Deficiency and excess of magnesium in cannabis plants

In the world of cannabis cultivation, each of the macro and micronutrients plays a crucial role in the healthy development of plants. In this sense, magnesium is a fundamental nutrient for harvest success, playing vital roles in key processes such as photosynthesis, chlorophyll formation, and nutrient absorption.

In this article, we invite you to explore the importance of magnesium in cannabis cultivation, from its basic functions to signs of deficiency and excess, as well as best practices to ensure an adequate supply of this essential nutrient.

 Advanced Magnesium deficiency in cannabis
 Advanced Magnesium deficiency in cannabis

Functions of magnesium in cannabis

Magnesium is a secondary nutrient that is very necessary in all phases of plant life and is required in large quantities. It is the central atom of chlorophyll and has a direct impact on the absorption of solar energy so that it can be subsequently processed and used by the plant to create sugars and carbohydrates.

The cannabis plant absorbs magnesium in the Mg +2 ion format, this being the formulation of magnesium that we can find in the earth. The absorption of this nutrient will be subject to the available format that this element is found in the marijuana substrate.

It is important to find a balance between available magnesium in the soil and unavailable magnesium. The unavailable magnesium has not yet been transformed by microbial life, making it unfit to be consumed by the plant. It is important to know the amount of magnesium available to the plant (which is very difficult to know without analyzing the substrate and the irrigation water used). Logically, in the case of inert substrates, the presence of magnesium will be zero.

So how do we proceed when we grow in soil or hydro to control the magnesium levels in the substrate? As it is a mineral, we can make a contribution through a Magnesium mono nutrient or other products that contain a large part of Mg for direct absorption by the roots without having to wait for the microbial life to carry out its transformation into assimilable fertilizer for the plant.

 Magnesium as a mineral
 Magnesium as a mineral

Magnesium deficiency in cannabis cultivation

If the pH of the substrate is less than 7.0, magnesium can be easily absorbed by cannabis plants, but if it is very acidic, below 5.0, its availability will be much less or non-existent and we must proceed to increase the pH value of the substrate. substrate using dolomite limestone before starting to grow in said substrate. If dolomite limestone is not available to mix with the substrate, there are also products on the market that incorporate this element in liquid format so that it can be dosed with each irrigation.

This is a mobile element, so deficiencies in this nutrient will first be visible in the oldest and lowest leaves of the plant. As the deficiency progresses, it will be the central part that, along with the lower part of the plant, will be affected by the lack of this element.

Magnesium is a nutrient that, like nitrogen, is easy to carry away with plenty of water. If we carry out root washing with enzymes due to having overfertilized the plants with other nutrients (N, P, K) we will have to replenish the levels of magnesium (and calcium, to maintain the balance) by providing two parts of Ca to one of Mg of EC. 0.0 to 0.4.

 Beginning of Magnesium deficiency in cannabis
 Beginning of Magnesium deficiency in cannabis

It is important to highlight that other factors reduce or cancel the absorption of magnesium even when it is present in the soil, such as continued humidity in the substrate, a low temperature, or also an acidic and cold substrate. If the substrate is almost always very humid, we will proceed to introduce a fan into the crop that points directly at the pot where the excessively moist substrate is located, so that we can facilitate the drying of the substrate and improve the assimilation of magnesium. . Using a thermal blanket under the pots can also work great.

Once the substrate has recovered, we will regulate the amount of irrigation to promote Mg absorption and also avoid possible root rot, which is usually very harmful to cannabis plants. These cases can occur in indoor crops when growing temperatures are not well controlled, especially when the plants are at night.

Overwatering cannabis plants

Overwatering is a frequent error that must be taken into account when you start growing cannabis and want to avoid losing many seeds or plants during cultivation. Learn how to prevent sudden deaths due to an excess of water when growing marijuana.

To solve this problem we must install a heat source that provides a higher temperature to the crop with a minimum temperature of no less than 18º so as not to slow down the development of the plants and solve the problem with the absorption of magnesium.

