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How To Fix Cal Mag Deficiencies in Cannabis Plants

Fixing calcium and magnesium deficiencies isn’t as easy as cannabis growers have been led to believe.

The common tactic is to add calcium and magnesium into the root zone or as a foliar spray. And while this could work, depending on what’s causing the deficiencies, it can also create more nutrient absorption problems than it solves.

Fortunately, there’s a better program for dealing with a suspected calcium or magnesium deficiency:

  • First, foliar spray a cal-mag additive mixed with a surfactant and B-52 vitamin booster at 240 parts per million and 5.7 pH.
  • Flush the root zone using reverse osmosis water and a full dose of Flawless Finish®, with a pH of 5.9 for the flushing water.
  • After the root zone has dried out from the flush, feed the plants a moderate dose of quality base nutrients, such as pH Perfect® Sensi Grow & Bloom or pH Perfect® Connoisseur® Grow & Bloom. Not only do they automatically balance your pH levels, they’re specifically engineered for cannabis with the correct ratios of macro, secondary, and micronutrients needed in each phase of growth.
  • Wait at least five days to see the effects of the flushing by examining leaf color and health, along with overall plant growth. The leaves that were already damaged won’t heal, but newer leaves will look better and growth rate and overall vigor should improve.
  • If you still suspect calcium and/or magnesium deficiencies, root feed Sensi Cal Mag Xtra® to your cannabis. In most cases, you wouldn’t do the cal-mag root feed supplementation for more than once a week, and only for two to three weeks maximum.

The Most Comprehensive Way to Deal With Cal Mag Deficiencies

There are many cal-mag additives on the market, most of which are generic and ineffective. Some are sold at discount stores, intended for fighting blossom-end rot in tomatoes, while others are available at hydroponic grow stores.

If you’re wondering how to fix cal mag deficiencies, these generic solutions are never the answer.

The swiss army knife of cannabis grows, Sensi Cal Mag Xtra is made specifically for cannabis plants and contains stable, absorbable forms of calcium and magnesium in the correct ratios. It also contains chelated iron — often undersupplied or marginally bioavailable in hydroponic root zones — which assists in calcium and magnesium absorption and utilization.

We know solving cannabis nutrient deficiencies can be endlessly frustrating, inducing panic at the first sight of leaf problems and glitches with plant growth.

And it can be tempting to subsequently start dumping cal-mag additives and other materials onto your crops, which ultimately makes the problems much worse.

By following the program laid out above, you’ll have the best chance of fixing calcium and magnesium deficiencies in your cannabis crops so they return to optimal performance and reward you with heavy harvests of potent, flavorful buds.

Why Do Calcium and Magnesium Deficiencies Arise in Cannabis?

Improper configuration of the root-zone media or nutrient solution is often the reason behind these deficiencies.

If you’re using well water or municipal water instead of reverse osmosis water, your H2O may contain elements that interfere with the absorption of other nutrients or negatively affect root-zone pH, resulting in such deficiencies.

In many cannabis gardens, the root zone becomes oversaturated with nutrients or drifts to the wrong pH, blocking absorption of calcium or magnesium.

Some soils also have physical or chemical structural issues that block calcium and magnesium absorption, while coco coir needs its own specialized feeding program to address nutrient lockout. Root-zone temperatures that are too warm or too cold also interfere with the uptake of nutrient elements.

You’re most likely to encounter calcium or magnesium deficiencies encountered in bloom phase, especially when using intense grow lights such as professional LEDs and double-ended HIDs. Intense lighting stimulates accelerated photosynthesis which increases your plants’ need for calcium and magnesium.

The Importance of Calcium for Cannabis

Deficiencies of either of these key nutrients can create unhealthy, vulnerable plants with slow or stunted growth, weak stems and stalks, and smaller, less-potent yields.

For example, cannabis research indicates that drought, extreme heat or cold, wind, pests, diseases and other stressors induce cannabis to defend itself by transferring calcium to affected areas of the plant. Calcium deficiencies in cannabis cripples this defensive response, making plants more susceptible to damage from garden attackers, like spider mites.

And just like calcium is an integral component of healthy human skeletons, it’s also integral to plant cell walls and membranes. Calcium deficiency weakens plants structurally, reducing their ability to build healthy new tissue, including roots, stalks, stems and buds.

Magnesium deficiencies often occur in tandem with calcium deficiencies.

Let’s take a look at…

Why Magnesium Matters

Stages of magnesium deficiency in cannabis plants

Why is a magnesium deficiency problematic for cannabis?

Simply put, magnesium is essential for the primary engine of all plant life: photosynthesis.

It’s a central element in chlorophyll, the substance that enables plants to absorb light and turn it into energy to power their metabolism. Magnesium is key to metabolic processes that assist your plants in nutrient absorption and processing, utilization of carbon dioxide, activating enzymes, and creating proteins and carbohydrates essential to floral and seed production.

