Help me clear this Mg deficiency

600w Ushio opti-blue mh apprx 18”-21” from canopy
FFOF/FFHF/Perlite medium in 7gal smart pot
Blue Plant Nutrients
Tap water pH between 6.4-6.8
water/feed/water/feed schedule

Long time lurker, first time poster. I’m all out of ideas guys/gals and need your help as I’m fucking panicking. About 10 days ago, I noticed some of her lower leaves turning yellow within the margins of the veins. It seems like it happened overnight; particularly after I fed nutes at 1/2 strength. This week is 9th week of Veg. Did some research and came to the determination that she has a Mg deficiency. Tested the pH of my soil and it was at 5.5. I added some dolomite lime a couple inches under the top of the soil, and also top dressed and gave her a good watering; runoff was at 6.4, incoming water was at 7.5. This was done about 7-8 days ago. I foliar fed with Epsom salt solution (2 tsp/gallon, 6.5) 3 days ago, as well as this morning because I know the lime is going to take time to break down. I snipped off the damaged leaves over the weekend.

At this point, I’m really bummed because I feel like I’m chasing my tail. This is my first time growing only 1 plant so if I lose her, I’m going to be devastated. I feel that she should have bounced back by now. I made sure I sorted my I’s and crossed my T’s prior to germinating the seed so to see her having issues is puzzling. Did I miss anything in my game plan/remedies? How long does this usually take to clear up? Using tap water the cause of this? Am I just being impatient? My new growth (what little bit I’m getting nowadays) appears to be a pale, lime green color and the leaves that have yellowed, are showing advanced signs of the deficiency because brown spots are now present and I don’t remember them being there previously. I’m fucking lost. Please help because I’m this close to tossing this bitch in the garbage and starting over from scratch.

By 9 weeks in veg you should be feeding full strength, you could try adding some cal mag to you nutrient solution. If it’s mag def you need to increase the mag and make sure the ph is in the range for optimal uptake of it.

swisha said:

By 9 weeks in veg you should be feeding full strength, you could try adding some cal mag to you nutrient solution. If it’s mag def you need to increase the mag and make sure the ph is in the range for optimal uptake of it.

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My understanding is that cal-mag has more calcium than magnesium in it with cal to mag ratio being close to 3:1. I’ve been foliar feeding with mag sulfate thinking the plant would absorb it quicker than putting it in my water.

No need to start from scratch!

5.5 is a bit on the low side so could be blocking Mag.

You’ve done the right thing with the dolomite lime. Will take a while to buffer the soil pH but better late than never.

Don’t remove leaves unless they are basically dead. Use them to see if the deficiency stops or it’ll just consume more leaves.

Why not add some Epsom salts to your water only days?

Some say 1tsp others say 2tbsp. I go with 1tbsp per 1 gal of water and it worked wonders for me. Might have heavy Mg dependant strain here.

Here is before and after Epsom salts. Stalks went from purple to bright green on both plants!

Nutty sKunK said:

No need to start from scratch!

5.5 is a bit on the low side so could be blocking Mag.

You’ve done the right thing with the dolomite lime. Will take a while to buffer the soil pH but better late than never.

Don’t remove leaves unless they are basically dead. Use them to see if the deficiency stops or it’ll just consume more leaves.

Why not add some Epsom salts to your water only days?

Some say 1tsp others say 2tbsp. I go with 1tbsp per 1 gal of water and it worked wonders for me. Might have heavy Mg dependant strain here.

Here is before and after Epsom salts. Stalks went from purple to bright green on both plants!

Click to expand...

Thanks for the reply Nutty. I’m almost positive the issue I’m experiencing is because of the low pH in the root zone which sucks but the lime should help with that. Can you further share you experience on how you used Epsom salts to help with your deficiency? I know you said you used 1 tbsp in your water, but how often did you use it and what was the turn around time for the recovery of your ladies? Guy at my hydro store suggested I flush but running 21 gallons of water through my 7 gal smart pot is going to be a pain.

Nutty sKunK said:

No need to start from scratch!

5.5 is a bit on the low side so could be blocking Mag.

You’ve done the right thing with the dolomite lime. Will take a while to buffer the soil pH but better late than never.

Don’t remove leaves unless they are basically dead. Use them to see if the deficiency stops or it’ll just consume more leaves.

Why not add some Epsom salts to your water only days?

Some say 1tsp others say 2tbsp. I go with 1tbsp per 1 gal of water and it worked wonders for me. Might have heavy Mg dependant strain here.

Here is before and after Epsom salts. Stalks went from purple to bright green on both plants!

Click to expand...

Are you sure about the 1 Tbsp of Epsom salt per gallon.
I thought 1 tsp per gallon was more of a standard. I prefer 1/2 tsp per gallon but I tend to underfeed.
Can you point me to the thread were you got the 1 or 2 tbsp info from?
Did you mean tablespoon or teaspoon?

RollABluntLightABlunt said:

Thanks for the reply Nutty. I’m almost positive the issue I’m experiencing is because of the low pH in the root zone which sucks but the lime should help with that. Can you further share you experience on how you used Epsom salts to help with your deficiency? I know you said you used 1 tbsp in your water, but how often did you use it and what was the turn around time for the recovery of your ladies? Guy at my hydro store suggested I flush but running 21 gallons of water through my 7 gal smart pot is going to be a pain.

