What is Calcium Deficiency in Cannabis Plants and How to Fix it?

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Added 21 April 2022

The doctors often ask us to eat a calcium-rich diet — it’s good for bones and muscles. Similarly, your beloved cannabis plant needs calcium to grow big and strong. 

A plant grows healthy through a complex interaction of various compounds, including macronutrients like nitrogen, potassium, and phosphorus (NPK) and micronutrients like magnesium, boron, iron, etc. 

Among such compounds is calcium which benefits your plants in various ways like growing strong branches, developing roots and flowers, and more. However, sometimes, calcium may fail to reach your plant properly, leading to a deficiency.

Lack of calcium is problematic as it can significantly harm your plant’s health, and you must fix the problem before it’s too late. In this article, learn how you can identify and solve this problem and become a better cannabis grower.

Why Does Cannabis Require Calcium?

calcium deficiency cannabis

Calcium is a vital compound that your plant uses to grow healthy roots, stems, leaves, and buds, but it is required for many other processes. And it is a semi-fixed compound (unlike nitrogen), so it remains in the cell walls until it runs out.

Below are some of the ways calcium aids in your plant’s development:

What is Calcium deficiency?

ca deficiency

Sometimes, your plant may not get adequate calcium, known as calcium deficiency. This problem is less common in outdoor plants since natural soil has enough calcium. However, in hydroponic systems, it occurs often.

The lack of calcium in cannabis plants causes the following problems:

  • Slower nutrient uptake since calcium is responsible for decomposing organic matter in the root zone
  • Slower bud development during the middle flowering phase
  • Nutrient lockout — calcium deficiency inhibits the roots from absorbing other minerals properly
  • Weaker plant tissues since calcium is vital for sturdy growth

Calcium may also cause other problems, including other deficiencies in your cannabis plant.

What are the Symptoms of Calcium Deficiency?

Calcium deficiency

Like many other forms of deficiencies, calcium deficiency is tricky to identify since its symptoms look akin to other deficiencies. However, since calcium is an immobile nutrient, deficiency symptoms occur the most at the top tiers.

In addition, before confirming if it’s a calcium deficiency, always make sure it’s not a heat burn. Since calcium plays a key role in helping the plants transpire, it can indirectly lead to heat burn. Heat burn can look similar to calcium deficiency in its initial stages.

Plants transpire through their stomata and roots. Stomata are tiny pores. If the pores are closed due to excess heat, it causes a superficial heat burn, similar to calcium deficiency. Adequate calcium allows the plant to resist heat. 

Below are some of the common symptoms of calcium deficiency.

  • Slower Growth of New Leaves and Branches

cannabis calcium deficiency

One of the significant symptoms of calcium deficiency is slow growth, where new leaves grow weak, twisted, or curled up. Even the new branches grow weak, and in severe cases, they may appear purple or yellowish.

Sometimes, the young leaves turn very dark green. This often looks like nitrogen toxicity, but since nitrogen is a mobile nutrient and affects the bottom leaves at first, you should be able to differentiate between the two deficiencies easily. 

  • Compromised Root System

Since lack of calcium causes the roots to absorb fewer nutrients, calcium deficiency can also slow down the roots’ development. Due to this deficiency, your plant’s roots may also turn brown and become more susceptible to root rot, especially if it’s growing in a hydroponic system.

  • Brown or Necrotic Spots on the Leaves

Cannabis calcium deficiency

Due to calcium deficiency, the older leaves may turn brown or develop necrotic spots on them. Plus, the edges may also mottle. Finally, as the problem develops, the leaves may begin to turn yellow.

This symptom can be confused for fungus infection, too. So, look carefully at the entire plant — the brown spots will travel through the plant gradually, and the brown blotches will spread towards to edge over time — if you see these signs, you can be sure it is calcium deficiency.

