Deficiency and excess of magnesium in cannabis plants

Functions of magnesium in cannabis plants

Magnesium as mineral
Magnesium as mineral

Magnesium is a very necessary secondary nutrient in all the stages of the plants life, and it’s needed in large quantities. It’s the central atom of chlorophyll and has a direct impact on the absorption of solar energy to be subsequently processed and used by the plant in the creation of sugars and carbohydrates.

The cannabis plant absorbs magnesium in ion Mg+2 form, being this the magnesium formulation normally found in most soils. Thus, the absorption of this nutrient will be determined by the available form of this element in the substrate for marijuana plants.

It’s important to find a balance between the available and the unavailable magnesium in the soil. Unavailable magnesium still hasn’t been transformed by the microbial life yet, so plants can't absorb it. It’s important to know the amount of magnesium available for the plant (which is very difficult to know without analyzing the substrate).

Then, how should we proceed when growing in soil/hydro to have the magnesium levels in the substrate under control? As it’s a mineral, we can supply our plants with it by using a mono-nutrient in Mg + 2 form or other products high in Mg, so we achieve a direct uptake by the roots without having to wait for the microbial life to transform it into assimilable elements for the plant.

Magnesium (Mg) deficiency in cannabis plants

Onset of Magnesium deficiency
Beginning of Magnesium deficiency

If the Ph range of the substrate is lower than 7.0, then magnesium can be easily absorbed by cannabis plants. But if the soil is very acid - lower than 5.0 - magnesium won't be assimilable by the plants. In this case, we should increase the Ph level of the substrate by using limestone dolomite. There are other products on the market that contain magnesium in case of not having limestone dolomite to mix with the substrate.

As magnesium is a mobile element, any deficiency of this nutrient will be first visible in the oldest leaves and those of the lower part of the plant. As the deficiency advances, the central part will also be affected.

Magnesium - as nitrogen - is easily flushed through abundant watering. If we wash the roots to solve other other nutrient excesses (N, P, K) we must add magnesium and calcium to maintain a correct nutrient balance. Normally, growers use two parts of Ca for one of Mg (EC=0.4).

Advance of magnesium deficiency
Advance of magnesium deficiency

It’s important to note that there are other factors that may reduce or lock-out the magnesium uptake, such as a permanent humidity, low temperatures, or acid and cold substrates. If our substrate is costantly humid, we will proceed to put a fan in the growing space pointing directly to the pot's substrate, thus providing a more dry substrate and improving magnesium uptake. And, of course, we won't water our plants so ofently.

Once the substrate has the correct humidity level, we should adjust the amount of nutrient solution per irrigation to favour Mg absorption while avoiding a possible root rot, which is usually very harmful - even deadly - for cannabis plants. These cases are more usual in indoor crops, especially when temperatures aren’t well controlled during the night period.

To solve this problem, we should install a heat source to raise the temperature of the growing space to a minimum of 18 °C during the night period. In this way, the development of plants is not slowed down and we solve the problem in regard with magnesium uptake.

Progressed magnesium deficiency
Serious magnesium deficiency

Remember that during the flowering stage, plants need a lot of nutrients such as phosphorus, potassium and of course magnesium to develop large flowers and the original smells and flavours naturally present in the plant.

Magnesium during the flowering stage of cannabis plants

Many of the deficiencies that we can observe during the flowering period are given due to an excess with the bloom fertiliser. Most fertilisers for this phase contain high amounts of phosphorus and potassium: it should be said that potassium and calcium are two nutrients that can cause magnesium lock-out when added in excess.

Usually, this lock-out occurs during the first weeks of flowering - when the plant is creating buds - approximately during the third-sixth week of flowering, depending on whether it’s an Indica or Sativa strain. During these weeks, and if we use too much fertilizer, we'll cause an excess of potassium (K) that will block the absorption of magnesium (Mg). To solve this nutrient imbalance, flush the roots to lower the nutrient excess in the substrate so that the plant can absorb Mg again.

It’s also necessary to emphasize that the deficiency often appears because the substrate doesn’t have enough Magnesium; in these cases, we should provide our plants with magnesium during the above-mentioned weeks, either directly in the soil - by adding limestone dolomite - or through irrigations, using a Mg supplement in he nutrient solution and via foliar with a Ph level of 7.1. You'll notice a quick recovery of marijuana plants.

Magnesium (Mg) deficiency

How to quickly detect a Mg deficiency in cannabis plants?

  • It's difficult to detect in the early stages
  • First signs are shown in the oldest or lower leaves of the plant
  • The tips of the leaves turn brown and curl upwards
  • The brown spots increase in number and size, advancing from the lower part to the top of the plant
  • The youngest leaves, located in the top part of the plant, are also affected, showing brown spots and possible discoloration of the veins
  • The deficiency may come preceded by an accumulation of other nutrients such as calcium, hydrogen, and potassium
  • It’s possible to apply magnesium sulphate via foliar for better absorption (2 % of Mg with a PH level of 7.1)
  • Temperatures shouldn’t be under 18ºC during the night period and 24°C during the day
  • Adjust the Ph value of the substrate to 6.5 (soil) and 5.5 ( hydroponics)

Magnesium (Mg) excess

How to quickly detect a Mg excess in cannabis plants?

  • Its detection is difficult since having magnesium excesses is rare as long as we use substrates suitable for growing marijuana.
  • Magnesium ions come into conflict with calcium ions, causing Ca lock-out (look for symptoms of calciumdeficiencies)
  • If there is excess of Mg you should flush the roots with triple the amount of water than the capacity of the pot. After that, water your plantswith a balanced fertiliser

Magnesium deficiency
Magnesium deficiency

Imagen 6
Mg deficiency

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” (19)


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!


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!


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.


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!


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!


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!


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.


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!


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.


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!


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!


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.


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


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?


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


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!


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: 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!


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


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!

Queries about shipping and payment

For doubts relating to deliveries and payments you can check the sections shipping cost and payment methods.

Do you want to give your opinion on "Deficiency and excess of magnesium in cannabis plants" or ask a question abut this post?

Watch out, it will be published!

Make sure that it is a valid email address. It will not be published

About this Cannabis Blog

This is the official blog of Alchimia Grow Shop. This blog is intended exclusively for the use of adults over the age of 18 years.

To buy equipment for growing cannabis at home you can consult our catalogue of cannabis seeds, grow shop and paraphernalia

Subscibe to the blog

Do you want to receive all the latest developments, news and curiosities from the world of cultivation?