Without it, your plants can’t come anywhere close to their fullest potential.

Signs & Symptoms of Calcium & Magnesium Deficiencies

Although it’s difficult to definitively diagnose deficiencies based on leaf appearance alone, the following indicators taken together may indicate a calcium deficit:

  • Cannabis plants grow slowly and fail to reach normal height.
  • Leaves initially turn dark green.
  • Leaf edges may turn brown or dry out. The same can happen to leaf tips, which may also exhibit slight downward curling.
  • Root growth is slow. Root mass is small. Calcium-deficient roots suffer increased risk of diseases such as root rot, and are less efficient at intaking nutrients, oxygen and water.
  • In late-stage calcium deficiency, rust-colored spots or larger-sized blotches appear on leaves. On the undersides, blotches appear to be red, pale, transparent or white. These problems start on younger leaves and later appear on older leaves.
  • Stems and stalks are weak and brittle, leading to branches that break if heavy buds form on them.


Signs of magnesium deficiency somewhat overlap with but aren’t always identical to those of calcium deficiency.

These signs include:

  • Growing shoots that are spindly, narrower, or weaker than normal. Yellowing of new growth may also be present.
  • Root growth is slower and root mass is smaller than normal.
  • Necrotic (dark, dead, reddish-brown) spots appear on leaf margins. Parts of the leaf begin to die off.
  • Yellowing between leaf veins, with the central area of the leaf still green.
  • Magnesium deficiency shows first in older, larger leaves, usually the ones that are lower on the plant. If the deficiency isn’t corrected, leaves may die and fall off the plant.


And remember, you can always reach out to…

Advanced Nutrients Grower Support

If you suspect you have a calcium or magnesium deficiency — or if you need help troubleshooting any other cultivation issues — our Grower Support Team is here to help you.

And they’re available Mon-Fri 8AM-5PM PST.

Just call +1 (800) 640-9605 or email support@advancednutrients.com.

For more practical cultivation advice that will help you get even more from your crops, subscribe to our newsletter.


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About the Author

The Advanced Nutrients Team

Since 1999, Advanced Nutrients has been committed to educating the community and bringing the most up-to-date knowledge to the forefront of grows across the globe. Every article you read here has been curated by Advanced Nutrients’ industry experts, so you can continue raising your bud weight… and your reputation.

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Over time, components within the nutrients settle on the bottom of the bottle. When you pour a bottle without shaking it, you can potentially be pouring out an unbalanced solution. Shaking up the nutrient bottle ensures you are pouring the right blend into your reservoir, giving your crops the nutrition they need.

Usually, it’s not a problem. But some products are incompatible with each other and can damage your plants when they’re used together. For example, bloom boosters with high PK numbers or low-grade silica products can destabilize your nutrient mix and cause nutrient lockout.

Nutrient manufacturers design their products to be used with each other. In doing so, they include specific portions of each nutrient in each product, so crops receive the right nutrition through each stage of the grow cycle. When you stick with one manufacturer, you don’t have to worry about inadvertently destabilizing your nutrient mixture. One example of why you need to stick with only one manufacturer’s products is Advanced Nutrients’ pH Perfect line. These products are designed to keep your crops’ root zone within a specific pH range. When you use them alongside products from another manufacturer, we can’t guarantee your root zone will maintain its optimal pH level.

Yes! If you’re growing with Advanced Nutrients products, you can use our easy nutrient calculator to generate the correct nutrient chart for your crops in seconds. Check out Advanced Nutrients’ nutrient calculator here. Another great resource is our library of free custom-growing recipes. Try our expert grower-tested nutrient schedules here.

You can also download our official BudLabs app to generate nutrient schedules and receive real-time notifications for specific tasks, including feeding your crops. Upgrade to BudLabs Pro to maintain profiles on an unlimited number of crops.

If you are a grower, visit this link to find the closest Advanced Nutrients Authorized Retailer. If you are a retailer, visit this link to identify our official distributors.

Our greatest passion is growing great cannabis—and helping other growers do the same. That’s why we offer complimentary cultivation support 24/7 from Monday to Friday and from 9 AM until 7 PM PST on Saturday… With the same level of advice licensed producers would pay consultants tens of thousands of dollars to unlock.
Just call 1-800-640-9605 or email support@advancednutrients.com. For Spanish-speaking growers, email spanishsupport@advancednutrients.com.

Yes! If for any reason you are not absolutely thrilled with the results you achieve when you use our products, just bring the unused portion, along with your original sales receipt, back to the place of purchase within six months, and ask for your money back. Find out more about our Grower Guarantee here.

Advanced Nutrients has spent many thousands of man-hours developing a technology that automatically balances your pH for you — putting it in the “sweet spot” and holding it there for one week.

And the technology is so “smart” that it can account for many of the aforementioned variables in your grow room. Our proprietary pH buffering agents and stabilizing mechanisms became the foundation of a new system aptly called pH Perfect® Technology. Learn all about it here.

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