Get your free CO2 calculator
Grow the best plants with your FREE CO2 calculator.
Learn how to spot and treat Cal Mag deficiency in cannabis
Cal mag deficiency is one of the most common nutrient problems cannabis growers face
If left unchecked, it can wreak havoc on your plants, causing stunted growth, weak stems, and lacklustre yields.
But don’t panic! This guide has everything you need to spot and fix the problem before it ruins your grow.
We’ll walk you through the symptoms of cannabis cal mag deficiency, explain why deficiencies happen (especially under LEDs), and provide practical solutions to restore your plants’ health.
Disclaimer: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis, you’re doing so by the law and subject to appropriate permissions and licenses of the applicable country.
Get expert growing advice – sign up for the Grow The Best newsletter now!
What is Cal Mag deficiency?
Cal mag deficiency happens when your plants don’t get enough calcium and magnesium - two key nutrients that are vital for healthy growth.
Calcium strengthens cell walls and membranes, supporting healthy root growth and preventing weak stems and stalks.
A lack of calcium weakens the plant's structure, making it more susceptible to stress and poor nutrient uptake.
Magnesium, a central element in chlorophyll, is essential for photosynthesis.
Without these nutrients, you’ll notice slow or stunted growth, discoloured leaves, and weak stems in your cannabis plants.
Under LED grow lights, Cal Mag deficiency in cannabis plants is particularly common.
These lights are brighter and cooler than other types, making it harder for plants to absorb calcium & magnesium properly.
The problem is so widespread that many manufacturers produce Cal Mag supplements specifically for growers using LED setups.
How to spot early signs of calcium and magnesium deficiencies
Identifying calcium and magnesium deficiencies early can save your cannabis plants from lasting damage.
Look out for these common signs and symptoms of calcium magnesium deficiency:
Calcium deficiency symptoms:
- Yellow or brown spots on younger leaves.
- Weak or brittle stems.
- New growth that appears distorted or curled.
Magnesium deficiency symptoms:
- Yellowing between the leaf veins on older leaves.
- Leaf tips curling upwards.
- Gradual spreading of yellowing to the edges.
These issues often appear during the vegetative stage, but they can also occur in the flowering stage, where nutrient demands increase.
While symptoms of magnesium deficiency somewhat overlap with calcium deficiency symptoms, careful observation of which parts of the plant are affected is key to accurate diagnosis.
Magnesium typically affects older leaves, while calcium impacts new growth.
Why Cal Mag deficiency happens
There are several reasons why your cannabis plants might struggle with magnesium and calcium uptake:
Many nutrient deficiencies arise in cannabis because of environmental factors or imbalances.
1. Environmental factors:
- Incorrect pH levels can negatively affect the root zone, making it harder for plants to absorb calcium and magnesium. For cannabis, the right pH is crucial; fluctuations can negatively affect root zone pH and can lead to nutrient lockout and poor nutrient absorption, especially in coco coir setups. Aim for a pH of 6.0 - 6.5 in soil and 5.8 - 6.2 in coco coir to avoid Cal Mag deficiency in coco.
- Reverse osmosis water (RO water) lacks the natural minerals found in tap water, making calcium and magnesium deficiencies more likely.
2. Growing medium:
- Coco retains magnesium poorly, making a Cal Mag deficiency in plants more likely if you’re not supplementing correctly.
- Cal Mag deficiency in soil happens when there’s low nutrient content.
3. LED grow lights:
- Magnesium deficiencies arise frequently under LED lights because of increased nutrient demands and cooler temperatures.
How to fix Cal Mag deficiency
The good news is that Cal Mag deficiency is fixable.
Here’s how to get your cannabis plants back on track:
1. Adjust your feeding regime:
Most of the time, increasing your main nutrient feed solves the problem.
Many base nutrients already contain enough calcium and magnesium to meet your plants’ needs.
2. Supplement with Cal Mag (if necessary):
If your plants are still struggling, a Cal Mag supplement can help.
Be cautious, though; many of these products contain nitrogen, which can lead to overfeeding during the flowering stage.
