Now Reading
Magnesium (Mg) and Cannabis Growing

Magnesium (Mg) and Cannabis Growing

Five days from harvest showing perfect magnesium deficiency

HEY HEY EVERYONE; I hope this article finds you all baked and happy. Today I wanted to babble a bit about magnesium (Mg) and cannabis growing. Cannabis is a serious consumer of Mg, especially during the flowering cycle, but … too much Mg is highly destructive to your living soil, especially effecting soil structure, which will mess with the aerating abilities of your soil, hence hurting all the soil microbeasties; if the microlife is not happy and healthy, neither are your plants. So, what I’m going to do here is just give you all a rundown addressing Mg, how to safely add it if needed, and how to avoid an Mg overdose. I’ve been doing this growing thing now for almost half a century (44 years) and I am a decent observer of detail and patterns when it comes to cannabis, so hopefully I can help many of you out here. Let’s check this out shall we…


Looking at that photo with those flowering plants 5 days from harvest showing ‘awesome’ Mg deficiency; and I say awesome, because Mg is one of those elements in cannabis that does not smoke well (and is in fact really harsh). So, you always love to see the Mg deficiencies this close to harvest if you are aiming for thee tip-top shelf finished product. You growers that are all tea crazy, using decent amounts of molasses in your teas, you need to know that molasses packs a big Mg punch; so, translated that means you should lay off the molasses usage by about halfway through flowering. You always want to have a grip on all (globally) your mineral type inputs—like the Mg in the molasses—because other elements can get deadly too, fast, when a bit too much is added, like potassium (K), or sulfur (S), as two other examples.

Garden grade epsom salt

Again, referring to those plants in the photo with the good kind of Mg deficiency, you will see how the larger leaves in about the middle of the plant are really getting yellow/chlorotic. These are classic Mg signs here folks; and those leaves aren’t just yellowing, they show ‘interveinal chlorosis’ yellowing, which simply means that the veins of the leaves stay green while the rest of the leaf yellows between those veins. That’s how you can spot a Mg deficiency on flowering plants especially. It should be noted here that I run rich levels of Mg in my own soil blends, with my largest contributor being dolomite lime. I occasionally need to amend with additional Mg; some genetics just “crave” more Mg than others, so there’s always that to keep in mind.

Curled up and burnt tips strongly indicate early Mg issues

See in the photo how the tips of the leaves are curling upwards? This is highly likely in my experience to indicate an early stage of an Mg issue. This is what I would call an early warning flag, and no need to fear if you see this, just keep an eye on the newest growth for the next 10 days or so. I normally find this situation fixes itself as the plant and the soil life work in concert to make more of the Mg in the soil available to those plants. Put your thinking caps on and go over everything going into your plants, mineral-wise, and especially Mg & calcium (Ca). Dolomite lime in your soil mix is the best way to proactively handle this. But I have found other ways to amend and check for Mg issues, so let’s have a look at those now.

My epsom dosage per gallon of water

Epsom Salt (garden grade) is a great tool to have but must be used with great caution. If you are seeing Mg issues the first week or two into flowering you may want to amend your water with some Epsom Salt. In the photo you can see the amount of Epsom Salt I add per gallon of water. Your water must have some calcium dissolved in it, like well or spring water, but less than 100 PPM on a TDS meter. I have used bottled spring water for this purpose (many times) in the past; these days I use reverse osmosis water that I have dissolved some calcium into the same way you would make a tea, with an air pump and stone, bubbling for about 24 hours. Turns out about 60 PPM before adding the Epsom. Well water works great just blend with rain or distilled water to reach desired PPM range if too high.

USAGE: What I do is add this to my water for my plants—stirring until dissolved—for the next 3 times I water, and that has always handled it wicked good for me if I am showing signs of an Mg issue too early into flowering. In the past I have tried adding Epsom Salt to my soil blends and it always seemed to cause me some issues, I have plenty of Mg these days in my soil from multiple sources and I don’t normally need any amended via the water as above, but now and then I do.

See Also
Weird Cannabis Growing Problems Conquered

Happy healthy plants look like this

If you are not sure you are experiencing an Mg deficiency/issue there is an easy way to check without potentially overdosing your soil with Mg; as I mentioned before, an OD on Mg is deadly to your plants and they will suffer an ugly death. If you think this has happened, you need to do a thorough flushing of your containers and cross your fingers, because it is fairly tricky to come back from that. Let me share this with you next…

My favorite mister

Just get yourself a hand sprayer. Dissolve ¼ teaspoon of Epsom Salt per liter/quart of water and spray on your plants leaves every morning (during first hour or two of light) for about 5 days in a row. Within just several days (max) you should be able to see by looking at the newest growth whether or not some Mg is in order. All nutrients work with all other nutrients, but those three secondary nutrients, magnesium, calcium, and sulfur, all seem to especially catalyze each other symbiotically, so make sure you are covered with all the secondary elements. I’ll share more on this in my next article amigos. Happy Trails!

View Comments (0)

Leave a Reply