Calcium Deficiency and Toxicity – Cannabis Growing Guide

This Growing Guide helps growers identify calcium deficiency and toxicity in cannabis plants, explaining their causes and providing effective solutions.

Calcium’s role in cannabis plants

Calcium helps build strong cell walls, supports important plant processes, and makes plants more stress-resistant. Calcium also plays a crucial role in pistil development during flowering. Without enough calcium, pistil formation can be weak or abnormal, potentially affecting bud development and yield.

Calcium deficiency can lead to weak stems, distorted new growth, and necrotic brown spots on leaves, while excess calcium disrupts the uptake of essential nutrients like magnesium and potassium.

Whether you’re seeing twisted leaves or brown spots, understanding how calcium works can help you prevent problems before they impact your harvest.

Calcium is an essential secondary macronutrient for cannabis. 

Essential: Calcium is considered an essential nutrient because cannabis plants cannot complete their life cycle without it.

Secondary Macronutrient: While needed in smaller amounts than primary macronutrients (nitrogen, phosphorus, and potassium), cannabis still requires calcium in relatively large quantities compared to micronutrients like iron or zinc. Calcium falls under this category, along with magnesium and sulfur.

How does cannabis take up calcium?

Plants absorb nutrients through passive or active uptake. 

Passive uptake means the nutrient enters with the water. Cannabis plants absorb calcium passively, moving along with water through the roots via the transpiration stream. This means calcium uptake depends on water flow and transpiration, not energy, and is affected by environmental conditions like humidity and root health.

In contrast, nitrogen, phosphorus, and potassium (NPK) are absorbed actively, meaning the plant uses energy to transport them into the roots.

Calcium is an immobile nutrient in cannabis plants, meaning once it’s deposited in plant tissues, it can’t be moved to newer leaves or stems. This is why calcium deficiency symptoms typically appear first in the newest leaves. Since the plant can’t move calcium to other areas, it must always absorb new calcium through the roots to support ongoing growth. Some authors recently have defined Ca as “semi mobile”.

Calcium is an immobile nutrient taken up passively

Understanding this helps identify deficiencies because it reveals that plants rely on steady transpiration for calcium transport, which is disrupted by low humidity, poor airflow, poor lighting, or excess of other nutrients, leading to calcium deficiencies in rapidly growing plants. Each cause is explained in detail in the sections below so you can identify and address possible environmental issues before “adding more calcium”, which can worsen the situation.

Cannabis calcium deficiency symptoms

How to tell if your cannabis plant has a calcium deficiency

Early symptoms of calcium deficiency

• Small brown or rust-colored spots on newer and sometimes older leaves
• Slight leaf curling or distortion
• Weak new growth
• Stunted development
• Roots discolored from white to brown

Advanced symptoms of calcium deficiency

• Spots turn necrotic (brown, dead tissue)
• Leaves become brittle
• Twisted, malformed growth
• Weak stems
• Poor root development
• Roots are prone to bacterial and fungal infections, pests, and diseases

Causes of cannabis calcium deficiency (and how to fix them)

This section covers the main causes of calcium deficiency in cannabis plants and how to fix them. While the symptoms may look similar, the underlying issues can vary greatly, from substrate and nutrient imbalances to environmental factors like humidity, lighting, and airflow. Each cause is explained with its corresponding solution, so you can take accurate and effective action.

Cause: Low calcium in water or soil

Some soils naturally contain calcium, while others, especially inert or hydroponic substrates, lack it. Water can also be a source of calcium. Still, if you use reverse osmosis (RO) water, distilled water, or soft water, these contain little to no calcium, increasing the risk of deficiency.

Over time, soil can also become depleted of calcium, especially if using organic amendments that break down slowly or if plants have been growing in the same soil without replenishing nutrients.

How to fix low calcium in water or soil:

• Check your water source. If you use RO (Reverse Osmosis, recommended) or distilled water (not recommended), supplement with a calcium-magnesium (cal-mag) product to replace missing minerals.
• Hard water, on the other hand, may already contain calcium, so always check water quality reports.
• Amend the soil if needed. If growing in soil, using dolomitic lime, gypsum, or oyster shell flour can provide long-term calcium. For a quicker fix, liquid calcium supplements or calcium nitrate can be added to the watering schedule.
• Make sure your nutrient mix matches your growing medium. Coco and hydroponics don’t provide calcium on their own, so it’s up to you to include it through cal-mag products or balanced nutrient formulas. Skipping this step is a fast track to deficiency.

Cause: Excess potassium, magnesium, ammonium nitrogen, or salt buildup causes nutrient block or lockout

Calcium uptake is closely linked to the availability of other nutrients. If potassium or magnesium levels are too high, they can compete with calcium, preventing the plant from absorbing it. 