In the flowering stage, the plant requires a large amount of nutrients such as phosphorus and potassium to create large flowers along with the original smells and flavors that the plant contains.

Magnesium and marijuana flowering

Many of the deficiencies that we can observe during the flowering period are caused by excessive flowering fertilizers. Fertilizers for this phase of cultivation contain high amounts of phosphorus and potassium. In this case, it should be noted that potassium and calcium are two nutrients that in excess will cause a blockage in the absorption of magnesium.

 Progress of Magnesium deficiency
 Progress of Magnesium deficiency

It is common for this blockage to occur during the first weeks of flowering, when the plant is starting to form buds, between the 3rd and 6th week of flowering, depending on whether it is a faster or slower variety. During these weeks, and if we exceed these nutrients, the plants will try to absorb all the possible available food, causing overfertilization of K that will block the absorption of Mg. To fix this nutrient imbalance, we must carry out a root wash to reduce the excess nutrients in the substrate so that the plant can absorb the available Mg again.

It is also worth noting that many times the deficiency will appear because the substrate does not have enough magnesium. In these cases, where there is no overfertilization, we can therefore make a small extra contribution of magnesium during the weeks mentioned, either in the soil directly with Dolomite Limestone or by irrigation with a supplement of Mg and use the spraying of this same element in the leaves of plants with a Ph 7.1 for rapid recovery of plants.

How to quickly detect a Mg deficiency in cannabis plants?

  • It becomes difficult to detect during the early stages of deficiency
  • Deficiencies occur in the oldest and lowest leaves of the plant.
  • The tips of the leaves turn brown, curling upwards
  • The brown spots increase, progressing from the bottom to the top of the plant
  • The youngest leaves, located at the highest part of the plant, are affected by brown mottling along with possible discoloration of the veins
  • The deficiency may be preceded by an accumulation of other nutrients such as calcium, hydrogen and potassium.
  • Mg sulfate can be applied foliarly to help eradicate the deficiency in less time, with a dose of a concentration of 2% Mg with pH 7.1
  • The root and environmental temperature should not be lower than 18º during the night and 24º during the day.
  • Adjust the pH of the substrate in soil to 6.5 and in hydroponics to 5.5 for about 5 days if the deficiency is important.

-15%
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Excess Magnesium (Mg)

Too much magnesium in cannabis plants can manifest itself through various symptoms, including poor growth, dark green leaves, or even brown spots on the edges. Additionally, it can interfere with the plant's ability to absorb other nutrients, resulting in secondary deficiencies.

To correct this problem, it is essential to balance soil pH and adjust fertilization levels to reduce magnesium concentration. Additionally, you can choose to water your plants with purified or rain water along with enzymes to help remove excess nutrients from the soil.

How to quickly detect excess Mg in cannabis plants?

  • It is difficult to detect since excesses of magnesium are not very common as long as substrates suitable for growing marijuana are used.
  • The magnesium ions conflict with the calcium ion-producing a Ca blockage (look for symptoms of Calcium deficiency)
  • If there is an excess of Mg, a root wash must be carried out, applying at least three times as much water as the capacity of the pot and then watering with a balanced fertilizer.

Happy harvest!


The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “Deficiency and excess of magnesium in cannabis plants” (20)

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Dude 2023-07-23
I'm seeing a dark purple form throughout the tops of some plants, it's happened before and pretty much destroyed the plants, not sure if it's a deficiency or an infestation, I have searched everywhere and have not seen this in any pictures throughout the web

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Alex 2021-09-30
Thanks, one more thing bothering me. I have more than one plant and the issues are only affecting half of them even though they are all grown in the same substrate using the same nutrients and are watered with the same water. Any thoughts on why it affected some plants and not all?

Alchimia Staff

Tim Alchimia 2021-10-05
Hi Alex, I'm guessing that you're growing a few different varieties at the same time, which would explain why some plants might have more need of Calcium and Magnesium than others. Even if you're growing seeds of the same variety there can be big differences in nutritional requirements fro one phenotype to another. Other factors that could affect nutritional requirements are things like the intensity of light, where plants at the edge of the light footprint can "eat" less than those in full light, or ventilation, when plants nearer to the fan are more likely to transpire at a higher rate, increasing the nutrient uptake. I hope that helps you work out what the problem is. Best wishes and happy growing!