Click to expand...

I used it once 6th week veg. Turnaround was literally overnight. That was at a dose of 2tbsp. Pretty high I know but they loved it..

I suspected another Mg about a week or so ago (5th week flower) and fed just 1tbsp.

That took a bit longer to stop, about a couple of days after feeding I noticed the symptoms stop. Apparently they can be Mg deficient week before any visual signs..

@Buba Blend

I know man! It was confusing to read. Most weed sites say tsp, this garden site said tbsp.

All I know is that in my experience it worked very well.

I did dose at 1tsp but perhaps wasn’t enough. As a while later deficiencies started appearing again..

1 tsp 5ml
1 tbsp 15ml.

Here’s the link;

https://bestplants.com/epsom-salt-garden-cheats/

Nutty sKunK said:

I used it once 6th week veg. Turnaround was literally overnight. That was at a dose of 2tbsp. Pretty high I know but they loved it..

I suspected another Mg about a week or so ago (5th week flower) and fed just 1tbsp.

That took a bit longer to stop, about a couple of days after feeding I noticed the symptoms stop. Apparently they can be Mg deficient week before any visual signs..

@Buba Blend

I know man! It was confusing to read. Most weed sites say tsp, this garden site said tbsp.

All I know is that in my experience it worked very well.

I did dose at 1tsp but perhaps wasn’t enough. As a while later deficiencies started appearing again..

1 tsp 5ml
1 tbsp 15ml.

Here’s the link;

Click to expand...

Thanks for clearing that up. It seems my solution is too diluted. Everyone whom I read online that has foliar fed, has seen improvement overnight!! I feel left out lol. I’ll up it in 3 days to see if it clears it up.

RollABluntLightABlunt said:

Thanks for clearing that up. It seems my solution is too diluted. Everyone whom I read online that has foliar fed, has seen improvement overnight!! I feel left out lol. I’ll up it in 3 days to see if it clears it up.

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That points to a soil issue then.. pretty sure even if you foliar feed and it’s locked out it don’t absorb it..

Don’t count me on that, I’m not 100% certain..

Did your temperatures drop a lot leading up to the purple stems and yellowing? Are you letting the soil dry out good enough before you water again? Also Happy Frog is a coco based mix which requires you to supplement cal/mag. Were you giving supplement cal/mag?

When I see this kind of thing in my plants it is usually because temperatures fall below 65F or when I over water or don't let enough time lapse between watering and the roots suffer from lack of oxygen.

I don't think dumping a lot of fertilizers or things will do you any favors. Water or feed with extra cal/mag, warm the area up in night cycle and let them roots breathe and I think you'll be golden. With soil I think you're fine PH wise since you added lime, 5.5 is a little acidic for soil but its the other end of the spectrum that will give you more issues.

She just seems hungry for mag..also seems to need some extra nitrogen. Purple stems will come from lack of phosphorus getting to the plant, which is what happens when the plant is deprived of magnesium as the two count on each other. What exactly have you been feeding it and at what ppm's? Too much potassium can lockout both magnesium and nitrogen, but im guessing you may just need to increase your dosage/frequency of feeding. Does your new growth have suddenly thinner leaves than before?

Your leaves are showing conflicting signs. Stay with me for a minute. It looks on the one hand as if your nitrogen deficient the lower leaves are starting to fade out, but all your Leaf tips are burnt which is a slight nitrogen excess. The problem is not nitrogen per se. That is a very large pot, and I think that you may be having some over-watering issues. A wet medium can swing pH , locking out nutrients, and the minor elements that contribute to the uptake of nitrogen, or even the nitrogen itself.

I'd let her get really dry before you take your next step. Just my two cents ,really hope the best of luck for you. Peace

thenotsoesoteric said:

Did your temperatures drop a lot leading up to the purple stems and yellowing? Are you letting the soil dry out good enough before you water again? Also Happy Frog is a coco based mix which requires you to supplement cal/mag. Were you giving supplement cal/mag?

When I see this kind of thing in my plants it is usually because temperatures fall below 65F or when I over water or don't let enough time lapse between watering and the roots suffer from lack of oxygen.

I don't think dumping a lot of fertilizers or things will do you any favors. Water or feed with extra cal/mag, warm the area up in night cycle and let them roots breathe and I think you'll be golden. With soil I think you're fine PH wise since you added lime, 5.5 is a little acidic for soil but its the other end of the spectrum that will give you more issues.

Click to expand...

Temps are pretty consistent..low 70s with lights off, mid 70s with lights on. I haven’t been feeding with cal/mag as I was under the impression my tap water was sufficient enough. I’m probably going to make the change to RO water and supplement it since I’ll know exactly what my plant is getting.

Blitz35 said:

She just seems hungry for mag..also seems to need some extra nitrogen. Purple stems will come from lack of phosphorus getting to the plant, which is what happens when the plant is deprived of magnesium as the two count on each other. What exactly have you been feeding it and at what ppm's? Too much potassium can lockout both magnesium and nitrogen, but im guessing you may just need to increase your dosage/frequency of feeding. Does your new growth have suddenly thinner leaves than before?

