Potassium Deficiency?

Discussion in 'Marijuana Plant Problems' started by algebraist, Mar 13, 2017.


    algebraist Active Member

    What do you think? Growing in soil; 3 1/2 weeks into flower. I recently started giving FF Big Bloom every other watering; could increase to every watering, or give a higher concentration.



    Bear420 Well-Known Member

    FF is a very potent product, I have a shit tonne of that i just quit using, it burns plants fast. Try using smaller amounts, I use a 1/4 teaspoon instead of a full one and it still worked, but still had problems with it. They really don't look to bad right now but it will continue unless you cut back. Try cutting back and see if it improves.

    Good luck.

    algebraist Active Member

    I guess that's one vote for nutrient burn -- thanks. I have trouble believing it though -- they're in 7 gallon containers of Fox Farm Ocean Forest, cut significantly with perlite, and I fed no nutrients at all until they hit 2 months. Since then (about 3 1/2 weeks) I've fertilized more or less every other watering -- at first with "Botanicare Pure Blend Pro Bloom Soil Formula" (which is 1-4-5 NPK), and now with Big Bloom (which is 0-0.5-0.7, which seems reasonably mild to me).

    Other opinions?

    Bear420 Well-Known Member

    *Maybe Cal-Mag?

    *BUDS Well-Known Member

    Dont increase the feed, the tips are already brown.

    NaturalFarmer Well-Known Member

    That is the start of potassium burn from deficiency IMO. The burned tips are part of that deficiency. Are you giving calcium an mag to this plant? Too much of one of those will make K def or at least decrease the potassium ratio in your soil.
    Tangerine_ and Bear420 like this.

    algebraist Active Member

    I haven't been giving calcium or magnesium. I checked the pH yesterday and found that it was low -- 5.5 when I tested the runoff; 5.8 when I mixed some soil with water and tested that. That can't be good, and I'm going to work on bringing that up.
    OldMedUser and Bear420 like this.

    Bear420 Well-Known Member

    Cool sounds like you're on the track.
    get er back
    good luck keep on keeping

    NaturalFarmer Well-Known Member

    I would get topdress some dolomite lime for mag and calcium, as well as raising your pH as fast as possible.
    Lots of people seem to have issue with these mass produced bagged soils. People would be better off making their own by the sounds of it.

    Stipulus Active Member

    I was thinking cal-mg as well.
    Bear420 likes this.
    Dr. Who

    Dr. Who Well-Known Member

    The pH of run off is only the pH of the run off! NOT the pH of the soil!

    When you water soil. It drops in pH by about a whole point!
    As the soil dries back out. It will rise in pH, back to the soil's "normal" pH.
    All nutrients are best available at differing pH value's.
    This is natures way of making nutrients available at the best rates during that "swing".

    Always let your soils dry out pretty well between waterings!

    I would put your soils pH at around 6.5 and maybe a bit higher....

    Soil self pH's! If it was built properly, and has sufficient Ca in it. You don't attempt to change that....

    Simple ways to ensure proper soil pH in synthetic use - Simply pH all in-going liquids to 6.5 in veg and 6.7 in bloom...

    Use of large amounts of Phosphoric acid (main ingr. in most pH downs) will do damage to the roots in a progressive way..

    Lean towards a multi acid mix if you must use a down (after adding all nutrients and metering).

    MAD FARMER has a good mix for pH down. You use like half of what you would with other brands to get to the same point. With less phosphoric too!

    WHAT SOIL is it?
    What is the water's pH before you use it?
    What do you set it to before watering (after you add nutrients)?
    What are you feeding and how much at a time?

    It looks to be possibly several things. The answers to the questions will sort that out, and we can be specific in helping.
    travisw, OldMedUser and Tangerine_ like this.

    algebraist Active Member

    Truly appreciate your help.

    First, the soil is mostly FoxFarm Ocean Forest. To be precise: 67% FFOF, 27% perlite, 6% vermiculite.

    I had assumed my water would be pH 7.0 or thereabouts; this morning I set out to answer all your pH questions and my first reading showed my water at 8.09. I figured that couldn't be right, and it's been a couple of weeks since I calibrated my pH pen, so I recalibrated and checked again: this time I got 7.66. Which still seems high, but maybe possible. While checking the pH of my fertilizer solutions I checked my water twice more; I got 7.37 and 7.44. This is water from the tap; I've been testing it "lukewarm," but just by feel. I know temperature affects pH, but I wouldn't think that drastically, and in any case these (quart) samples were at least about the same.