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Alex 2021-09-29
Is there a water test kit that tests for calcium and manganese? would like to take the guess work out of diagnosing my issue as I'm not sure if I have a excess or deficiency in calcium or manganese.

Alchimia Staff

Tim Alchimia 2021-09-30
Hi Alex, thanks for your comment and question. Yes, you can apply Calmag as a foliar spray but I wouldn't recommend using it on a flowering plant as it may leave residues and can easily encourage bud rot. Also, if your plants have a Manganese deficiency then I'm afraid that applying CalMag will not help much because it contains Calcium and Magnesium, and not Manganese. Plus, if you're almost ready to harvest, to be honest, it's a little late to be able to do anything about deficiencies, it's unlikely the plant's metabolism will be able to bounce back in the remaining time available to it. For future reference, deficiencies of Magnesium, Manganese and Calcium can be easily avoided by using a quality substrate and feeding with a complete nutrient line but being especially careful to control the pH of the nutrient solution, taking great care to stay above 6 and below 7 pH. As for testing kits, we don't have any that test for Calcium or Manganese but you can probably find water tests for Calcium and Magnesium in most pet shops as it is used to test aquarium water. I hope that helps. Best wishes and happy growing!

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Alex 2021-09-29
I am struggling with what I thought was a calcium and maganese deficiency so I have been adding calmag to my watering for about 3 to 4 weeks now (almost ready to harvest) but the problem seems to be getting worse. I'm wondering now if I have too much calcium instead of a deficiency. It started at the top leaves, new growth but is spreading down to the lower leaves as well. The leaves have all turned brown and curled up. Also, is it ok to foliar feed with calmag? I just tried that and am wondering if that is making it worse.

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Mucci 2021-08-31
Hello.. I have a question.. Before I put some girls into flower and after their last feeding in veg, I believe I over furtalized. The effects weren't too bad and they all recovered, except for one strain. Now I'm in the first week of flower and on the the plant and the clones I took from it that are now veging, it seems like it has self terminating growth. By that I mean it only grows one node with one leaf at a time it's kinda hard to explain pictures would make it more understandable. But I'm worried that it won't produce buds because it hasn't stretched at all in Flw so far. Also it doesn't look like the clones will produce any branches, they are only producing funky leaves. It's kinda like the plant is topping its self, it has no main growth and the only growth is where the side branches would grow if that's comprehendable. Iv never seen this happen before on any plant and iv been doing this a long time and also feel stupid for explaining it like this. If anyone could understand the problem I'm describing, help would be appreciated.

Alchimia Staff

Tim Alchimia 2021-08-31
Hi and thanks for your comment. It sounds like this particular plant is going through serious shock/stress as a result of the excess nutrients. It also sounds a little like the clones and the plant are trying to revegetate and recover - at least the only time I've seen such strange growth on a plant is during revegetation. The clones will most likely recover with time and care, although I doubt the same can be said for the plant that's currently flowering. If plants begin the flowering cycle under stress then it's very difficult for them to get back on track and give good results - they're investing so much energy in getting back to normal that flowering is not a priority. Personally, I'd be tempted to get rid of it and give more space to the plants that are doing well rather than keep a sickly plant in my flower room where it could become a vector for pests and pathogens. I hope that helps. Best wishes and happy growing!

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Mr p 2020-10-11
My leaves have got yellow veins in them, they are just in pre flower this week, but its been developing a while , still growing strong but the tips of the leaves are now going light green, so slight you could miss it tbh, im using led lights and the manual mentioned I might need to use more cal mag than usual, so I'm wondering if this is the issue? Thanks

Alchimia Staff

Tim Alchimia 2020-10-13
Hi and thanks for your comment. Yes, it sounds like your plants are experiencing a slight Mg deficiency, I've heard the same thing from a few growers using LED lighting, and they simply increase the dosage a little to compensate. I've also read that this deficiency is more commonplace when using older or cheaper LEDs and shouldn't be a problem when using the latest, high-quality LED lamps from Lumatek or Gavita. I hope that helps, all the best and happy growing!

