When to Reduce, Increase, or Stop Cal-mag

Discussion in 'Growing Organic Marijuana' started by the3rdman, Apr 17, 2023.

  1. #1 the3rdman, Apr 17, 2023
    Last edited: Apr 17, 2023
    I am in 6-week flower. I had been feeding 1/16 tsp cal-mag each time I water but in week 5 I had a calcium deficiency on Blue Dream. So, increased cal-mag to 1/8 tsp. Should I continue to feed at 1/8 tsp rate and when should I start decreasing or stopping cal-mag feed?

    I am growing three strains: Blue Dream, Kosher Kush, Skywalker OG. Blue Dream is a 9-10 week strain while Kosher Kush and Skywalker are 8-9 weeks.

    I am using Happy Frog soil, Dr. Earth Flower Girl 3-9-4, and NPK Industries Raw Calcium/Magnesium.
  2. Its probably not the answer you want to hear, but it depends.

    I use pretty hot soil so its often 4-6 weeks into a grow that I start seeing early stage Ca deficiency, which is when I tend to start adding it ( am currently using the bloom city version of cal-mag and tbh I don't know how the concentrations or feeding amounts may differ between your brand and mine.

    Anyway my stuff is 2-4ml per gallon, or 4-6ml per gallon in flower. As soon as I start seeing leaves look like the left most pic, I begin slow, 2-4ml per gallon 2 times per week. (each gallon I then split between up to 4 plants, so I'm feeding a good bit less than they recommend).

    Anyway, in flower I tend tho give each plant about 2ml of cal mag per week, usually with other nutes and always in at least 1 liter of water, although by mid to late flower my plants at least are fine sucking down 2-2.5 liters of just water every day.

    If things continue to get worse, I may double the cal mag amount but rarely go over 4-5ml per week in mid to late flower as if the plant is still struggling, there is likely something else that needs attention as well, and adding more nutes are just going to choke out the root zone.

    Lastly, once you are few weeks into flower, your basically racing against the plant's biological clock, so if you come out of stretch with deficiencies, sometimes the best you can do is fix them up so they won't get worse, coupled with the natural yellowing and fade of plants as they close in on harvest time.

    Best of luck mate and I hope you get it sorted!

    PS blue dream is a personal fav of mine, hopefully it will do you well also.
  3. Thanks for the reply. Can I reduce or stop the amount near the end of flower, like in week 7, 8, or 9 because the plant uses less? I don't want to have a calcium toxicity issue.
  4. If you are confident that you will cut the plants on or about a certain week, then its probably not going to make a bunch of difference either way.

    If its a new strain and you aren't sure on the grow time (breeder estimates of like 8-10 week flower time can be fairly inaccurate imo but thats at least partly because the wide variety of ways people grow)

    One of the main issues with cal mag (brand dependent) is that is also usually has a good bit of Nitrogen in it (typically because its made from calcium nitrate), for example the bloom city calmag I use is actually 2-0-0.

    My understanding is that once the plant is well into flower, there is less need for calcium in general (not to mention perlite can degrade and release some as well as iron or other oxides).

    the problem is the plant can only uptake so many nutrients at once, and prefers some to others based on life cycle stage or PH of the medium. If the plant wants potassium but your medium is flooded with nitrogen, calcium, or anything else, the plant will suffer, so in that respect less is usually more.

    Personally I run all nutes right up until about the final 7-10 days before I cut, and I may still feed some light stuff like earthworm castings during that week.

    Basically once I can see the trichs are about ready, I'll set a cut date, and switch to just tap water and castings for the final few days (I do not "flush" the plants though, just regular watering)
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  5. #5 the3rdman, Apr 17, 2023
    Last edited: Apr 17, 2023
    An internet search suggests growers stop cal-mag in week 4, 5, or 6. Some feed charts stop cal-mag in week 7. Some growers say the calcium deficiency pops up when following that advice. So, it may be better to reduce rather than eliminate.

    I may start reducing the amount by half in week 7 or 8 and see how that works out and, eliminate when I see mostly cloudy trichomes.

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