3. Try alternatives:
For magnesium, dissolve a small amount of Epsom salts in water.
To add calcium to address a calcium deficiency, add garden lime or gypsum to your growing medium.
4. Correct pH levels:
Ensure your root zone is within the correct pH range to avoid nutrient lockout.
5. Use tap water if possible:
Unlike RO water, tap water often contains enough magnesium and calcium to prevent deficiencies.
Want to grow like a pro? Subscribe to the Grow The Best newsletter!
Preventing Cal Mag deficiency in the future
The best way to deal with Cal Mag deficiency is to stop it from happening in the first place.
Here’s how:
Maintain ideal environmental conditions:
- Use a grow room monitoring device like the Grow Sensor to keep your grow room at the right temperature and humidity levels.
- Avoid extreme pH fluctuations in your medium.
Choose the right growing medium:
- If you’re using coco, consider pre-treating it with Cal Mag or using nutrient blends designed for coco.
Feed appropriately:
- Follow the manufacturer’s feeding schedule, and don’t over-rely on Cal Mag supplements.
- Follow the manufacturer’s feeding schedule to maintain balanced nutrients, and avoid common nutrient deficiencies.
Monitor your plants regularly:
- Look for signs of deficiency during critical growth stages, especially under LEDs.
Common myths about Cal Mag supplements
There’s a lot of debate about whether Cal Mag supplements are necessary for every grower.
Some see them as a miracle cure, while others argue they’re overhyped. The truth lies somewhere in the middle.
In most cases, feeding more of your main nutrient will solve the problem without needing extra supplements.
However, if your plants are in coco or under LEDs, you might need a little boost.
Remember to use Cal Mag sparingly. Overdoing it can lead to nutrient imbalances, particularly during the flowering stage.
Takeaways
Cal mag deficiency in cannabis plants might seem like a headache, but with the right approach, you can spot and fix the issue before it impacts your harvest.
Pay close attention to your plants, keep your environment dialled in, and don’t rush to supplement if it’s not necessary.
Sometimes, a few minor adjustments are all it takes to keep your cannabis plants thriving.
Happy growing!
FAQs
What are the signs of Cal Mag deficiency?
Calcium and magnesium deficiencies in cannabis plants can show up in different ways, depending on the plant's stage and growing conditions.
Early signs of Cal Mag deficiency include yellow or brown spots on leaves, weak stems, and poor growth.
If you’ve got a Cal Mag deficiency in veg, you’ll often notice leaf tips curling or discolouration on older leaves.
During the flowering stage, the deficiency may lead to reduced bud development and overall poor plant growth.
Specific symptoms of calcium deficiency include distorted new growth and fragile cell walls, while magnesium deficiency shows as yellowing between the veins, particularly on older leaves.
Be especially vigilant if you’re growing in coco or under LED lights, as these setups often exacerbate Cal Mag deficiency symptoms.
How to fix Cal Mag deficiency fast?
Fixing calcium and magnesium deficiencies in cannabis plants requires quick action to restore your plants’ health.
Start by adjusting your nutrient solution to ensure it contains enough magnesium and calcium, most base nutrients already include these, so increasing the dosage often solves the problem.
If the deficiency persists, consider adding a Cal Mag supplement, but be cautious; many include extra nitrogen, which can cause issues, especially during the flowering stage.
Alternatively, use Epsom salts for magnesium deficiency or lime for calcium deficiency.
Check your growing conditions, too. Ensure the pH in your root zone is within the ideal range (6.0 - 6.5 for soil; 5.8 - 6.2 for coco).
If you’re using RO water, switch to tap water to add natural minerals. These steps will fix the deficiency and boost your plants’ growth fast!
How do I know if my plant needs Cal Mag?
Determining if your cannabis plants need Cal Mag starts with observing the signs of deficiency.
Look for early signs of Cal Mag deficiency, such as yellow or brown spots, curled or discoloured leaf tips, and poor growth.
Pay attention to the older leaves - yellowing between veins often indicates a magnesium deficiency, while brittle stems and distorted new growth suggest a calcium deficiency.