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mickeyG 2020-09-07
Is diatomaceous earth a good source of calcium?

Alchimia Staff

Tim Alchimia 2020-09-08
Hi Mickey, thanks for your question. Diatomaceous Earth is a great source of Silica and does contain around 20% Calcium but as I understand it the Calcium is bonded to the non-water-soluble portion of the Silica and would need to be decomposed (composted) before it could be available to plants. Personally I like to use Dolomite Lime in my garden, it's easy to apply and almost impossible to overdose the plants with it. All the best and happy growing!

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Glyn Hunt Is an Alchimia client 2020-09-02
To all persons concerned with deficiencies: Nitrogen, Magnesium and Phosphorous all overlap with symptoms of red stems and flashes and streaks of red on leaves when severe. From what I understand this is a pigment that forms when the plant is very stressed. I've observed this with my own plants, particularly Magnesium and no amount of epsom salts will cure it! The quickest remedy is cal mag nitrate for real problems with magnesium. If it's mild you can try epsom but sulphates are slow to uptake.

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Chris 2019-08-22
Hi Tim again!!! Forgot to ask....if spraying with epsom salt what is the mix? Thanks

Alchimia Staff

Tim Alchimia 2019-08-27
Hi again Chris, for the Epsom Salts I use 1 tablespoon per gallon. I tend to apply it mixed with a cup of freshly blended Aloe Vera Juice in there too, apart from anything else it works as a good surfactant and generally promotes plant health. It's not essential though. Another thing I forgot to mention was that some soil tests will probably come in very handy to find out exactly what the problem could be, I'd recommend testing the soil before you amend it for next year, otherwise, you may unknowingly be making the problem worse! All the best!

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Chris 2019-08-22
Hi Tim...The purple is in the new growth at the ends and the stems at tips are streaked purple. No purple in main stems or bigger leafs. One strain can have purple traits but the other two do not, even at that this purple isn't normal looking and stunts the flower growth. Our strains are Gorilla Glue, Blue headband and MSD (Sour diesel), the blue headband can be purplish. We have wondered about the over watering, had a very wet spring here even though they are covered their in a high water content area, we water once a week. Phosphorous is something we wondered about, I have some nutrient with guano in it will add some today when watering. We haven't had too many cool temps, so kinda ruling that out! This fall we are going to condition the soil with some manure and give it a good turn, hopefully won't have this problem next year. We've been told so many different things, someone said could be bug in soil...but too many others say "not"! I will see if I can get some pics sent today.

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Chris 2019-08-21
We have outdoor grow 3 different strains, problem started last year in one end of greenhouse, now this year has spread through all plants! New growth purple with some purple streaks, only in end of stem where new growth is. Planted them in a peat moss w/coco, worm castings and soil all blended together. Was using just regular nutrients during veg, now have switched to a recommended nutrient high in cal and mag for the problem and a blooming nutrient. Geez they are just starting to bloom and the purple is not going away, tomorrow will be third watering with new nutrient. There are a few shoots that are normal and look great....but not many. Recommendations? Too late to trim off the purple? I have sniped some ends a few weeks ago helped some , some grew back purple! I think we're looking a a failed crop. Last year the plants that grew purple ends just quit growing and produced very little and little nugs! We think a deficiency or excess something? We water with a ph of 6.00. HELP!!!

