How to fix magnesium deficiency in cannabis?

We all dread the idea of our favourite green girls running into any issues while they are under our care. The amount of time, passion, and effort that goes into growing display-worthy weed is immense, but, at some point in your cannabis cultivation journey, you are going to run into problems, no two ways about it.

One of the more common nutrient deficiency issues in weed plants is magnesium deficiency, especially if you are growing in coco-coir or with a full hydro set-up. There comes a time in a weed plant’s life when it really needs a decent hit of magnesium, and if you aren’t there ready, things can go south pretty quickly.

Why do cannabis plants need magnesium?

Magnesium is one of the many secondary macronutrients that all plants need to develop properly, and that includes cannabis. It is classed as a mobile nutrient, meaning the plant can essentially move it around to where it is needed.

In this case, if a leaf needs more magnesium, then the plant can redirect from older leaves that may not be performing as much photosynthesis. That’s why you’ll often see signs of a magnesium deficiency starting from the bottom of the plant and working its way upwards.

But what exactly does magnesium do for cannabis plants?

Magnesium forms the ‘backbone’ of cannabis plant energy production. It is the powerhouse that drives photosynthesis, the process by which plants convert light energy into chemical energy to fuel growth and development. Without enough magnesium, this process is severely slowed to the point where a bunch of issues can arise.

It also plays a crucial role in:

• Transporting and metabolising of other nutrients around the plant structure, especially phosphorus
• Facilitating the production of nucleic acids, which are essential for the growth of healthy plant cells.
• Helps in producing plant proteins.
• Acts as an enzyme activator in several key reactions within the plant.
• Stomata function regulation.

If any of these functions are impaired due to a lack of magnesium, it can quickly have a detrimental effect on your cannabis plants.

What are the symptoms of magnesium deficiency in cannabis plants?

The first thing you are likely to notice if your plant is low on magnesium, is Interveinal chlorosis. You have probably seen this before in either weed or other plants without really realising it. It is a yellowing of the tissue between the veins on leaves, leaving a dark green vein structure behind.

Other signs include:

• Browning and curling of leaves
• Stunted growth
• Twisted or distorted new fan leaf growth
• Reduced bud development
• Red stems (especially the leaf stems)

What are the main causes of cannabis magnesium deficiency?

That depends on which growing medium you are using. If you are growing in soil, magnesium deficiencies are much less common as most cannabis-specific soils contain a decent amount of the nutrient to start with. That said, it’s not totally unheard of, and if you started with poor soil, you could definitely run into this issue.

Soil grown cannabis plants may also run into pH issues, but again, this is less common than for plants grown in coco coir or hydroponically. If your soil is too acidic, it can lock out magnesium and other essential nutrients, leading to deficiencies.

For those growing in coco coir or hydroponics, magnesium deficiencies are more common. The reason for this is that these growing mediums have no natural reserves of magnesium, so it’s up to the grower to ensure they are providing enough through nutrients or supplements.

In hydro systems especially, magnesium can be easily depleted as plants can take up a lot of water quickly, and with it goes a portion of the nutrients in solution.

Over or underwatering can also lead to nutrient issues, as can using the wrong type of water (e.g. RO water or hard water)

How to treat magnesium deficiency in cannabis plants?

First up, you need to make sure it is a magnesium deficiency you are dealing with. The symptoms listed above can be similar to other nutrient deficiencies, so it’s always best to double-check.

No matter which grow media you are using, my first stop on the deficiency fixing express is always pH city. Too often, new growers will run into issues that look like deficiencies but are actually caused by incorrect pH levels.

Re-calibrate your pH tester before you do anything else. If you don’t have a pH tester, go and get one, right now.

Next, flush your medium with a pH-balanced solution (5.5 to 6.5 for coco/hydro, or 6.0 to 7.0 for soil). This means draining out all the old water/nutrient mix and replacing it with fresh water (not recycled).

Give the plant a day to recover, and then feed as usual but also add a magnesium supplement to the solution. Most good cannabis nutrients will have some in there already, but check to be sure. Cal-mag supplements can be found at any grow store and are a quick fix for magnesium deficiencies.

If you want to go the organic route, Epsom Salt is a great option. Simply mix 1 tablespoon per gallon of water and use it to water your plants every other watering until symptoms disappear.

You can also do some foliar application, but that’s probably not something you want to be doing too often.

How to prevent cannabis magnesium deficiency?

You’ll hear it time and again, but prevention is always the best cure. Even if you haven’t run into a magnesium deficiency before, then chances are there is something in your grow that needs tweaking.

• Be strict with your pH (and EC or TDS) – be sure to check it at least once a week, even if you are using good soil. Every water if you are using coco or hydro.
• Start with good soil – don’t skimp on good quality organic soil. It’s worth spending a bit extra to avoid headaches later. Good drainage is key.
• Don’t over/under-water – ensure your plants get the right amount of water for their size and age. This will vary between plants, but generally speaking, you want to see about 25% of the water drain out of the pots.

What does too much magnesium look like in cannabis?

This is rare but still worth mentioning. Excess magnesium can cause deficiencies in other nutrients. You may see signs of toxicity if you use supplements for too long or do not monitor.

Symptoms include:

• Dark green leaves
• Slow growth
• Brown spots or tips on leaves

How can I tell the difference between magnesium and other deficiencies?

With magnesium deficiency in the early stages you should see yellow stripes along the leaves of the cannabis plant. When you compare this to other common deficiencies such as potassium or calcium you can clearly spot the difference as other deficiencies often cause yellowing at the tips or brown patches.

Magnesium deficiency FAQs

How long does it take to correct cannabis magnesium deficiency?

You should see improvements within a week or two of addressing the issue, but it can take a good few weeks for new growth to appear healthy.

How often should I give my cannabis plants Cal Mag?

I like to give it once a week during the vegetative growth stage and then twice weekly during flowering.

Can Epsom salt help with magnesium deficiency?

Yes, Epson salt is a great organic option that can be used to amend soil. Do not use it in hydro setups.