plant ph and mag Issues after switching to cobs

Discussion in 'LED and other Lighting' started by mcnasty_nug, Nov 14, 2017.


    mcnasty_nug Member

    If this should be in the sick plant forum im sorry, I thought this would be more apt since this seems to be an LED related issue.

    Short story, since i switched to cobs my PH rises crazy fast and Im seeing classic magnesium issues

    Long story:
    I recently build some LED lights out of Luminus cxm-22 cobs. Im running 2x lights in a 4x8 tent. Each light is 10 cobs mounted in a xxxl hood running at about 72w each. So Im looking at 1440w per 4x8. They are 3000k and a 50/50 mix of 80 and 90 cri.

    My plants are in titan flow and grow system, Ive run this system before, and i have another 4x8 right next to this one that is still 1khps and all is fine. I had another 4x4 with one of these lights in it, i had the same issue. One of my drivers died so I have 2x 315 CMH in there now and the mag and ph issue is gone. (the 4x4 is a flood and drain table)

    I use jacks hydro and calnit with some added microbes and geolite/rocks as my medium.
    my room temp is 80-84 with lights on, my leaf surface temp is 76-79.

    Ever since I moved to cobs, this tents rez PH is rising rapidly. IE I PH it down to 5.6 with botanicare phosphuric acid ph down and by the evening im at 6.5+

    I am also seeing classic magnesium def signs.

    Ive washed out my rez, I dont see signs of algea or anything crazy going on in the root zone, although my plants have filled up the buckets with roots, could it be root bound?

    I just recently saw on PLC site that mag should be in the 75-90ppm range, my jacks is sitting at like 25-30ppm so i JUST started running my nutes as such:
    jacks 3g
    cal-nit 2.5g
    mag sulfate 2g
    diammonium phosphate .25g (i read the ammonium base helps buffer PH lower, doesnt seem to be working tho)
    growsil .25g

    This has my mag at 75ppm exactly I'm pretty sure.

    so, any ideas whats going on here? Like I said I have several other tents going with HPS, CMH, and even a veg area with citizen 6500k cobs and none of them are giving me these issues. Its JUST the cob tents.
    Hybridway likes this.

    kushedy Active Member

    Lots of people have reported either calcium or magnesium deficiencies when switching to cobs.
    I haven't seen any solid explanations as to why this happens as yet. Everyone seems to be just adjusting their nutrient profile to include extra calcium & mag to avoid deficiencies setting in.
    Your ph issue is odd. The only time I have seen ph swing that much was during the onset of a root infection. Whats your res temps?
    GreenLogician likes this.

    DrBlaze Well-Known Member

    Just so we're all on the same page, what exactly are the signs you are seeing?

    * bonus points for pictures :)
    Last edited: Nov 14, 2017

    nfhiggs Well-Known Member

    Raise ambient temp by 5 degrees and add extra calcium/magnesium.

    DrBlaze Well-Known Member

    I think 85-89 degrees would be too warm, especially under his hid lights. I think he's already in a pretty ideal range for led.

    Edit- just reread it and I missed the mention of tents the first time. I thought these were tables next to each other. In that case I agree, maybe bring up the temps a few degrees in that tent and maybe the plants will transpire more.

    I still want to see pics, as I sometimes see people say magnesium deficiency when its really calcium, or its just some purpling of the stems where the light is hitting it (although they would probably do that under the cmh light as well)
    Last edited: Nov 14, 2017

    PurpleBuz Well-Known Member

    Calcium:Magnesium ratio is low. aim for a 2:1 ratio.

    NPK is about 1:1:1.60 too much phosphate. drop the ammonium dibasic phosphate and use ammonium nitrate instead. I'm currently running with NPK 2:1:3

    are you using RO or distilled water ?
    Last edited: Nov 15, 2017

    wietefras Well-Known Member

    Less evaporation reduces uptake for elements (like calcium and magnesium) which are taken up passively.
    Randomblame likes this.

    mcnasty_nug Member

    Alrighty, thanks for the replies guys! Now, I JUST switched my nutrient profile to what I posted a couple days ago. Before then I was running my usual jacks 3g cal-nit 2g and my microbes. Thats it. so currently im looking at 155-51-171-75-125 N-P-K-mg-ca Like I said though this issue started just running the usual jacks 3-2 setup. So I should add more calcium? My P is really too high? cannastats tells me I can be 70-100ppm P without issue? For now I think ill let it go a few days and see if the adjustment helps.