Alchimia Staff

Tim Alchimia 2019-08-22
Hi Chris, thanks for your comment and questions. Purple growth in cannabis can be due to several factors, the most likely being genetics. A nutrient deficiency can cause some purple stems to appear, but I've never heard of the leaves turning purple due to a fertilising issue. Thirdly, low temperatures will cause some plants to develop purple leaves and flowers. Can you confirm that this is leaves or just stems we're talking about here? Some info about the "strains" you're growing might be useful too. Are you sure this isn't a variety with some purple genetics in it? As for the purple plants not yielding much, that could also be due to the genetics being grown, some strains produce very little, although of very high quality. It sounds like you're doing the right thing as far as nutrients go, although it may need more than just three irrigations with the new nutrient solution for you to see any change. If you're seriously worried about an Mg deficiency then I'd be spraying Epsom salts as a quick fix, it will take effect much faster than changing the feed nutrients in the irrigation. Sometimes this purple can be caused by a Phosphorous deficiency, so I'd maybe try feeding some guano or other high-P fertiliser, especially as the plants are beginning to bloom and will need more and more P as flowering progresses. Also, check you aren't over-watering as this can also sometimes cause purpling in new growth. I wouldn't recommend doing any pruning of the purple growth, especially if the plants are starting to flower, you'll just end up stressing them and delaying the flowering, which will also affect final yields. Hopefully, things will get back on track soon, don't give up! All the best for the rest of the season, and happy growing!

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Russell Eldridge 2019-05-17
I have constant problems indoors where my leaves are yellow in between the veins exactly like the picture. When I was outside in 5 gallon pots the plants looked incredible despite me following the bottle recommendation of GH VEGETIVE feeding which is 3:2:1 teaspoons. That’s 15ml+10ml+5ml per gallon! and puts the nutrient at 3.0Ec! Now that I’m indoor and follow their food chart online which is much different, in having this problem on 2 different strains. Ready to give up.

Alchimia Staff

Tim Alchimia 2019-05-27
Hi Russel, thanks for your comment and question. Before you give up, try and address what could be a magnesium deficiency by foliar or root feeding with Epsom Salts or applying CalMag to the roots. Foliar feeding will give the quickest results, simply dilute 1 tablespoon/15ml Epsom salt in a gallon of water and apply to the whole plant, paying special attention to the undersides of the leaves. Different types of lighting can cause some plants to use more or less of certain nutrients, changing their fertiliser requirements. It may be the case that these two strains are responding to the light spectrum or potency indoors and it seems like they're telling you they need more Mg at this point. Remember that the nutrient schedule provided by fertiliser manufacturers is a guideline and that pants will have differing needs depending on many factors, including genetics, conditions, lighting etc. I hope that helps, please let us know what happens All the best and happy growing!

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High rokker 2019-02-02
Magnesium is absorbed as the Mg2+ ion and is mobile in plants, moving from the older to the younger leaves. It leaches from the soil like calcium and potassium. Magnesium is the central atom amid four nitrogen atoms in the chlorophyll molecule, so it is involved in photosynthesis. Manganese is one of nine essential nutrients that plants require for growth. Many processes are dependent on this nutrient, including chloroplast formation, photosynthesis, nitrogen metabolism and synthesis of some enzymes. This role of manganese in plants is extremely crucial. If top leave look good and lower leaves are yellow, it is magnesium.

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Burnt sienna 2018-08-12
Cant quote me but, happen to dealing with similar issues. hoping GH didn't mess with their formulas after being bought out