  • Weak Branches

Additionally, the plant’s branches may also turn weak and brittle. This is because a lack of calcium can turn the branches hollow. This problem usually occurs when the plant already doesn’t have enough calcium, and the roots begin to deplete, which further inhibits calcium uptake. 

  • Lower Heat Resistance

Lastly, if your plant does not respond well to heat, whether it is during the hot hours of the afternoon or due to your light panels, it is a sign of calcium deficiency.

What Causes Calcium Deficiency?

Calcium deficiency occurs when your plant does not get enough calcium due to various problems. Below are the most common causes of calcium deficiency.

  • Wrong pH in the Root Zone

Like most other nutrient deficiencies, the most probable cause of calcium deficiency lies in choosing the wrong growing medium. If the medium’s pH is too low or high (acidic or basic grow medium), it can inhibit the roots from properly absorbing nutrients, including calcium.

  • Using RO Water

Water is one of the primary sources of micronutrients for plants, especially calcium. However, reverse osmosis (RO) water is devoid of all minerals. Therefore, if you feed your plant RO water, it can cause calcium deficiency.

  • Excess Manganese and Potassium

The roots absorb manganese and calcium similarly, so their ratios need to be balanced. But if there is too much manganese in the growing medium, it can prevent the roots from absorbing adequate calcium (and vice versa).

Similarly, too much potassium can also hamper the roots’ ability to absorb calcium along with nitrogen and other nutrients.

  • Strain Specific Problems

Each cannabis strain has unique genetics. So, some strains may require less calcium or higher levels of the same. If you’re facing calcium deficiencies repeatedly with a specific strain, do some research and try to get more details from the manufacturer. 

How to Fix Calcium Deficiency in Cannabis?

If your plant is experiencing calcium deficiency, you must act quickly. Even though calcium is a micronutrient, lack of it can cause many problems for your plant, ranging from wilting leaves to a dead plant in severe cases. 

So, here are some techniques you can use to fix calcium deficiency in your plant.

  • Check and Rebalance the Grow Medium’s pH Levels

Soil pH chart

The first step to solving calcium deficiency in your plant is to check the growing medium’s pH balance. It can stop the roots from properly absorbing calcium if it's improper.

Invest in a good pH meter and check the growing medium’s pH levels. To do this, measure the pH of the nutrient solution and the runoff water. If the runoff water has higher pH levels, the growing medium is basic or vice versa. 

Hydro pH chart

Ideal pH levels for your plant for calcium absorption, depending on your growing system, are:

  • 6.2 to 7.0 for a soil-based plant
  • 6.0 to 6.5 in a hydroponic system

If the pH levels are not in line, you need to rebalance them. 

An easy way is to flush your system with RO water to remove all minerals from the growing medium. However, remember that flushing isn’t a miracle cure because the plant would have absorbed the nutrients already. Once you flush, give your plant a while to adjust and start feeding your plant a pH-balanced low-strength nutrient solution. Less is more, so don’t stress the plants too much. 

Keep a close eye on the pH in the coming weeks to ensure the problem does not arise again.

  • Add Dolomite Lime to the Soil

dolomite lime

If you are growing cannabis in soil, you can add dolomite lime, a natural supplement, to the soil to give your plant a calcium boost. 

Dolomite lime is nothing but calcium magnesium carbonate, with 20% calcium (in most cases). It leeches calcium slowly and is quite cheap. You can purchase it at your local gardening supply store.

Plus, it has a pH of 7.0, which helps the soil maintain its pH levels. But still, go through the packaging to see if the dolomite lime you’ve bought has the same pH levels, as it can vary sometimes.

Dolomite lime comes in various textures, but we recommend using finer dolomite soil as it works faster in the soil.

To use dolomite, mix 6 to 7 teaspoons per gallon of soil and let the soil dry properly. Once it’s dry, add some water with a pH balance of 6.5. Let the soil settle for a few days, and then transplant your plant into it.

Remember, this is just a general rule. Read the manufacturer’s instructionproperlyer to know how to add the supplement.