This is common when growers add excessive magnesium sulfate (Epsom salt) or overuse bloom (PK) boosters high in potassium. This happened to me even when I used the PK booster at the recommended dose, some strains just don’t handle high potassium levels well. 

High levels of ammonium nitrogen can block calcium uptake in cannabis plants. If this is the case, a better strategy is to reduce ammonium and use more nitrate-based nitrogen instead of adding more calcium. This shift boosts calcium absorption and supports stronger plant development.

Additionally, salt buildup from fertilizers can accumulate in the root zone, disrupting nutrient absorption and leading to deficiencies, even when calcium is technically present in the medium. When salt builds up in the growing medium, the plant reacts as if it’s experiencing water stress. This can trigger stomata, which are tiny pores on the surface of the leaves, to close, reducing water flow and blocking calcium from reaching developing leaves and buds.

How to fix nutrient lockout causing cannabis calcium deficiency:

• Flush the growing medium if you suspect salt buildup, fertilizer burn, or nutrient lockout. Use pH-balanced water to wash away excess salts before resuming feeding.
• Adjust nutrient ratios. Avoid excessive potassium in bloom-phase fertilizers and ensure magnesium is balanced with calcium (a 4:2:1 potassium-calcium-magnesium ratio is ideal).
• Check the ammonium-nitrate rate of your nitrogen fertilizer. Choose balanced formulas over ammonium nitrogen ones.
• Use a well-balanced feeding schedule that provides nutrients in the correct proportions rather than adding extra supplements without considering interactions.

Cause: Improper pH levels

The pH of the growing medium affects how nutrients dissolve and become available to plant roots.

Calcium and magnesium become increasingly unavailable to the plant at lower pH levels, while some micronutrients may reach toxic levels due to higher availability. In many cases, nutrient disorders are caused by imbalanced root zone pH rather than an actual deficiency or excess.

• In soil mixes, calcium is best absorbed between pH 6.2 and 7.0. If pH drops below 6.2, calcium uptake declines, and deficiency symptoms appear.
• Calcium deficiency is uncommon in outdoor grows unless the soil is highly acidic.
• In hydroponics and coco, calcium is absorbed efficiently between pH 5.8 and 6.2. A pH lower than this leads to lockout, preventing the plant from taking in calcium.

How to fix improper pH levels:

• Measure pH regularly. Use a pH meter to check both your water/nutrient solution and the runoff (excess water draining from the pot) to ensure it’s within the correct range. Use the PourThru method, 1:2 dilution method, or SME (Saturated Media Extract Method) for better results.
• Adjust pH as needed. If pH is too low in hydroponics, use pH-up (potassium hydroxide or calcium carbonate) to bring it up. If it’s too high, use pH-down (phosphoric acid or citric acid) to lower it.
• Outdoors, calcium needs to be added as dolomitic lime (or other amendments) to bring the pH range to 5.8-6.3. Gypsum also works for calcium deficiencies, but won’t affect the pH as much as dolomitic lime.
• Don’t use Gypsum on soils with a pH below 5.5, because it makes aluminum (Al) soluble and poisons the plants.
• Monitor pH stability. Sudden fluctuations can cause deficiencies, so keep a consistent pH range throughout the grow cycle.

Cause: Growing in coco without calcium supplementation

Calcium deficiency is a common issue in coco, especially if you’re using unbuffered coco or relying only on basic nutrient solutions. To prevent problems from the start, it’s essential to supplement with calcium early and consistently throughout the grow.

Because coco doesn’t contain calcium naturally like soil does, and because it can bind to added calcium before the plant can absorb it, unbuffered coco can quickly cause deficiencies. Pre-buffering with calcium or using coco-specific nutrient lines helps reduce this risk.

Frequent fertigation is key when growing in coco. Since coco doesn’t store nutrients the way soil does, cannabis plants rely on a steady flow of nutrient-rich water to meet their needs. If feeding is too infrequent, calcium levels can quickly drop, leading to deficiencies. 

When coco dries out, salts can concentrate and raise the EC (electrical conductivity) in the root zone. This can lead to nutrient burn, with symptoms like leaf tip burn, brown spots, and overall stress in the plant.

How to fix growing in coco without calcium supplementation:

• Always use a cal-mag supplement when growing in coco. Start adding it from the beginning to prevent deficiencies later.
• Pre-buffer the coco before planting. If using unbuffered coco, soak it in a calcium-magnesium solution before use to saturate the cation exchange sites and reduce calcium lockout.
• Monitor EC levels. If the coco dries out too much, salt concentrations rise, leading to nutrient lockout. Keep the medium consistently moist and always fertigate with 10%-20% runoff to prevent buildup.

