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Coco growing, hard water & CalMag

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    Coco growing, hard water & CalMag

    I've been growing for about 3 years now using Fox Farm Happy Frog. I'm going to now try growing in Coco. GWE website is where I get all my advice. I'm going to use Canna Coco for my nutrients. Their website says you do not need CalMag if you have hard water (see screenshot). My water is hard water. Will the bottle of CalMag be something that is just not needed? I always pH my water so that's not an issue. Any advice is appreciated.
    Mother Earth Coco/Perlite 70/30
    Canna Coco Or
    Happy Frog soil
    Fox Farms nutrients trio
    Smart pots
    Vivosun tent, LED lite & fan

    Semper Fi brothers and sisters!

    Do u know exactly how hard your water is? And, it's not going through a softening device prior to where u get it for the plants, right?
    It's also my understanding that it is hard to give to much Ca and Mg to a cannabis plant. Though I could be wrong.
    Let's see what others have to say.
    Back to playin in the dirt!
    Currently growing 8 Scarlet Grape. Check it out here:
    I do not currently partake. I grow for fun. Someday!


    • jamesphoto
      jamesphoto commented
      Editing a comment
      No soft water. Looked on the towns website and no water report there. Will keep looking. Thanks

    This is an excellent write up from but before you read on, whether you can add too much cal-mag supplement is highly dependent on whether or not it also includes other nutrients and chemical such as IRON as many do.

    Cal-Mag for Plants – What Is It and Do You Need It?

    Cal-Mag stands for Calcium and Magnesium, two of the macronutrients that are rarely discussed when we talk about plant fertilizer, but they are critical for plants. You can buy Cal-Mag fertilizer to add to your regular fertilizer, or you can buy fertilizer that contains calcium and magnesium. Which option is better and do you need either one?

    If calcium and magnesium are so important to plants why are they not included in all fertilizer? It is more complicated than you think.

    The Calcium Story

    Calcium (Ca) is a mineral that is important for plant growth where it plays a major role in developing cell walls. It’s also critical for activating enzymes that carry out all kinds of functions and it plays an important role in fruit development. Plants use it in fairly large amounts making it a macronutrient. A lack of calcium shows up as localized rot and stunted growth at the tips of plants. Leaf tips, buds and fruit don’t develop properly, leaves are dark green and stems are weak. A common symptom of a deficiency is the black end on tomatoes, called blossom end rot.

    Plant nutrients can be immobile meaning that the nutrient can move up the plant through the xylem, but once in the leaves it can’t move back down or to other parts of the plant.

    A calcium deficiency in plants can happen because there is not enough calcium in soil, or because there is not enough water moving up the plant, which is usually the cause of blossom end rot in tomatoes. Except for sandy soil, most soil has enough calcium because it sticks to clay.

    The Magnesium Story

    Magnesium (Mg) is also an important mineral for plants. This macronutrient is the central atom in chlorophyll, it’s used to activate enzymes, it helps with the transportation of other nutrients and it stabilizes nucleic acids.

    This nutrient is mobile, so if a plant has low levels it will move magnesium away from older leaves to where it’s needed. This results in a yellowing of older leaves.

    Limestone rocks decompose in soil and slowly release magnesium. As a result most soil, except for sandy soil, has enough magnesium.

    The Importance of The Calcium/Magnesium Ratio

    Studies done in the 1940s showed that plants grow best with a particular calcium to magnesium ratio, however more recent research has shown that this is not true. The calcium to magnesium ratio is not important for plant growth, but this myth is still very prominent in agriculture and gardening circles.

    It is much better to think of these as two independent nutrients and that plants need different amounts of each.

    Commercial Fertilizer Lacks Cal-Mag

    Why does most fertilizer not include calcium and magnesium?

    As mentioned above, most soil contains enough of both nutrients so there is no reason to add more when you fertilize. Sandy soil or very acidic soil may lack these nutrients in which case adding some extra can be beneficial. People with acidic soil tend to lime it to raise the pH and if they use dolomitic lime they are adding calcium and magnesium as part of the liming process and don’t need to add more when they fertilize.

    A second reason for leaving it out of fertilizer is that a lot of tap water is hard which means it contains calcium and/or magnesium.

    Some products do contain Cal-Mag. Scotts 15-5-15 Cal-Mag fertilizer contains 5% calcium and 2% magnesium.

