 mS/cm2

Different Nutrient Deficiencies

Now, let's go over some of the minerals you might encounter when facing a nutrient deficiency. These can be split into two categories: macronutrients and micronutrients.

Macronutrients are minerals heavily consumed by a plant, and micronutrients are trace nutrients used in very small quantities. Hydrogen, oxygen and carbon are considered macronutrients but we will not be discussing these here as we assume your plants have enough air and water.

Macronutrients:

Nitrogen

Nitrogen deficiency is a common issue in cannabis cultivation. Nitrogen is used primarily in the vegetative phase and necessary for the development of chlorophyll. Without it, plants do not photosynthesise properly and leaves turn yellow as the reserves are taken from lower fan leaves.

Deficiency Symptoms:

• Yellowing leaves in lower areas of the plant
• During vegetative phase
• Leaves eventually dry, curl and fall off
• Slow growth

Tip: Yellowing (chlorosis) is normal towards the end of the flowering stage and a healthy sign a cannabis plant is nutrient-free before harvest.

Phosphorous

Phosphorous plays a big role in a cannabis plant's ability to photosynthesise and reproduce. As a macronutrient, it is needed in large quantities during flower for the production of resin. Deficiencies in phosphorous are not too common as it is usually abundant in feeding solutions and grow mediums, however lock outs from pH imbalances can still happen. 

Deficiency Symptoms:

• Stunted growth
• Deficiency works inwards from leaf tips
• Fan leaves turn a rusty purple/brown/yellow
• Parts of leaf curl

Tip: Phosphorous can become locked out with low temperatures and overwatering. Maintain grow medium above 15°C.

Potassium

Potassium is especially important during the blooming period for the production of flowers. As well as being essential for plant growth, potassium helps to protect against pests and disease, improving defences and the overall function of a cannabis plant. Potassium is also responsible for a large part of how plants disperse sugars or evaporate water.

Deficiency Symptoms:

• Increased temperature of plant
• Accelerated transpiration
• Dull/desaturated leaves
• Scorched leaf edges turn yellow/brown

Tip: Adding nitrogen can decrease potassium absorption, and vice versa.

Micronutrients:

Calcium

Cannabis plants need calcium for cell development and strength. Calcium deficiency is less of an issue in soil grows because pH levels are higher than they are in hydroponics. Shortages usually occur because of cold, wet conditions when the soil or grow medium is too acidic. Results in lack of transpiration and other nutrient problems.

Deficiency Symptoms:

• Weak stems
• Newer growth affected first, become distorted
• Brown spots on upper fan leaves
• Deficiency shows in areas exposed to light

Tip: Use Dolomite Lime for soil grows to help prevent calcium deficiency.

Magnesium

Magnesium is an immobile nutrient that enables a cannabis plant to convert light into useable energy, making it a key element in the process of photosynthesis. Magnesium is the main part of chlorophyll that allows it to synthesize carbohydrates and sugars for the plant. If pH is too low, not enough magnesium gets absorbed and a cannabis plant will quickly suffer.

Deficiency Symptoms:

• Lower leaves affected first
• Light leaf margins
• Crispy leaf edges
• Brown spots and red stems can develop

Tip: Epsom salts can be used to remedy a lack of magnesium.

Sulphur

On the rare occasion cannabis plants can experience a sulphur deficiency. Sulphur promotes efficient use of nitrogen as well as the forming of chlorophyll, oils and terpenes. The symptoms of sulphur deficiency are very similar to nitrogen, and is most noticeable through the yellowing of newer foliage. It can be particularly problematic during flowering, as heavily affected buds start to die due to the lack of protein.

Deficiency Symptoms:

• Yellowing begins at the base of leaf fingers and works its way up
• Discolouration under leaves
• Hints of red and pink

Tip: Mycorrhizae fungi are beneficial in soil to increase sulphur content.

Iron

Issues with iron often stem from deficiencies or excess of other minerals such as calcium or zinc. The symptoms look like a magnesium deficiency, although if the problem is related to iron, yellowing will start from the top of the plant. High pH levels or over watering are also often the cause of an iron deficiency. Fortunately, cannabis plants can recover quickly after the correct treatment.

Deficiency Symptoms:

• New growth completely yellow
• Deficiency starts at base of fan leaves
• Chlorosis between leaf veins
• Roots lose function

Tip: Blood meal is a great source of nitrogen and iron for soil growing. Use high quality grow mediums and avoid cheap nutrients. 

Copper

Copper is only necessary for a cannabis plant in trace amounts and is actually poisonous if too much is added. Grow mediums including soil do not usually contain any copper, but plants receive it anyway because most tap water contains a little. Copper helps to break down carbohydrates and is required for proper bud production. Yields can be heavily affected if the deficiency worsens.

Deficiency Symptoms:

• Deformed/twisted growth
• Green colours darken with hues of blue/purple
• Leaf edges turn yellow or pale
• Tips begin to curl under

Tip: Check or test your grow medium before use so you know what minerals it contains.