Cause: High humidity reduces transpiration and calcium uptake

Calcium is transported within the plant through transpiration, the process where water moves from the roots to the leaves and evaporates. When humidity is too high (above 70% RH in veg and flower), plants transpire less, reducing calcium movement. This can lead to deficiencies even if calcium is present in the soil or nutrient solution.

This is a common issue in grow tents with poor ventilation and air exhaustion.

How to fix high humidity reducing transpiration and calcium uptake:

• Improve airflow and ventilation. Use oscillating fans to increase air movement and encourage transpiration.
• Reduce humidity. Keep humidity about 60–70% for seedlings, 70% in the pre-rooted propagation stage, 55–65% for veg, and 50% for flowering to maintain proper calcium transport.
• Use a dehumidifier in high-humidity environments.
• Avoid overwatering and take out the runoff water. Overly wet soil reduces oxygen availability, weakening roots and further limiting calcium uptake. Ensure proper drainage and water only when needed. This doesn’t apply to coco coir.

Cause: Low humidity can increase the rate of transpiration causing calcium deficiency symptoms

Low humidity (below 40% RH) increases the transpiration rate, meaning plants lose water more quickly through their leaves. While this might seem beneficial for calcium transport, excessive transpiration can lead to water stress.

When plants lose too much water, their stomata (tiny pores on leaves) close to conserve moisture. This reduces the uptake of water and nutrients, including calcium, from the roots. As a result, calcium transport becomes erratic, leading to deficiency symptoms even if calcium is present in the growing medium.

Low humidity is especially problematic in seedlings and young plants, which have underdeveloped root systems and rely more on consistent moisture levels. It can also contribute to tip burn in fast-growing plants, where new growth develops small necrotic brown spots due to calcium deficiency.

How to fix low humidity causing cannabis calcium deficiency

• Increase humidity to optimal levels.
  • Keep humidity about 60–70% for seedlings, 70% in the pre-rooted propagation stage, 55–65% for veg, and 50% for flowering to balance transpiration and calcium uptake.
  • Use a fogger or humidifier if the humidity drops too low.
• Mist young plants (but not flowering ones).
  • Light misting in early veg can provide temporary relief, but avoid misting plants in bloom to prevent mold and bud rot.
• Use humidity domes for seedlings and rooting clones to keep high humidity levels.
• Monitor temperature and airflow.
  • High temperatures combined with low humidity can worsen calcium issues by increasing water loss too rapidly. Keep temperatures within an ideal range: Germination 21 – 25°C (70 – 78°F), Vegetative 20°C – 30°C (68 – 85°F), Flowering 20 – 30°C (68 – 85°F), Cloning 24 – 30°C (75 – 85°F), and adjust airflow accordingly.
• Ensure roots are healthy.
  • If plants are losing water too fast, their roots must be able to compensate. Make sure they have good soil moisture, proper drainage, and access to calcium-rich nutrients.

Cause: Fluctuating and low light conditions reduces transpiration and calcium uptake

Calcium moves through the plant via transpiration, meaning it is transported with water as the plant takes in moisture through its roots and releases it through its leaves. When transpiration is low, calcium transport is reduced, leading to deficiencies in rapidly growing tissues, particularly in young leaves and shoot tips.

When this happens:

• Periods of cloudy or low light conditions, especially in greenhouses, and indoor grows with inconsistent light schedules, reduce transpiration and calcium movement.
• If the plant continues growing during low transpiration periods, new leaves receive less calcium than needed.
• When light returns suddenly, the calcium-deficient tissues cannot keep up with the increased metabolic activity, leading to necrosis (tissue death) in young leaves.

How to fix fluctuating light conditions reducing transpiration and calcium uptake

• Maintain stable light conditions when possible to avoid sudden shifts in transpiration rates.
• Replace old light fixtures with more modern and efficient light fixtures.
• Ensure adequate humidity control to promote consistent transpiration.
• Use fans or ventilation to encourage transpiration in controlled environments.

Calcium deficiency week by week

This section tracks how calcium deficiency develops in cannabis plants week by week, helping you recognize early signs and understand how symptoms progress over time.

Calcium deficiency week 3

This study (Llewellyn & Zheng (2023) on calcium deficiency in cannabis found that symptoms first appeared around week three, starting on the lower fan leaves. These leaves developed small spots between the veins, with yellow rings around necrotic (dead) brown centers.

Calcium deficiency week 4

By week four, symptoms spread upwards but were still concentrated in the bottom two-thirds of the plant. The spots became more severe along the leaf edges and tips, eventually merging into larger necrotic brown areas. Some leaf margins also turned brown.

Calcium deficiency week 5

By week five, the deficiency spread to upper fan leaves and sugar leaves, causing upward curling of leaf tips. Some of the lowest fan leaves also developed general yellowing (chlorosis), which worsened over time.

Calcium deficiency week 6

By week six, the entire plant was affected, with severe necrosis along leaf edges and curling leaves at the top.