    Calcium and Magnesium for Plants

    How much calcium and magnesium should be used for plants growing in a soilless media? The above Scotts fertilizer suggests around 66 ppm Ca and 26 ppm Mg. The University of Massachusetts suggests an alkalinity range of 37 to 105 ppm, calcium in the range of 40 – 100 ppm, and magnesium in the range of 30 – 50 ppm. Suggestions for hydroponic solutions are 80-140 ppm Ca and 30-70 ppm Mg.

    A recommendation for agricultural loam and clay soil is 200 ppm Ca and 50 ppm Mg.

    Calcium and Magnesium in Tap Water

    If you use city water, check with the water department and they should be able to provide three values for you: alkalinity, calcium levels and magnesium levels. If you are on a well, but near a town that uses well water, you may have similar water as the town.

    The alkalinity value will give you a total hardness number that tells you how suitable the water is for plants. More on that in What is Alkalinity – It May Not Be What You Think.

    The calcium and magnesium values will tell you if you need to add Cal-Mag to fertilizer. My tap water has 114 ppm Ca and 34 ppm Mg. The calcium is a bit high and the magnesium is on the low side for a soilless mix, but I don’t need to use a Cal-Mag fertilizer and I should not use a fertilizer that includes calcium or magnesium, at least not for anything growing in a soilless mix.

    Homemade Cal-Mag

    There are numerous DIY recipes online for Cal-Mag. Most are some combination of eggshells and Epsom salts. The Epsom salts will add a suitable form of magnesium. Since most potting media has a pH between 6 and 7, the eggshells will not decompose and therefore won’t add much calcium. Your tap water probably has more. Therefore these recipes do not make a good Cal-Mag mixture.

    You can also make your own Cal-Mag using Epson salts and calcium nitrate, which is what commercial products use, and this will produce a suitable solution.

    Do You Need to Add Cal-Mag to Garden Soil?

    If the soil has a pH above 7.0, it probably contains enough calcium and magnesium and you should not add more. If the soil is acidic and you see symptoms of either calcium deficiency or magnesium deficiency it is a good idea to have the soil tested and then follow directions. Alternatively, you can lime with dolomitic lime and improve the pH and add both calcium and magnesium.

    If you irrigate a lot with hard water, you may not need to add any more.

    Do You Need to Add Cal-Mag to Potting Soil?

    The discussion in this section applies to any container that holds a soilless mix, including houseplants, outdoor containers and even raised beds.

    Soilless mixes are usually based on peat moss, coir or composted wood. None of these provide high levels of calcium or magnesium which means that they need to be added along with the fertilizer.

    If you are using high alkalinity water, that may provide all the Cal-Mag you need. In that case select a fertilizer that does not contain calcium or magnesium.

    If you are using rain water, RO water, or distilled water, you need to add Cal-Mag. This can be done using a separate product, or the calcium and magnesium could be part of your regular fertilizer.

    Buffering Coir

    As I was researching this material I came across the term “buffering coir”. The term is not used by reliable sources of information but it does seem to be popular with marijuana growers. The process itself does make sense. *emphasis added

    Coir is made from coconut husk and this raw material contains a lot of sodium (from salt water) and potassium. The coir has a fairly high CEC, which means that it holds on to cations such as sodium, potassium, calcium, magnesium, etc. Washing coir in fresh water will wash off a lot of surface salt (i.e. sodium), but it won’t wash out the sodium that is attached to the coir due to the CEC.

    When you use this material for plants and start to fertilize, the calcium and magnesium in the fertilizer, or in hard tap water, will cause the sodium and potassium to leave the coir and be replaced by calcium and magnesium. This process is exactly the same as the one that takes place in a water softener. The net effect of this is that plant roots are exposed to high levels of sodium and potassium, and they get no calcium or magnesium. That is not good for plants.

    *Addition by me - why is this important? Because The sodium in the soil or growing medium of your plant is what controls how much water your plant can absorb.

    To prevent this problem you can pretreat the coir with Cal-Mag before using it. To do this, put the coir in a pail with fertilizer water that contains calcium and magnesium or hard tap water or a commercial Cal-Mag product. The calcium and magnesium will stick to the coir, and the excess sodium and potassium will be released into the water, which is then discarded.

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    ​Sorry about borrowing Robert's article word for word but he says it much better with much less words than I could and I think the information is something that all growers should know but was afraid most wouldn't click the link and read it if given the option.
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    Last edited by BU2B; 01-16-2023, 07:21 PM.


    • jamesphoto
      jamesphoto commented
      Editing a comment
      Excellent article. Thank you!

    I run the Canna line presently on .3 Base E/c city water and do not use a calmag on the regular.