  • Use Calcium Booster

If you’re growing cannabis in a hydroponic system, you can add calcium boosters to the nutrient solution. There are various types of boosters you can find nowadays, including calcium magnesium (cal-mag) formula, calcium magnesium acetate, or calcium acetate.

You can purchase these products at any horticultural store. Ideally, choose a booster with calcium and magnesium in a balanced ratio, as a shortage of one can be balanced with the other.

Before adding the calcium boosters to your grow system, read the instructions carefully. Avoid overfeeding calcium to your plant as it can cause calcium lockout and make matters worse.

You can also use calcium nitrate (CaNO3), which can be easily dissolved in water, but avoid using this during the flowering phase. It is rich in nitrogen, but your plant doesn’t need ample nitrogen during the flowering phase. 

Other Calcium Supplements

You can use other kinds of supplements too, which are rich in calcium, to feed your plant. Popular ones include liquid calcium or liquid lime, fish bones, crushed eggshells, seashells, calcium sulfate, etc.

  • Switch to Non-RO Water

If you have been using RO water for your plants, you can switch to regular tap water as it is rich in calcium. 

The tap water must have an EC of 0.3 to 0.4 (150 to 200 ppm), as this is where the water would have enough calcium for your plant.

How to Prevent Calcium Deficiency in Cannabis Plants?

cal mag

Preventing calcium deficiency is tricky, so you must be cautious. Firstly, it’s essential to understand the types of nutrients necessary for the plant’s growth. You don’t need to dive into deep science, but a basic understanding will go a long way. 

Many growers place a lot of importance on preventing calcium deficiency — and rightly so — but they feed their plants with excess nutrients that can upset the balance. One of the most common supplements used to prevent calcium deficiency is Cal-Mag. 

As you can surmise, Cal-Mag consists of both calcium and magnesium. Most companies sell both nutrients together. Why? It’s because these two nutrients go hand in hand. Some Cal-Mag supplements from certain companies also contain iron (Fe) in minor quantities as the three nutrients (calcium, magnesium, and iron) often show up together in a cannabis plant. 

Thus, it’s critical to understand how these three nutrients can impact your plants. Magnesium and calcium play a vital role in regulating photosynthesis and maintaining the plant's structure and cell walls. Since these nutrients are interconnected, calcium deficiency will often result in a magnesium deficiency. 

Therefore, most growers who report a calcium deficiency will also report a magnesium deficiency. However, adding too much magnesium can also result in calcium deficiency. See how tricky that is? This is one reason why you shouldn’t add Epsom salts (magnesium sulfate) randomly. Plants growing in soil will somehow survive, but it’s essential to add a balanced nutrient solution if you’re growing in a hydroponic setup. 

Also, calcium deficiency can mimic other deficiencies and confuse the grower. Without experience, it’s difficult to pinpoint the exact deficiency. Moreover, overzealous growers use excessive amounts of Cal-Mag supplements that result in toxicity. 

So, what’s the best way to prevent a calcium deficiency? First, get a Cal-Mag supplement, but use it according to the instructions from the manufacturer. Nothing more, nothing less. And, do not use it as a growth supplement as it shouldn’t be used as a stimulant for fast growth. 

In addition, read the ingredients carefully so you balance the feeding schedule. Although most Cal-Mag supplements contain only magnesium and calcium, some may contain additional ingredients. In short, consistency is key when using Cal-Mag supplements. You can use it at any stage of the growing cycle for excellent results. 

Growers growing their cannabis plants in organic soil like Super Soil or composted soil may not need commercial Cal-Mag as they use dolomite lime during composting. In addition, calcium is usually available in abundance in natural soil. However, you will have to use it if the plants display deficiencies. 

In conclusion, you will need Cal-Mag if:

  • You’re using soilless mediums like coco peat or coir to grow cannabis.
  • You’re using soft water or reverse osmosis water containing no nutrients.
  • Your plant displays brown spots, or the lower leaves turn yellow in between veins. 
  • Your plant is turning bright or lemon yellow. 
  • You want to prevent calcium or magnesium deficiencies from the very beginning. 

