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How to Recognize and Treat Magnesium Deficiency in Cannabis Plants

By Leo Bear-McGuinness

Published: Aug 04, 2023   
A vector image depicting varying levels of nutrient deficiency in cannabis leaves
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Cannabis crops need magnesium. It’s an essential element for photosynthesis, the process of turning light into energy. So crops with magnesium deficiencies will have less mass and so, for the farmers, less yield.

This Analytical Cannabis article explores the importance of magnesium, how to spot magnesium deficiency, and how it can be prevented.

Why is magnesium important to cannabis plants?

Magnesium is a crucial element for most plants, not just cannabis. It’s a building block for chlorophyll, the compound that gives plants their green color and converts energy from the sun into energy for the plant. Magnesium also plays an important role in transferring this energy to areas of the plant that need it, such as emerging petioles.

“It’s a mobile element, which means it can be trans-located,” David Kessler, chief science officer at Agrify, tells Analytical Cannabis.

“We can move magnesium from the older growth [at] the bottom of the plant, up to developing areas of the plant higher up; they’re in higher photosynthetically active areas. You’re going to start seeing that that element is being moved into those areas where new growth is really taking place.”

Magnesium deficiency in weed, what are the symptoms?

Clearly, magnesium is crucial for healthy cannabis crops. So how can cultivators spot the signs of magnesium deficiency? Well, it won’t be immediately obvious. According to one study of nutrient deficiencies in cannabis crops, published earlier this year in plants, there are no visible deficiency symptoms in magnesium-starved plants for the first three weeks. But, by week four of the experiment, “mild interveinal chlorosis” – that’s a yellowing of the areas between the leaves’ veins – started to develop in lower fan leaves.

“A slight yellowing, intravenously, between the veins of the leaves – that is the most common problem, probably the beginning symptom that cultivators might see,” Kessler tells Analytical Cannabis.

“As that progresses, as the deficiency either continues in length of time or gets worse, that yellowing is going to become more pronounced on those leaves. And as the symptoms advance, you’re going to see a real contrast between dark green veins and very yellow interventional regions, which will eventually become necrotic.”

Kessler’s descriptions are in line with the plants study, which noted that, by week six of the experiment, most of the crop’s smaller leaves showed severe yellowing, while some older fan leaves remained largely unaffected.

Perhaps the ultimate symptom of magnesium deficiency, though, is a lighter crop come harvest. According to the plants study, inflorescence yields from the magnesium-starved crops were around 30% lower compared to the control crops.

How can you treat magnesium deficiency in your cannabis plants?

So, if a grower spots these yellowing signs early, what can they do? Well, their first response might be to top up the magnesium levels in their soil.

“Farmers looking to efficiently apply magnesium should probably target 50-to-75 parts per million in their nutritional mix,” says Kessler.

To get these figures, many growers would be tempted to use a standard cal-mag supplement pack. But this two-in-one nutrient bag might not be the answer, according to Kessler.

“Going and grabbing the most readily available cal-mag supplement might not be the best way,” says Kessler. “If you already have sufficient calcium, you don’t want to have too much calcium, which can be antagonistic, which can reduce, for example, the uptake of potassium.”

“The easiest garden remedy is magnesium sulphate,” Kessler continues. “People can buy it at their local pharmacy, and it’s fine.”

But what to do if your soil has already got enough sulfur? Well, there’s always magnesium nitrate.

“It might be more expensive,” Kessler concedes, “but something like magnesium nitrate, where you’re adding nitrogen and magnesium, can be a better option than say, magnesium sulphate.”

“If you’re in the vegetative side of growth, that might be a much better option,” he continues.

“You do have to look at what your nutritional formulation is, what you’re growing in the growing medium, and make sure that whatever compound you choose to add to, to address the magnesium deficiency, is going to not negatively impact the rest of the feed program.”

Leo Bear-McGuinness

Science Writer & Editor

Leo joined Analytical Cannabis in 2019. From research to regulations and analysis to agriculture, his writing covers all the need-to-know news for the cannabis industry. He holds a Bachelor's in Biology from Newcastle University and a Master's in Science Communication from the University of Edinburgh.


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