    Ive been using RO water. with this recent rez change i JUST switched to tap water, my hope was again to stop this PH drift. My tap is well water and it runs 50ppm. Im using a bluelab pen @ 500 scale btw

    My rez temp is 68-71 depending on if the pumps currently running or not. I do have air lines running to all the flow and grow buckets and in the rez. I had them running 15 on 15 off but i also just switched that to 15 on 45 off as I read too much air can be a cause of this ph swing?

    When i pull the plants up to look at the roots they look normal. They arent crazy white but my roots never are from the small bit of humic acid in the microbes I figure they are stained a bit. They arent flimsey, dont smell, none of that jazz.

    Heres the pics: (the glossy marks are from a foliar I sprayed last night)



    PurpleBuz Well-Known Member

    my reference to excess phosphate is based on the NPK ratio of the weights that you posted.

    These weights
    jacks 3g
    cal-nit 2.5g
    mag sulfate 2g
    diammonium phosphate .25g

    Have these nutrient ratios
    N: P2O5: K2O 1: 0.99: 1.56
    N: K 1: 1.3
    N (NO3-): N (NH4+) 1: 0.12
    Ca: Mg 1: 1.03
    mcnasty_nug likes this.

    Randomblame Well-Known Member

    What's the distance between canopy and lights? That looks like too much intensity, especially in the middle. Maybe it's already enough when you hang it a few notches higher. If that's not possible, dimming it a bit and slowly approach the maximum the strain can handle.
    1440w in a 4x 8' is a lot of light, that's 45w/sft...
    In this area you can easily get bleaching, if the distance is not big enough.
    24" is minimum to avoid that, maybe more...
    Do you have a lux meter? Could be helpful to test if there is an hotspot and to find the sweet spot.

    CobKits Well-Known Member Rollitup Advertiser

    i go a little higher with mag with jacks in veg

    instead of 3-2-1 i run 3-2-1.5

    my well water has plenty of calcium so i dont supplement
    mcnasty_nug and thetr33man like this.

    wietefras Well-Known Member

    Agree with Randomblame that looks more like bleaching than magnesium deficiency.

    Althouh that could be an advanced stage of Magnesium deficiency. I have never seen it like that on my plants at least. They wouldn't get that far. I just up the dosage of MgO and it's gone.

    I have never had Mg deficiency under COBs though. Only Ca. I do always get Mg deficiency under fluorescent tubes.
    mcnasty_nug and Randomblame like this.

    Greengenes707 Well-Known Member

    NPK and N: P2O2:K2O. P2O5 and K2O get reduced to ~.43 and .92 for P2O5:P and K2O:K respectively. The available NPK are what matters. Same when we want just S not sulfates. Ever one I have spoken with on nutrients uses actual NPK.

    Actual NPK= 1 : 0.43 : 1.3 for the OP's recipe...
    Screen Shot 2017-11-15 at 12.29.42 PM.jpg

    What are the antagonist signs for to close of a ratio for Ca:Mg? I have spoken with a few people and running 2:1 up to 1:1 has never been an issue. What am I missing? His issue is some what common and I haven't stuck a 100% nail into it.

    Veg+blooms forumla is interesting, and runs high on Ca(~145ppm), then up to the grower to supplement Mg. Just so hard to get the Ca up without getting N up too right?
    Last edited: Nov 15, 2017

    Greengenes707 Well-Known Member

    @mcnasty_nug The PH swing is the biggest issue I see. But I can't tell you why that rez is rising up and the others aren't with all the other variables being the same essentially.

    Are the rez/root temps the same?
    What microbe products? Might not be interacting with the rhizosphere the same.

    The tap water can help stabilize a little. sometimes, and can help with more Ca most of the time and helps your ratio PurpleBuz is talking about.
    If you were not adding any Mg should see them come around fast. You can foliar feed Mg to really get the pump primed since it's not mobile compared to npk.

