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Guides | 05.15.2024

How To Fix Calcium & Magnesium Deficiency In Cannabis Plants

Turn leafy frowns upside down with the right supplements.

Is your cannabis plant looking a bit under the weather?

Droopy leaves and yellowing can be signs that your plant is missing crucial nutrients like calcium or magnesium.

Calcium And Magnesium In Cannabis

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Every cannabis crop needs calcium and magnesium to thrive.

Calcium strengthens the structural integrity of cell walls, protecting them against weather, pests, and diseases. It also plays a large role in transporting nutrients throughout the plant and developing the roots and leaves.

On the other hand, magnesium is a critical component of chlorophyll – the molecule responsible for photosynthesis. Without enough magnesium, your plants may struggle to convert sunlight into the energy they need to grow.

Together, calcium and magnesium are a power duo that make sure your cannabis plants reach their full potential, from root to tip.

What Causes Magnesium And Calcium Deficiencies In Weed Plants?

Many factors can lead to poor calcium or magnesium levels in your weed plant:

Improper Soil pH

Calcium and magnesium are best absorbed by the roots when the soil pH level is around 6.0 to 7.0, with an ideal range of 6.2 to 6.5. If the soil is too acidic (low pH) or too alkaline (high pH), these nutrients can be unavailable for root uptake, leading to deficiency.

Growing Medium

The plants will inevitably become deficient if the chosen growing medium doesn’t contain sufficient calcium or magnesium. Some mediums, like certain hydroponic setups, naturally contain lower levels of calcium and magnesium. Others, such as coco coir, may not retain these nutrients effectively, leading to deficiencies unless supplemented.

Too Many Other Nutrients

Loading up your plants with high levels of nutrients like potassium or ammonium can disrupt the uptake of calcium and magnesium. Balance is key to avoiding nutrient antagonism.

Overwatering or Poor Drainage

Overwatering or poor drainage can wash away essential nutrients from the soil, including calcium and magnesium, or cause waterlogged conditions that decrease oxygen availability, further impairing nutrient uptake.

Root Problems

Healthy roots are essential for nutrient absorption. Damage from pests, disease, or underdevelopment can significantly impair a plant’s ability to take up and transport calcium and magnesium effectively.

Environmental Stress

Other elements, like high temperatures, low humidity, or drought conditions, can increase the plant’s demand for calcium and magnesium but limit its ability to absorb and move these nutrients throughout.

Overfertilizing During Flowering Stage

Excessive use of flowering fertilizers, which are typically high in phosphorus and potassium, can inhibit calcium and magnesium uptake, especially critical during the early flowering stage (weeks 3-6).

Signs Of Magnesium Deficiency In Cannabis

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Symptoms of magnesium deficiency in weed plants include:

  • Yellowing of lower leaves with progression toward upper leaves

  • Rusty brown spots on leaves

  • Leaves begin to curl upwards at the edges, known as “cupping”

  • If severe enough, leaves may start to drop off the plant

The yellowing of leaves from a magnesium deficiency will continue to progress, affecting newer growth. The yellowing can be seen between veins of the plant leaves, and small, rusty brown spots might appear scattered on more middle-aged leaves. There might also be cloudy flecks appearing throughout the leaves.

Calcium Deficiency Symptoms Cannabis

  • Brown spots on new leaves

  • Yellowing between veins

  • Stunted growth

  • Weak stems, causing plants to droop or fall over

Calcium deficiencies start slowly. You may notice stunted growth at the top of the plant and new leaves. Newer leaves may become light green at first, turning yellow towards the base of the plant leaves and between veins. Brown spots might appear on the edges and tips of leaves, which then might die off.

How To Fix Calcium Deficiency In Weed Plants

  1. Adjust soil pH – First, test your soil’s pH and adjust it to stay within the optimal range of 6.2 to 6.5, which is ideal for calcium uptake.

  2. Add calcium supplements – Using calcium-based nutrients like calcium nitrate or cal-mag supplement (containing both calcium and magnesium) can help make minerals more abundant in your plant.

  3. Top-dress with gypsum – Apply calcium sulfate, also known as gypsum, to your soil. It’s a slow-release calcium source that can be top-dressed and watered in, gradually improving calcium levels.

  4. Foliar feed with calcium – For a quick fix, foliar spray your plants with a calcium-rich solution such as calcium nitrate. This method delivers calcium directly to the leaves, where it’s quickly absorbed.

  5. Improve drainage –  Make sure your pots or grow beds are well-drained. Proper drainage helps prevent issues with calcium uptake, keeping your plants healthy and well-nourished.

How To Fix Magnesium Deficiency In Weed Plants

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  1. Adjust soil pH – Just like with calcium deficiency, the best soil pH range for magnesium is around 6.2 to 6.5. Adjust the soil accordingly to make sure magnesium is available for uptake.

  2. Add magnesium supplements – Epsom salts (magnesium sulfate) are a popular and effective choice if needing to add more magnesium to your plants. You can dissolve it in water and apply it during your regular watering schedule for gradual absorption.

  3. Include magnesium in your fertilizer mix – Make sure there are adequate magnesium levels in your fertilizers – some mixes might focus more on nitrogen, phosphorous, and potassium.

  4. Foliar feed with magnesium – Directly spraying a foliar mix with a solution of Epsom salts directly to the leaves can quickly provide magnesium to the plant and bypass the need for it to be uptaken by the soil.

Other Factors To Consider

Temperature Check

Make sure your growing space isn’t too chilly. Low temperatures can slow your plants’ transpiration rates, which is kind of like putting a kink in a straw when they’re trying to sip up nutrients, including calcium and magnesium.

Relative Humidity and Airflow

Keep an eye on the relative humidity (RH%) in your grow area. High humidity, when coupled with poor airflow, can greatly reduce transpiration and, thus, the amount of water your plant loses. Transpiration is a critical process for nutrient uptake through the roots and their distribution throughout the plant. Aim for optimal airflow to encourage healthy transpiration and nutrient absorption.

Vapor Pressure Deficit (VPD)

Vapor Pressure Deficit measures the difference between the moisture present in the air and the maximum moisture the air can hold when saturated. A proper VPD makes sure there’s an optimal environment for transpiration to occur, promoting efficient nutrient uptake. Use a VPD chart to help you optimize the right conditions for your cannabis plant.

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