Zinc

As part of the proteins and hormones, zinc aids in the growth of branches, stems and overall plant structure. It fortifies the membranes of leaves and gives plants their robustness. Lack of zinc results in poor enzyme function, leading to small plants with low yields. Signs can be clearly seen through changes in leaves, indicated by tight groups of withering, yellow shoots.

Deficiency Symptoms:

• Leaf veins shrivel/shrink
• Interveinal chlorosis
• Plants stop growing vertically/bush out
• Tips eventually dry out

Tip: Using a cal/mag solution with trace elements can help to resolve zinc deficiencies.

Manganese

Manganese deficiency isn't too common. However, this micronutrient is a key element for vigorous plant growth. Photosynthesis and nitrogen metabolism are two of manganese's main uses. It also utilizes iron for the production of chlorophyll, meaning high iron levels can cause lock out.

Deficiency Symptoms:

• Mottled grey/brown/purplish spots
• Upper growth affected early
• Similar to magnesium deficiency
• Heavily affected leaves wither

Tip: Careful when adding manganese as iron levels can become imbalanced.

Molybdenum

Although consumed in small quantities, molybdenum is necessary for enzyme processing in a cannabis plant. This mineral deficiency can be hard to detect and the similar chlorosis development is not to be confused with magnesium or nitrogen deficiencies. Low pH and cold conditions are usually the cause of a molybdenum deficiency.

Deficiency Symptoms:

• Red/pink leaf edges
• Interveinal yellowing
• Veins can turn a darker, deep green
• Spots and mottling

Tip: Flush the system and adjust pH if cannabis plants develop molybdenum deficiency.

Boron

Boron is essential for plants to develop a healthy structure, from its cell walls to the protective leaf membranes. Normally a problem occurs when plants dry out too much and do not receive enough water. Cannabis plants with a boron deficiency tend to experience other deficiencies at the same time, making it tricky to diagnose.

Deficiency Symptoms:

• Plant's stems and branches soften/weaken
• New shoots curl, turning grey/brown
• Fresh growth does not expand
• Dry spots appear on leaves
• Roots start to die

Tip: Maintain higher humidity levels to avoid boron deficiencies. Boric acid can also be mixed with water.

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Grow Tips For Cannabis Deficiencies

Testing Runoff

Problems with nutrient deficiencies are often caused by an unbalanced pH. Nutrients can be available in a growing medium but not be absorbed by roots if the pH is not sitting within the proper range. The first thing to do if you think you have a nutrient deficiency is to check the pH and EC of your runoff.

Testing the runoff gives you an indication of how much of the nutrient solution is being used by plants when you water. If the PPM or EC of your runoff is drastically lower than when you watered then plants may need more nutrients. However. If the EC stays more or less the same or is higher, it could mean the concentration is too high.

Tap Water and RO

Test the PPM or EC of your tap water. Tap water already contains some micronutrients, however the concentration and quality depends on your region. Most of the time tap water is fine to use but in some cases the mineral content can be too high for a cannabis plant. If your tap water is not potable (suitable for drinking) then it is probably not good for plants either.

Some growers will filter their water using reverse osmosis (RO) to remove almost all the salt content. Reverse osmosis strips over 95% of the dissolved minerals, leaving you fully in charge of the water profile. Filtering your water is beneficial if it has high levels of chlorine or fluoride. Water filtered using reverse osmosis needs buffering with cal/mag to replenish minerals lost otherwise deficiencies are likely to occur.

Flushing

Because deficiencies are sometimes caused by excess of other minerals, flushing before you treat a problem is recommended. Bear in mind that deficiencies can also be caused by a lack of minerals elsewhere in the plant. Flushing is the safest way to avoid further build ups or lockouts, and gives you the ability to adjust the pH properly before feeding again.

Humidity & Temperature

Cannabis plants cannot absorb nutrients correctly if the humidity and temperature are off. If humidity levels rise, moisture is taken through the leaves. If it is low, less moisture is absorbed from the atmosphere and the uptake of water happens primarily through the roots.

When humidity levels are too high, plants use less of the nutrients available around the root zone.

Temperatures determine how thirsty your plant will be. High temperatures mean faster water uptake and low temperatures won't encourage drinking. The key is to get the balance right. Usually, cannabis plants prefer temperatures between 20 and 25 degree celcius with a relative humidity of 55 - 60%. Nutrient deficiencies and other problems can occur easily in colder climates if the humidity is above average. 

Conclusion

Cannabis nutrient deficiencies should not be a problem if you are feeding your plants the right amount of minerals. Luckily, there are plenty of different fertilizers on the market that will cover these basic requirements, providing your cannabis plants with everything they need to produce high quality yields. Just remember to not over feed, keep a stable environment and monitor the pH and you will be fine.

There is much more to discuss on the topic of cannabis nutrient deficiencies so feel free to share your thoughts with the community down below!

External References

Expert System for Diagnosing Mobile and Immobile Nutrients Deficiency of Plants. - Saleem, Husnain & Khan, Abdur Rashid & Jilani, Tehseen. (2020)

This article was updated September 2020