How do you know if you have a Cal Mag deficiency?

December 29, 2025 •

4 min read

A staggering number of plant problems are misdiagnosed, with Cal Mag deficiency being a common culprit, especially for gardeners using filtered water or certain soilless media. Knowing how to spot a Cal Mag deficiency early is crucial for preventing stunted growth and poor yields in your plants.

Quick Summary

Distinguishing between calcium and magnesium problems involves observing specific plant symptoms, from brown leaf spots on new growth to interveinal yellowing on older leaves, and addressing underlying environmental or nutrient issues.

Key Points

Location Matters: Calcium deficiency first affects new growth and younger leaves, while magnesium problems show up on older, lower leaves,.
Check the pH: A pH imbalance is the most common cause of a Cal Mag deficiency, even if the nutrients are present in the medium.
Know the Symptoms: Look for stunted or distorted new growth (Ca) and yellowing between the veins of older leaves (Mg),.
Water Quality is Key: Using filtered (RO) or soft water without supplementation can cause deficiencies, as these sources lack essential minerals.
Use a Supplement: Commercial Cal Mag products are often the most straightforward solution for correcting and preventing deficiencies in various growing media.
Consider Epsom Salts: A quick foliar spray with Epsom salts can provide a fast-acting dose of magnesium to correct interveinal yellowing on older leaves.
Fix the Underlying Cause: Addressing the root of the problem, whether it's pH, water quality, or nutrient antagonism, is essential for long-term plant health,.

The Essential Roles of Calcium and Magnesium

Calcium (Ca) and Magnesium (Mg) are secondary nutrients vital for plant health, playing distinct but interconnected roles. Without sufficient amounts, a plant's entire structure and metabolism can be compromised.

The Importance of Calcium

Calcium is a relatively immobile nutrient within the plant, meaning it does not easily move from older leaves to new growth. This is why deficiency symptoms first appear on the youngest leaves and tissues. Its primary functions include:

Cell Wall Structure: Calcium is the 'glue' that holds cell walls together, providing rigidity and structural integrity. A deficiency leads to weak stems and stems that crack easily.
Root Development: It is critical for the growth and health of root tips. Poor root growth or root tip necrosis is a key sign of a calcium issue.
Nutrient Transport: It helps regulate the uptake and transport of other nutrients throughout the plant.

The Role of Magnesium

Magnesium is a mobile nutrient, so when a deficiency occurs, the plant will move its available magnesium from older leaves to support new growth. This is why magnesium deficiency symptoms are typically seen on older, lower leaves first. Its functions are primarily metabolic:

Photosynthesis: Magnesium is the central atom in the chlorophyll molecule, the pigment responsible for capturing sunlight for energy conversion. A lack of magnesium severely inhibits photosynthesis.
Enzyme Activation: It activates hundreds of enzymes within the plant that are crucial for producing carbohydrates, sugars, and fats.
Energy Transfer: It helps transfer and store energy for various plant activities.

Spotting Cal Mag Deficiency Symptoms

Recognizing a deficiency depends on both the type of nutrient lacking and the plant's growth stage.

Early Stage Symptoms (Seedlings)

In the sensitive seedling stage, a Cal Mag deficiency can have lasting negative effects.

Stunted, slow growth
Weak stems and stalks
Leaves with yellow and brown discoloration
Leaves that curl upward or downward

Vegetative Stage Symptoms

During the vegetative phase, plants are putting energy into green growth, making a deficiency particularly harmful.

Small, brown, or yellow spots on leaves that can expand over time
Tips of leaves become brittle and crispy
Interveinal chlorosis on older leaves (yellowing between the green veins)
Overall slow growth rate

Flowering Stage Symptoms

For flowering plants, a deficiency during bud formation can seriously impact yield and quality.

Rusty-colored spots appear on leaves and can develop on buds
Leaves become brittle and may drop prematurely
Flower and bud development slows or stops entirely
Final yields are significantly reduced

What Causes a Cal Mag Deficiency?

Beyond a simple lack of nutrients in the soil or medium, several factors can trigger a Cal Mag deficiency, often related to environmental conditions.

Incorrect pH Levels: This is the most common cause. In soil, a pH below 6.2 can inhibit calcium absorption, while a high pH (above 7.5) can lock out magnesium,.
Water Source: Using soft water, purified water (like Reverse Osmosis), or rainwater with low mineral content can starve plants of these essential nutrients,.
Nutrient Imbalance: Over-fertilizing with potassium, for example, can create an antagonistic effect that blocks the uptake of magnesium.
Growing Medium: In soilless media like coco coir or perlite, which contain no native nutrients, supplementation is crucial. Sandy or light soils also leach nutrients quickly.
Environmental Stress: Low humidity or uneven watering can restrict the transpiration process, limiting calcium distribution throughout the plant.

