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How to Recognize and Treat Calcium Deficiency in Cannabis Plants

By Aimee O'Driscoll

Published: Sep 21, 2022   
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Every cannabis crop needs calcium. The element is essential for cell wall strength and plays a role in regulating water uptake and metabolism. The former factor is especially critical during the vegetative growth stage when plants are actively growing and expanding.

But calcium deficiency is a common problem in cannabis plants. Symptoms include yellowing leaves, stunted growth, and brown spots. If left untreated, calcium deficiency can severely damage your plant.

This article explores the importance of calcium and how to spot calcium deficiency in cannabis plants. We also offer tips to help prevent and treat this problem.

Why is calcium important to cannabis plants?

Calcium is an essential element for all plants, providing structural support and helping regulate a number of metabolic processes. David Kessler, chief science officer at Agrify, explains that plants require 16 to 24 elements (depending on which literature you read). According to Kessler, macro elements, including nitrogen, phosphorus, and potassium, are required in the largest amounts. Still, plants also require significant amounts of secondary elements, which include calcium, magnesium, and sulfur.

Kevin Rampelberg, vice president of cultivation at Ascend Wellness Holdings reveals there are many reasons for the necessity of calcium. “Calcium is needed to strengthen the cell walls and cell membranes. This will create a defense against pests and disease and build strong plant structure.” Without adequate calcium, plants can become stunted and produce weaker leaves and stems that are more susceptible to damage.

Rampelberg adds that calcium helps to increase nutrient uptake and assists the plants in photosynthesis. Calcium also helps to promote root development, which is essential for the absorption of other nutrients and water from the soil. For optimal growth, cannabis plants need a steady supply of calcium throughout their life cycle. By providing adequate calcium, growers can help ensure their plants are healthy and productive.

Calcium deficiency in weed, what are the symptoms?

As Rampelberg explains: “Calcium deficiency is when the plant has an insufficient level of available calcium.” Symptoms include interveinal chlorosis (yellowing or browning in between leaf veins), tip burning or necrotic leaf margins, pale leaves, stunted growth, and weak plant structure.

Rampelberg advises that symptoms usually start on newer growth or younger leaves and tissue. And Kessler details that this is because calcium is an immobile nutrient. An immobile nutrient stays where it is deposited, for example, in a cell wall, as opposed to a mobile nutrient that can be relocated from one area of the plant to another. “And this is really important,” says Kessler. “Because if I’m going to see a calcium deficiency, and there is not sufficient calcium in the media, the fertigation water, then you would expect to see calcium issues at the top of the plant.”

Calcium deficiency symptoms usually progress, and Kessler explains that there are about five stages to the process. “Typically, you’re going to start to see stunted growth at the top,” says Kessler, explaining that the plant will grow a little bit more slowly at the tips and the new leaves. From there, you might see the new leaves or leaflets become light green towards the base. “And then over time, that light green might intensify to a yellow.”

The next step you would see if the problem continues to progress and get worse is interventional chlorosis, a yellowing in between the veins of the leaf. “From there, you’re going to see something to the effect of extreme stunting and marginal necrosis,” says Kessler. This means the edges and tips of the leaves may start to die, and you’ll see a significant slowdown in the overall speed of growth of the plant. “And then the final thing that you would really see would be those leaf tips that are yellow, those blades at the edges, they’re going to transition from yellow to brown becoming necrotic. And the tissue just continues to die.”

A plant will show similar symptoms at various stages of growth. “You would see it still expressed at the top of a flower in the genitive stage,” says Kessler. He adds that you wouldn’t see any change in symptoms, but would just notice them around the leaflets of a flower cluster instead of a vegetative branch.

How can you treat calcium deficiency in your cannabis plants?

Kessler notes that it can take about five weeks for a calcium deficiency to show visible signs, so once you see symptoms, the plant has already been suffering for a significant period of time. That said, there are ways to address the issue. Rampelberg outlines the key steps to take when you start to suspect calcium deficiency:

  • Test the pH of your medium. If it’s too low, it won’t utilize the nutrients in the soil efficiently.
  • Check the temperature. If it is too cold, then the transpiration rate of your plants will be low. This can cause a calcium uptake issue.
  • Check the percent of relative humidity (RH%). High RH and poor airflow will also have a negative effect on transpiration rates and can cause a lack of calcium.
  • Vapor pressure deficit (VPD). Find a VPD chart online and try to target your set points to achieve the most desirable climate.
  • Foliar sprays. Make sure to pH the calcium mix to 5.8 before spraying. A pH of 5.8 will help the absorption rate. It also helps to add a surfactant (an additive to help the mix stick to and stay on the leaves).
  • Add the correct amount of calcium to your nutrient feed. Some genetics may require more or less. When plants are struggling with calcium deficiency, you can also give your plants a calcium drench to help kickstart their recovery.

Although treatment can help save calcium-deficient plants, growers should ideally use preventative measures to avoid deficiencies in the first place.

How can you prevent calcium deficiency?

Since calcium deficiency shows visual signs well after the onset, growers must take a proactive approach.

“What I want cultivators to do is pay a lot of attention to their nutrient profile and their pH and, overall, just use an appropriate amount of calcium from the onset,” says Kessler. He also notes that you may notice some varieties tend to run calcium deficient, so you can take extra care to be proactive with those.

Rampelberg outlines some key proactive steps. First, he reiterates the importance of VPD. Then, as outlined in the treatment advice, he says you should always aim for the most desirable climate. Airflow is especially important to avoid calcium deficiency. “The right amount of airflow and air mixing will help with transpiration and move calcium through the plants,” says Rampelberg. Kessler agrees and explains that hindering moisture transpiration from the plant – which can happen if the environment is too humid – can diminish the pace at which calcium enters the plant through the roots.

Rampelberg also advises checking your medium pH with a meter at least weekly to be within the ideal range, depending on your choice of medium. As Kessler explains, overly acidic or alkaline conditions will lower calcium intake.

Cal mag deficiency

A well-balanced fertilizer is important, too. “Make sure your nutrient recipe is ideal for your genetics and medium you choose to work with,” advises Rampelberg. Kessler agrees, adding that knowledge of antagonists is important. In this context, antagonists (including magnesium and potassium) can prevent calcium uptake.

“If your magnesium is out of balance, then you might actually see a calcium deficiency, even though it’s your magnesium that's in too high a quantity, not your calcium too low,” explains Kessler.

Another piece of advice Rampelberg offers is that in the absence a reverse osmosis system, you should test your water for parts per million (PPM) or total dissolved solids (TDS) levels. “Without knowing what your water may have in it, you can at least identify if your levels are too high with a cheap PPM or TDS meter. I’ve always used a threshold of 300-400 ppm or less as acceptable. Anything higher I filter.”


Calcium is an important nutrient for cannabis plants, and a lack of it can lead to visible deficiency symptoms. To prevent calcium deficiency, growers should take a proactive approach by paying attention to their nutrient profile, pH levels, and the overall amount of calcium used. Additionally, maintaining the correct airflow, temperature, and humidity is crucial to preventing calcium deficiency.

Aimee O'Driscoll

Freelance Science Writer

Aimee is a freelance science writer with over a decade of experience as a development chemist. She has written for Analytical Cannabis since 2020.


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