If you’re using RO water, add two parts calcium and one part magnesium till you arrive at an EC value of at least 0.3 or 0.4. Then, you can start adding other nutrients. 

You can also prevent calcium deficiency by maintaining a healthy environment for the plants, no matter what medium they grow in. As soon as transpiration reduces in the plant, it will display calcium deficiencies despite adequate calcium levels in the growing medium. 

And when does transpiration slow down? Transpiration slows down when the plants are exposed to cold temps and high humidity. Maintaining the humidity is important, but you must understand that the levels will vary depending on the plant's growth stage. 

Also, instead of reducing the humidity and temperature levels drastically, decrease it by 5% per week to ensure the plants adjust to it. 

Humidity and temperature levels to be maintained:

Growth Stage




65 to 70%

20-25 C°

Vegetative Stage

40 to 70%

22-28 C°

Flowering Stage

40 to 50%

20-25 C°

Calcium Toxicity

Calcium toxicity

Calcium is a vital nutrient for the plant to grow healthy — you already know that by now. Calcium isn't toxic in itself, but an excess — like every other nutrient — can be detrimental to the plants.

How? For starters, excess calcium prevents the plant from absorbing other nutrients. Eventually, the plant will struggle to intake magnesium, zinc, boron, iron, etc. 

Often, the top tier or young leaves of the plant turn yellow. You may think it's iron deficiency but it could be calcium toxicity. One way to differentiate is to carefully check the leaf tips. If they look burnt and yellow, calcium could be the culprit.

It’s often challenging to identify calcium toxicity since it can cause various issues including iron, manganese, magnesium and potassium lockout. When there’s excessive calcium in hydroponic setups, it usually reacts with sulfur. Then, it precipitates and sinks to the tank’s bottom. In such situations, check if the water has turned cloudy. 

Since that calcium is unavailable to the plants despite its excessive presence, you will need to change your nutrient solution. 

So, what does calcium toxicity look like? Again, it appears very similar to a heat burn because the leaves react to excessive calcium and try to push it through the tips. The leaf tips will turn yellow and eventually become brown and start withering. 

Thus, you will have to figure out whether it’s a heat burn or excess calcium. If your lights are too close to the plants, it’s probably heat burn. However, if the lights aren’t too close and the plants are located in a grow tent with ample ventilation, you will have to check for calcium toxicity. 

Besides leaf burn, calcium toxicity can also cause a lockout of other nutrients, including potassium and magnesium. The plants will start displaying various symptoms related to other deficiencies, so it can be slightly challenging to identify the root cause of the problem. 

How to Treat Calcium Toxicity?

Treat calcium toxicity like you’d treat a deficiency. First, try to correct the pH, and the problem will be solved. Just like you flush the medium and correct a deficiency, you can flush the medium and treat the toxicity as well. 

However, try not to flush too much, or the plant will struggle from overwatering. Typically, a 2x flush should work. For instance, if you’re growing in a 5-gallon pot, you can start by flushing with 10 gallons. Pour water into the medium until you see water draining out at the bottom. 

Summary: What is Calcium Deficiency in Cannabis Plants and How to Fix it?

Calcium deficiency is tricky to identify but easy to fix. Give your plant enough calcium supplements, regularly test the pH, and use tap or spring water to fix calcium deficiency. 

In addition, keep an eye out for toxicity. You can easily prevent calcium toxicity by following the instructions provided by the manufacturer. For example, if you’re using Cal-Mag, use it consistently to avoid issues. 

If you follow these techniques, your plant will recover within a week!

Calcium is crucial for a strong plant, just like it is for your bones. So, ensure your plant always has enough calcium to feed on. You will be rewarded with bushy buds that are a joy to consume.



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Super read again. Thank you, and keep it up.