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Magnesium Deficiency in Cannabis & How to Fix it

By Jessica McKeil May 04, 2020

Cannabis has a greater need for magnesium than other plants. Indoor growers especially must remain vigilant for the signs of magnesium deficiency. Most basic nutrient products from your hydroponics supply store have the N-K-P covered, but micronutrients, like magnesium, are equally as important (and sometimes overlooked).

Magnesium deficiencies can strike both soil-based and hydroponic growers and throughout all stages of the growth cycle. Careful attention to the signs of magnesium deficiency can help you catch this issue and others (ex: calcium and potassium), before it damages the flower.

The Role of Magnesium in Growing Cannabis

Cannabis is a nutrient hog, and based on the current understanding, requires more magnesium than other commonly cultivated crops. Without magnesium, cannabis (and all plants) cannot feed itself through photosynthesis.

Magnesium is literally at the core of the plant's energy production. It sits squarely in the center of the chlorophyll molecule. Without magnesium, cannabis will stumble in the production of that vibrant green color, and fail to provide enough food (energy) for healthy growth

Magnesium is a mobile nutrient, meaning the plant can move the nutrient from old leaves into new leaves. If not caught early, your plants might begin making this rapid movement of magnesium, killing all older, lower leaves.

What Are the Signs of a Magnesium Deficiency in Cannabis?

The signs of magnesium deficiency may not appear until three to four weeks after the problem began. Daily rounds and careful assessment of discoloration and other symptoms is key to catching this nutrient deficiency as early as possible.

During all stages of the cannabis growth cycle, you'll want to be in the grow room, with safety glasses on, spending quality time assessing the health of your plants.

The most notable signs of magnesium deficiency include:

  • Lower leaves begin to yellow, starting at the outer edges and bleeding inward. It almost appears like a tie-dye effect.
  • Left unchecked, the full leaves turn yellow with slight green along the veins.
  • Serious deficiency turns the edges of yellow leaves brown and dry.
  • Stems begin to redden.

How to Fix a Magnesium Deficiency in Soil?

Although magnesium deficiency is most common with soilless growers, there are times when soil-bound growers become susceptible to this nutrient issue as well. Under certain conditions, an imbalance with other nutrients can lock cannabis out of micronutrient absorption.

Elevated levels of other nutrients, like potassium and calcium, may trigger the nutrient lockout of magnesium. Other environmental influences can also impact nutrient absorption, most critically pH level.

Once you've identified the issue, test the soil for pH, and adjust as needed to fall back within the ideal range of 6.0 to 7.0 pH with a water flush. Once flushed, assess the nutrients you have been using to better understand the magnesium supply.

Epsom salt is the most common way to apply straight-magnesium to your grow, but it must be diluted into water at a 20-25 ppm ratio. Dolomite is a combination of calcium and magnesium, also useful for an immediate correction. 

With a market full of cannabis-specific nutrients, catering to all types of substrate and stage of growth, it may be time to refresh your current supply. If you've been mixing nutrients yourself, or adapting non-cannabis varieties, consider a switch to improve magnesium uptake in your cannabis plants. 

How to Fix a Magnesium Deficiency in a Hydroponic System?

A hydroponic grow requires similar corrections, but slightly different parameters. First, test the pH of the system. A hydroponic grow needs to sit between 6.0 and 6.5 pH at all times. If your grow sits outside this range, its time for adjustment.

Once your system is fully flushed, and balanced back to the ideal pH level, restart the nutrient feeds with a hydroponic cannabis-specific product. Again, it is possible to provide additional feedings with an Epsom salt or dolomite lime mixture.

For the experienced grower, with the confidence to build a well-balanced nutrient profile, these products offer a much more affordable way to supplement cannabis growth than the premixed options. But, for newbies, stick with premixed cannabis nutrients.

A Final Word on Calcium Deficiency in Cannabis

With the two the three-week delay in symptoms, you may be caught off guard when the signs of magnesium deficiency appear. Finding yellowing leaves, bleaching out from the bottom up, is quite alarming. Although it is not possible to reverse the damage done by a severe magnesium deficiency, with quick action, you can prevent further losses.

Take the time to test for pH, rebalance if needed, and update your nutrient collect with cannabis-specific and substrate-specific products. If you know what you are doing, provide more immediate and directed magnesium relief with either cal-mag mixtures or Epsom salts. 


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