Interestingly, this progression differs from typical calcium deficiencies in other plants, where symptoms usually start in younger growth. Researchers found that the lower leaves still had higher calcium concentrations than the upper leaves at the onset of symptoms.

Calcium excess in cannabis: causes and solutions

While calcium deficiency is a common issue in cannabis cultivation, excessive calcium can also create serious nutrient imbalances. An excess of calcium primarily affects the plant’s ability to absorb other essential nutrients, such as magnesium, potassium, and iron, leading to secondary deficiencies that may slow growth and reduce overall yield.

Cannabis calcium toxicity symptoms

• Nutrient lockout of other cations like magnesium, potassium, and iron
• Interveinal chlorosis or mottling on young leaves due to magnesium or iron deficiency
• Rust-like spots or necrosis on leaves, especially if potassium uptake is affected
• Stunted growth or distorted new leaves
• Burned tips or edges from overall nutrient imbalance
• Leaf curl or deformation
• Unusually dark green foliage in some cases due to nutrient buildup

Calcium excess often causes secondary deficiencies, so symptoms may reflect imbalances more than direct calcium toxicity.

Causes of calcium toxicity

Overuse of calcium-rich fertilizers

Applying too much calcium in the form of supplements like calcium nitrate, gypsum, or cal-mag solutions can lead to an oversupply, especially in systems with frequent fertigation. This can disrupt the balance of other essential nutrients, such as potassium and magnesium, by competing for uptake at the root level. Over time, this imbalance may cause visible deficiency symptoms and reduce overall plant health and yield.

Hard water with high calcium levels

Many tap water sources contain significant amounts of calcium. If growers use cal-mag supplements without testing their water first, they risk overloading their plants with calcium.

This excess can lead to nutrient imbalances, particularly with magnesium and potassium, which may cause deficiency symptoms even when those nutrients are present in adequate amounts. Testing your water helps you adjust your feeding plan and avoid unintentionally stressing your plants.

Imbalanced nutrient ratios (shown as magnesium or potassium deficiencies)

Excess calcium disrupts the uptake of magnesium and potassium due to competitive interactions at the root level. This is particularly problematic in hydroponics and coco coir, where nutrients must be carefully balanced.

Excess calcium can often show up as symptoms that resemble potassium deficiency, such as leaf edge browning or tip burn, because too much calcium can interfere with potassium uptake. It’s important to keep track of your nutrient schedule to identify whether the issue is a true potassium deficiency or a calcium excess causing the imbalance.

How to fix calcium excess

Flush the growing medium

If signs of calcium excess appear (such as magnesium or potassium deficiency symptoms), flushing the substrate with clean, pH-balanced water can help remove excess calcium and restore nutrient balance. In coco coir, ensure the flush includes a light nutrient solution to avoid shocking the plant.

Check water quality

Test the EC and calcium content of your water source. If using hard water, consider filtering it or adjusting fertilizer inputs to avoid additional calcium supplementation.

Adjust nutrient formulations

Reduce or eliminate cal-mag supplements if using hard water or calcium-rich fertilizers. Ensure magnesium and potassium levels remain sufficient to prevent secondary deficiencies.

Take away

Calcium plays a fundamental role in cannabis growth, influencing cell structure, root development, and overall plant resilience.

Calcium deficiency can cause weak stems and necrotic spots, while excess calcium disrupts nutrient uptake, leading to magnesium and potassium deficiencies.

Calcium deficiency causes permanent damage during cell development, so affected leaves and tissues won’t recover, even though the plant can still bounce back with healthy new growth.

Once calcium is properly supplied, new growth should return to normal within a few days to a week, but existing damage won’t heal.

By understanding how calcium interacts with other nutrients and adjusting feeding strategies accordingly, growers can ensure their cannabis plants remain strong, healthy, and productive throughout their life cycle.

I hope this guide helps you spot and solve calcium issues while growing. If you’ve dealt with calcium deficiencies in the past, share your experience in the comments, or join our Growing Community and Grower Support to get advice tailored to your specific case. And don’t forget to check out the rest of our nutrient series for more tips on keeping your plants healthy! Happy growing!

References and sources

Cockson, P., Landis, H., Smith, T., Hicks, K., & Whipker, B. E. (2019). Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences, 9(20), 4432. https://doi.org/10.3390/app9204432

Llewellyn, D., Golem, S., Jones, A. M. P., & Zheng, Y. (2023). Foliar Symptomology, Nutrient Content, Yield, and Secondary Metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies. Plants, 12(3), 422. https://doi.org/10.3390/plants12030422

Rosenthal, Ed. Cannabis Grower’s Handbook: The Complete Guide to Marijuana and Hemp Cultivation (English Edition)

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Photos: Cited literature, Alicia Muzio (Author), Wikipedia