Click to expand...

New growth leaves are definitely smaller/thinner than what I’ve experienced on my other grows! Good call on that. I’ve been running some new nutes (Blue Planet). I got a ppm meter yesterday to measure the ppm of what I was normally feeding and it was above 2000 (that was using distilled water as well for the first time). Wayyy to high for the amount I was using. Going forward, I’m going to feed her around 700-800ppm, see how she reacts and increase if needed. Moral of the story: I believe that ppm’s matter in soil; contrary to what I’ve seen online. I won’t be so naive next time for sure.

HookahsGarden said:

Your leaves are showing conflicting signs. Stay with me for a minute. It looks on the one hand as if your nitrogen deficient the lower leaves are starting to fade out, but all your Leaf tips are burnt which is a slight nitrogen excess. The problem is not nitrogen per se. That is a very large pot, and I think that you may be having some over-watering issues. A wet medium can swing pH , locking out nutrients, and the minor elements that contribute to the uptake of nitrogen, or even the nitrogen itself.

I'd let her get really dry before you take your next step. Just my two cents ,really hope the best of luck for you. Peace

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I agree, as I was thinking initially that it WAS a N deficiency so I up’ed the dosage to 3/4 strength and that’s how the subsequent lead tip burn happened; had some clawing going on too. I don’t think it’s underwatering only because I lift the pot to see when she needs water. With it being such a big pot, it makes it easier to determine when it’s dry vs. wet.

I ended up flushing her last night. Took her to the tub and ran 21 gallons of distilled water through her. pH = 6.5, used about 1/8 strength of my nutes, added cal/mag. Last gallon, I fed her at 800ppm and used 1/4 tsp of SuperThrive I had laying around. Raised my light a bit too to give her a break while she recouperates. I was turned off by the ppm reading I saw so I’m sure there is salt build up causing lockout. We shall see...

RollABluntLightABlunt said:

Temps are pretty consistent..low 70s with lights off, mid 70s with lights on. I haven’t been feeding with cal/mag as I was under the impression my tap water was sufficient enough. I’m probably going to make the change to RO water and supplement it since I’ll know exactly what my plant is getting.



New growth leaves are definitely smaller/thinner than what I’ve experienced on my other grows! Good call on that. I’ve been running some new nutes (Blue Planet). I got a ppm meter yesterday to measure the ppm of what I was normally feeding and it was above 2000 (that was using distilled water as well for the first time). Wayyy to high for the amount I was using. Going forward, I’m going to feed her around 700-800ppm, see how she reacts and increase if needed. Moral of the story: I believe that ppm’s matter in soil; contrary to what I’ve seen online. I won’t be so naive next time for sure.
Nm, didnt see your last post, you already did this..best of luck!



I agree, as I was thinking initially that it WAS a N deficiency so I up’ed the dosage to 3/4 strength and that’s how the subsequent lead tip burn happened; had some clawing going on too. I don’t think it’s underwatering only because I lift the pot to see when she needs water. With it being such a big pot, it makes it easier to determine when it’s dry vs. wet.

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If your new growth is thinner and appearing stalled, then it would make sense that you have either too much potassium or too much phosphorus laying around..which makes sense if you were feeding at 2000 ppm. Potassium will lock out nitrogen and magnesium, whereas phosphorus wouldn't. So my guess would be too much potassium and by now maybe even sodium, which also conflicts with magnesium. Too much phosphorus sitting around would in turn lock out potassium and calcium. Having your ph that low meant the plant may not have been able to uptake magnesium even if it was available. Flush it all out and re administer a balanced feed, see how she replies

Update: Appears the flush has helped tremendously. No chlorosis on new growth and the affected leaves haven’t gotten worse over the past 7 days. She’ll be going into flowering in 3 days. Thanks to the community and everyone who chimed in. I appreciate it.

Sidenote: Should I go ahead and remove the damaged leaves or leave as is?

RollABluntLightABlunt said:

I ended up flushing her last night. Took her to the tub and ran 21 gallons of distilled water through her. pH = 6.5, used about 1/8 strength of my nutes, added cal/mag. Last gallon, I fed her

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21 gallons of distilled water through a 3 gallon pot and she didn't drown? Probably wont water for a month. 21 gals of distilled h2o Thats 40-50 bucks from the store! Do you make it yourself? Distilled water and RO are not the same but are both 0ish ppm.

Cannabis prefers 5:1 Ca:Mg. But varies with strain.


Mg is not as effective foliar as watering it in. Top dress is most effective.

I would have add some kelp meal or legbenite (sp) or trade name supplimate instead of the dolomite. But with what you have already done, Blitz35 is accurate in his conclusion.

You can lower the pH of the water a bit more to raise the soil pH. How did you pH test soil?Measuring pH of runnoff doesn't tell you anything. Measuring the ppms will.

Lay off the superthrive. Once or twice in early veg or at transplant. Search superthrive cannabis complications.

Don't over feed, over tea!