    I remember @captainmorgan having PH rising issues at one time, not sure what he did. I know veg bloom has that issue in RO sometimes without the right buffer too.
    mcnasty_nug and 714steadyeddie like this.

    thetr33man Well-Known Member

    Dont mean to hijack but Ive got a plant thats has some issues under cob also, not MJ but plants r plants ehh? (maybe not) This is using a home made organicish soil and it looked great for a month or two, but something isnt right now. Suggestions on how to correct this damage?
    mcnasty_nug likes this.

    PurpleBuz Well-Known Member

    yes agree with all you stated.
    standard fertilizer NPK expresses phosphorus as P2O5 and potassium as K20

    NPK ratings consist of three numbers separated by dashes (e.g., 10-10-10 or 16-4-8) describing the chemical content of fertilizers.[6][7] The first number represents the percentage of nitrogen in the product; the second number, P2O5; the third, K2O. Fertilizers do not actually contain P2O5 or K2O, but the system is a conventional shorthand for the amount of the phosphorus (P) or potassium (K) in a fertilizer. A 50-pound (23 kg) bag of fertilizer labeled 16-4-8 contains 8 lb (3.6 kg) of nitrogen (16% of the 50 pounds), an amount of phosphorus equivalent to that in 2 pounds of P2O5 (4% of 50 pounds), and 4 pounds of K2O (8% of 50 pounds).

    so if you want to compare NPK ratios it would be nice to stick to standard fertilizer conventions.

    regardless as that's just "a convention" I have actually seen the ph creep up that the OP is talking about.

    Several years ago I switched to Jacks 3-2-1 in flood and drain hydro and my first round had constant ph creep up and very similar nute issues (yellowing, interveinal patterns, etc.). I added ammonia and increased my N to P205 ratio to about 2:1 and all my problems went away. In general, despite all the commercial hype about phosphorous boosters cannabis does not need a lot of phosphate, especially in hydro.

    factoid: Excess phosphorous in hydro can be antagonistic to calcium and magnesium. Calcium Nitrate + phosphoric acid => Calcium phosphate which is practically insoluble in water.

    To this day I am running jacks hydro at about NPK 2:1:3 and about a Ca:Mg ratio of 2:1 and substituting some of the calcium nitrate with ammonium nitrate and/or ammonium sulfate. The amount of ammonia and Calcium nitrate varies depending on EC and calcium content of my water source.

    churchhaze Well-Known Member

    More like 3:1 or 4:1. (still agree though that it's too low obviously)
    mcnasty_nug and jarvild like this.

    churchhaze Well-Known Member

    I always mean N, P2O5 and K2O when refering to NPK just because that is the standard in the US for packaging nutrients and I feel like I would be confusing people if I gave them true NPK. Giving them both seems to confuse people even more. I'm not disagreeing with your reasoning though.

    When giving ratios, I always put the : between the values (like you) because NPK rating is percent by mass, but most people don't understand this distinction anyway.

    Honestly if we were talking about available NPK, it would make more sense if the standard was NO3-, KH2PO4-, K+ since those are the most available ions. Too bad the standards don't make much sense, especially for hydro users.
    mcnasty_nug and Greengenes707 like this.

    churchhaze Well-Known Member

    If your pH is rising that quickly, it's possible your plant is deficient in either K+, Ca++ or Mg++. (probably Ca)

    I think you need more calcium nitrate and less magnesium sulfate. P is fine. (60ppm is higher than needed, but if it drifts below 30ppm, you will see reduced growth. Starting at 60ppm makes P def nearly impossible as time goes by)

    I had the exact same 'pH swings up' problem when I was testing how low I could go with calcium nitrate. I think it was fixed by putting it back up again, but it wasn't exactly a controlled experiment.
    Last edited: Nov 15, 2017
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    DrBlaze Well-Known Member

    A 2:1 Ca to Mg ratio is what I aim for and never have mag problems. Also for someone wanting to add Ca without adding a lot of N, Calcium chloride is an excellent tool to have in the arsenal. It also adds chloride, which is a micronutrient most people never think of and that a lot of gardens are probably a bit deficient in.
    mcnasty_nug likes this.

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