Growing conditions also provide clues. If you’re using LED lights, the brighter intensity can make calmag deficiency symptoms more likely.
Similarly, plants in coco or on RO water may struggle to absorb these essential nutrients.
If you’ve adjusted your nutrients and pH but still see symptoms, supplementing with Cal Mag might be necessary to restore healthy plant growth.
Should I add Cal Mag to every watering?
Adding Cal Mag to every watering isn’t always necessary and depends on your cannabis plants’ needs, growing setup, and water source.
Tap water often contains enough calcium and magnesium to prevent deficiencies, so supplementing at every watering could lead to nutrient imbalances.
However, regular Cal Mag supplementation may be needed if you’re growing in coco or using RO water since these setups can lack essential nutrients.
Even then, monitor your plants closely. Overusing Cal Mag can result in excess nitrogen (common in many Cal Mag products) or nutrient lockout.
The best approach is to test your solution of nutrients and observe your plants.
Adjust as needed during critical stages like veg or flowering, but don’t rely on Cal Mag as a one-size-fits-all solution.
Do I add Cal Mag first or last?
When mixing your nutrient, always add calcium and magnesium first.
This ensures it properly dissolves and doesn’t interact negatively with other nutrients.
Start by adding Cal Mag to your water, whether you’re using tap water or RO water, and mix it thoroughly before adding other nutrients specifically for cannabis plants.
By adding Cal Mag first, you give your plants access to essential calcium and magnesium, which are vital for nutrient uptake and integral to plant cell wall development.
Skipping this step or adding it later could lead to inconsistent results or nutrient imbalances, particularly in setups like coco, where deficiencies are more common.
Once you’ve added your Cal Mag and other nutrients, check the pH to ensure your root zone is within the ideal range for cannabis, avoiding issues like nutrient lockout.
What happens if I use too much Cal Mag?
Overusing Cal Mag can create more problems than it solves.
Excessive amounts often lead to nutrient imbalances, particularly a build-up of calcium or magnesium, which can block the uptake of other key nutrients like potassium and iron.
This can cause further deficiencies and harm your cannabis plants’ overall growth.
Many Cal Mag supplements also contain nitrogen, so overuse can lead to nitrogen toxicity, especially during the flowering stage.
This can result in dark, overly lush leaves and delayed bud development.
Burnt leaf tips, salt build-up in the growing medium, and poor root zone health are signs of overuse.
If you notice these issues, flush your system with plain water to remove excess nutrients and recalibrate your feeding schedule.
Should I give Cal Mag during flowering?
Yes, but only if your cannabis plants show signs of cal mag deficiency during flowering.
Calcium and magnesium are crucial for bud development and overall plant health, so deficiencies at this stage can reduce yields and quality.
Look for signs of deficiency like yellowing between leaf veins, feeble stems, or poor bud growth.
However, be cautious with Cal Mag supplements during flowering. Many contain nitrogen, which plants need less of in this stage.
Overusing Cal Mag can lead to nitrogen toxicity, delaying flowering and harming your harvest.
If you’re growing in coco or under LED lights, where deficiencies are common, adding Cal Mag sparingly may be beneficial.
Always monitor your plants and adjust your nutrient solution to meet their needs without overfeeding.
How often should I feed Cal Mag?
How often you feed CalMag depends on your growing setup and your cananbis plants’ needs.
If you’re using coco, which tends to retain magnesium poorly, or RO water, which lacks natural minerals, adding CalMag once or twice a week can prevent deficiencies.
In setups with tap water, you may not need to supplement as frequently since tap water typically contains enough calcium and magnesium.
Overusing CalMag can lead to nutrient imbalances, so only feed it when your plants show signs of deficiency.
During critical stages like veg or flowering, monitor your plants for early signs of calmag deficiency, such as yellowing leaves or weak stems, and adjust accordingly.
Always check the pH in your root zone to ensure optimal nutrient uptake.
Discover the best growing tips – sign up for the Grow The Best newsletter!
.avif)