    Anyway: Water: about 7.5, more or less.

    The "Botanicare Pure Blend Pro Bloom Soil Formula" (1-4-5), 1 Tbs in 1 gallon: 5.92. I have not been adjusting that at all, so that's what it's gotten (5 times over the past 4 weeks).

    The FoxFarm Big Bloom (0-0.5-0.7), 1/4 cup per gallon: 7.45. This I've fed twice over the past 4 weeks.

    I did no fertilization for the first two months, and then started the 12:12 light cycle and began fertilizing a few days later.

    Some thoughts. I have two plants (photos to follow), treated as much the same as I can. One is showing these necrotic spots on the leaves, the other is not. The pH of the runoff from these plants is not the same -- it might not be a good measure of the soil pH, but it does reflect something.

    Anyway, right now I am thinking that the pH is not the issue, and I am still guessing potassium deficiency. But I'm thinking that perhaps it's a matter of buildup of nutrient salts in the soil -- I've been watering with plain water between fertilizations, but I've been watering just until a little water comes out the bottom. I seem to recall a suggestion to let a significant amount of water come through, to clean up nutrient salt buildups -- which it seems can prevent nutrient uptake. No idea how this relates to potassium, or of that even is indeed my problem, but in any case today I watered more thoroughly than usual, letting a quart or two of water come out the bottoms (and painstakingly siphoning it away).

    Once again, thanks very much, @Dr. Who, for your help and suggestions.

    The problem plant:

    Her sister:

    The whole garden:


    algebraist Active Member

    P.S. Runoff from today's watering: 6.11 for the "problem" plant; 6.29 for her sister.

    And this is my pH pen, which I am suspicious of: [​IMG]
    OldMedUser likes this.

    algebraist Active Member

    ...and they're in 7 gallon cloth containers, that probably have closer to 6 gallons of soil in them.
    OldMedUser likes this.

    BuD_DizZLe Member

    What color are the stems? Green? purple/red? Strain?

    algebraist Active Member

    It's Northern Lights (from Sensi seeds). The stems of the plant itself are green, the stems of the leaves are a red/purple.
    OldMedUser likes this.

    Yodaweed Well-Known Member

    it kinda looks like cal/mg problem to me, but i could be wrong , usually rust spots mean cal i think, good luck

    OldMedUser Well-Known Member

    What you have there is the beginnings of toxic salts buildup. The high mineral content of your tap water has caused a buildup in your soil much like it builds up in a kettle. Add in the minerals already in a rich soil like you are using and it overloads the plants with salts. (The minerals in your water and the food your plants eat from the soil are all salts.)

    You need to flush out some of that buildup or it will only get worse. Check the ppm of your runoff if you have a ppm pen and it will be high. Most people don't check runoff properly. Soak the soil until it won't hold any more water like you do when watering and let it sit overnight. Then add a cup or so of water to get some runoff and check that to get a more exact reading of the pH and ppm in the rootball. It can take hours for the rootball to reach equilibrium after watering so the first runoff is totally out of whack to what the plant is living in most of the time.

    Tap water is a main cause of most indoor plant problems and getting a water report from your supplier is a good idea so you know exactly what you are working with. Most will email you a copy of their water analysis with a polite phone call. Tell them you want to raise fish or make beer if they ask tho it's none of their damn bizzness! :)

    Or do what I do and switch to RO water so you eliminate your water as a possible cause of problems.

    NaturalFarmer likes this.

    algebraist Active Member

    Thanks. I don't have a ppm meter, but can certainly get one. I use town water, and they publish their analysis, broken down by particular minerals. The sodium level is 62 ppm; nothing else comes close in terms of concentration (Sulfate: 2.2 ppm, Fluoride: 1.0 ppm are the next two). Does that sound like a lot?

    If I need to I can look into RO water, but damn how the costs have added up! I've easily spent over $1000 on this growing obsession -- first grow. Not going to save or make a penny either -- not selling, and haven't been smoking...

    algebraist Active Member

    Just ordered a TDS (ppm) meter; be here Saturday...

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