Comparison: Calcium vs. Magnesium Symptoms

Understanding the distinct symptoms is key to proper diagnosis. Use this table as a quick reference.


Symptom Type	Calcium Deficiency	Magnesium Deficiency
Location on Plant	Primarily on new growth and younger leaves.	Primarily on older, lower leaves.
Leaf Discoloration	Yellow or brown spots appearing between or along leaf margins.	Interveinal chlorosis, where yellowing occurs between the green leaf veins.
Leaf Shape & Texture	Young leaves may be distorted, misshapen, or curled.	Leaf edges might develop reddish or brown tints. Leaves may curl inwards.
Other Symptoms	Weak stems, poor root development, blossom end rot.	Purple stems or dark veins in some plants, leaf necrosis in advanced stages.

How to Correct a Cal Mag Deficiency

Once a deficiency is confirmed, a multi-pronged approach is often necessary for recovery.

Check and Adjust pH: Always start here. Use a pH meter to test your soil or hydroponic solution. Adjust as needed to bring the pH into the optimal range (e.g., 6.0-7.0 for soil).
Add Supplements: Use a commercial Cal Mag supplement. These are formulated to provide an ideal ratio of calcium to magnesium, often with other trace minerals. Follow product directions carefully.
Use Epsom Salts (Magnesium): For a quick boost of magnesium, a foliar spray of Epsom salts (magnesium sulfate) can be very effective. Dissolve a teaspoon of Epsom salts in a gallon of water and spray the leaves. Do this on a cloudy day to prevent leaf burn.
Soil Amendments: For long-term soil health, you can use dolomitic lime (which contains both Ca and Mg) or gypsum (calcium sulfate, which is pH neutral).
Flush Your Medium: If a nutrient lockout is suspected due to excess salts, flush the medium with pH-balanced water and a light nutrient solution before applying new fertilizers.

Conclusion: Timely Action for Healthy Plants

Timely identification and correction of a Cal Mag deficiency are critical for a plant's health and productivity. By paying close attention to where on the plant symptoms appear—young leaves for calcium, older leaves for magnesium—you can make an accurate diagnosis. Correcting underlying issues like improper pH and choosing the right supplements or soil amendments will ensure your plants get the nutrients they need to thrive. Proactive monitoring and regular testing are the best defenses against these common deficiencies.

Visit West Virginia University Extension for more information on plant nutrient deficiencies

Authoritative Resource

West Virginia University Extension: Nutrient Deficiencies in Plants

Frequently Asked Questions

The first signs typically depend on which nutrient is lacking. For calcium, look for distorted or small new leaves. For magnesium, the first signs are interveinal yellowing (yellowing between the veins) on the older, lower leaves of the plant,.

Calcium is an immobile nutrient, so its deficiency symptoms appear on new growth first. Magnesium is mobile, so its deficiency appears on older growth first. Additionally, calcium shows brown spotting on leaves, while magnesium primarily presents as yellowing between the veins,.

In soil, a pH that is too acidic (below 6.2) or too alkaline (above 7.5) can cause a Cal Mag deficiency by locking out the nutrients. In hydroponics, a pH below 5.5 can make these nutrients unavailable to the roots,.

Yes. Water from Reverse Osmosis (RO) systems, water softeners, and rainwater have very low mineral content and are often a primary cause of Cal Mag deficiencies, especially when used with soilless media,.

For magnesium, a foliar spray with Epsom salts offers a fast solution by allowing direct absorption through the leaves. For a combined issue, using a liquid Cal Mag supplement mixed into your nutrient solution and ensuring the correct pH is the fastest remedy.

Organic options for adding calcium and magnesium include using dolomitic lime or gypsum as a soil amendment. Bone meal and crushed eggshells can also provide calcium over time. For a quick boost, a compost tea can help.

Yes, an oversupply of Cal Mag can be problematic. Excess calcium can cause a nutrient lockout of other minerals, especially potassium, iron, and manganese. It is important to avoid over-supplementation and maintain a balanced nutrient regime.

Magnesium deficiency can be confused with other nutrient issues like iron or zinc deficiency, as all can cause interveinal chlorosis. Calcium deficiency can resemble problems like boron deficiency or blossom end rot, which it can cause.