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charles ward 2018-04-26
I am into the third week or so of flowering, using coco with four stains, Granddaddy Purple Seeds and its clones seem to have no problems what so ever. Cheesecake seems to have the worst luck on this run, uneven leaves, small leaves, chlorosis and some purple stems. Chronic Thunder is a close run to the Grandaddy Purple. The Acapulco Gold is very strong, but slightly burned tips and purple stems. I'm using Hydroguard, and this feeding. Notes: 4.) Maintain pH of 5.5-6.5 Coco Support Vegetative Stage FloraMicro FloraGro FloraBloom CaliMagic (when water is needed) Before Sprout Week 0 ~ ~ ~ 1 tsp Seedling Week 1 1/4 tsp 1/8 tsp 1/8 tsp 1 tsp Vegetative Week 2 1/2 tsp 1/2 tsp 1/8 tsp 1 tsp Vegetative Week 3 1/2 tsp 1/2 tsp 1/4 tsp 1 tsp Flowering Stage Transition Week 4 1/2 tsp 1/2 tsp 1/2 tsp 1/2 tsp Early Bloom Week 5 1/2 tsp 1/4 tsp 1/2 tsp 1/2 tsp Early Bloom Week 6 1/2 tsp 1/4 tsp 1/2 tsp 1/2 tsp Mid Bloom Week 7 1/2 tsp 1/8 tsp 1/2 tsp 1/2 tsp Mid Bloom Week 8 1/2 tsp 1/8 tsp 1/2 tsp 1/2 tsp Late Bloom Week 9 1/2 tsp ~ 3/4 tsp 1/2 tsp Late Bloom Week 10 1/2 tsp ~ 3/4 tsp 1/2 tsp Ripen Week 11 1/4 tsp ~ 3/4 tsp 1/2 tsp Flush Week 12 ~ ~ ~ ~ Coco Coir Nutrient Schedule General Hydroponics Flora Series + CaliMagic 1.) Never mix nutrients directly with each other 2.) Add "FloraMicro" to water first • Repeat Week 3 for a longer vegetative period 3.) Add all nutrients before testing pH • Repeat Week 8 for a longer flowering stage • Give enough water for 10-20% runoff, and always remove runoff! 5.) Give plain water every other watering • You can mix water up to a week ahead of time Amounts per 3.79 liters (1 US Gallon) Base Nutrients I have tilt tables with perfect temps and humidity. Why would I have some of these crazy problems on some plants? I can only think of maybe not letting the coco dry for long enough, and that is controversy. I'm keeping ph at 6 to 6.1 and around 700 ppm average. I did have a lower ph for a while but read the mag needed to be in the range I'm in. Have any clue what I'm doing wrong?

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John Murphy 2018-01-03
Sorry typo there, in did LST training.....

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John Murphy 2018-01-03
I'm not sure what exactly is going on with my Autoflower I did alot off Lostwithiel this lady to get the bonsai affect, but after all that about three weeks and I also tim it twice its is now almost 6 weeks in veg, but now my plants leaves are turning yellow and very purple stems I'm also thinking Mag but I'm not sure it has seemed to have started from the top any help would be much appreciated.

Alchimia Staff

Tim Alchimia 2018-01-03
Hi John, thanks for your question. Without knowing the substrate and nutrients you've used it's hard to say what the problem could be, it may be a deficiency or the purple stems could be from cold temperatures. However it could simply be stress of some kind due to the training and pruning you've done. We don't recommend doing training or pruning on Autos, they are simply too short-lived to benefit from these processes. Because these plants hove such short lives, they don't have enough time to recover from pruning etc, and unfortunately anything at all that slows them down or restricts their growth is going to affect yields negatively. I'd recommend just letting autos do their thing and save LST and other training/pruning techniques for photoperiod plants that will have time to recover before starting into flowering. All the best!

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A friend 2017-12-12
Hi I’m pretty sure I’ve got a calcium and magnesium deficiency. I would appreciate immensly if you had a quick look to confirm this:https://i.imgur.com/bj7GaZN.jpg Kind regards, Anon

Alchimia Staff

Tim Alchimia 2017-12-15
Hi, yes it looks like it to me too. You could try a foliar spray of epsom salts and top-dressing the soil with some dolomite lime and see if that helps. Best of luck!

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John C 2017-08-18
Ah ya ok thx for the reply though

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John C 2017-08-17
We have an excess magnesium in our well water plants are fine until half way through flower then plants go yellow and leaves die...could this be the cause? Thank you

Alchimia Staff

Dani Alchimia 2017-08-18
Hi John C, If the lower parts of the plant - the oldest leaves - are turning yellow, then it's more likely that your plants have nitrogen deficiency. When plants need nitrogen to produce new shoots and can't find it in the soil they use the nitrogen stored in their fan leaves. Also, keep in mind that excess magnesium causes calcium lockouts, so once your plants are flushed remember to use a fertiliser with calcium. Hope it helped!

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