    I hydrated new coco with it as the binding of calmag on coco is extreme so if it is setup right in the beginning and the water source includes calmag then unless I start having problems I do not use it.

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    You're killing me Smalls!


    • jamesphoto
      jamesphoto commented
      Editing a comment
      So you are using it in the very beginning getting your coco ready for plants? That's what I got out of the above article. Thanks for that good advice!

    No soft water. Looking on the city website I don't see any water report. I will reach out to them and ask though. Thanks!
    Mother Earth Coco/Perlite 70/30
    Canna Coco Or
    Happy Frog soil
    Fox Farms nutrients trio
    Smart pots
    Vivosun tent, LED lite & fan

    Semper Fi brothers and sisters!


    • Blowdout2269
      Blowdout2269 commented
      Editing a comment
      You can get home tests also.
      You could use this to compare to the city report.

    My well water is hard at about 450-500 ppm at ~90/10 Ca/Mg and a pinch of iron. So after going through the rainwater and hard water only grows with issues haunting me I went with a 50/50 mix last time. Worked very well and I did not add too much more Ca. I did continue to add some Mg. Next grow is the same but adding some Langbeinite to each mix, it's 2K/Mg sulfates. I'm short on the K in my nutes I use I suspect.


      I have not used there products but was recently considering, especially there coco, because it sounds like it is processed with higher quality, however at the end of the day, it is stii coco coir and will / probably require cal mag additive. I would have a bottle of there cal mag on hand just in case.


      • ZigZag
        ZigZag commented
        Editing a comment
        I've had similar experiences growing in coco, which I do to reduce insect and other pesky problems indoors. Never had any issues with pests or mold or anything else except PH and nutrient imbalances_ pretty typical in coco, especially if using new nutrients .
        I also add a good dose of Cal/Mag when i prep the coco before planting so I don't need to add it for a bit once the new seedlings are in. My experience has been that when using low concentrations of nutrients with seedlings that I can get a cal/mag deficiency that needs correction early on. This helps prevent that.
        I also have started using a slightly higher PH during the early veg stage_ about 6-6.5, it seems to help prevent Cal/Mag deficiencies since those are more easily accessible to plants between PH 6-7 or so. Then once the coco medium is well established I back the PH off to 5.8-6.2 for the remainder.
        I also flush with plain water at PH 6.0 every three weeks to clean out any accumulated salts.
        Our water runs about 7.8 PH out of the tap so it needs adjustments.
        Happy growing!
        ps;_ I prep the coco by washing it 3x in water at PH 6.0. Then I add a fairly strong Cal/Mag flush and then a nutrient flush at 1/2 strength. Seems to work pretty well.

      I am really glad to finally see this conversation! When I switched to coco, I added cal/mag as directed on this grow site for coco grows and immediately had issues with bronze spots taking over plants. This issued plagued me for several grows until I found a photo of cal/mag toxicity and it looked like my plants. I finally put it together that my municipal water was hard and I didn't need to add cal/mag. Haven't had any issues since I stopped adding cal/mag. I do use it when I rehydrate coco brick but no more after that. Good luck!
      DIY grow space 2.5' X 3.5' X 6.5'
      315 LEC
      6" Vortex exhaust fan w/speed control
      6" Carbon filter
      3- 6" fans
      temp and humidity gauge
      5 gal smart pots w/ FFOF soil
      FF trio nutrients


      • 3Berries
        3Berries commented
        Editing a comment
        My epiphany this year after many was my potassium issues were the nutes I was using being way unbalanced. And for the last two years I've had what I believe to be the cure on hand. I will find out in about 3 months.

      • changeofpace2014
        changeofpace2014 commented
        Editing a comment
        I think every feed chart is for RO water, with EC 0.00 and that is why when following the feed charts with tap water, it requires a "lot of dilution" at the end of mixing for the target EC.

      I’ve only ever grown in coco using canna products. I always add 1ml of cal mag per litre or water with no issues. I tend to stop after a few weeks of flower. Whether it’s right or not it seems to work for me and my set up


      • jamesphoto
        jamesphoto commented
        Editing a comment
        Thanks, your experience is helpful. I've decided to use the Canna CalMag. Same ratio. If I begin to see crazy shit happening I'll make a change. I'm excited. I'll have some new plants in the pots in a few weeks.

      • ZigZag
        ZigZag commented
        Editing a comment
        Yeah, I prep my coco with cal/mag and add 2 ml/quart too, but on alternate feeding days. Seems to work well though right now I have 2 out of 8 that are showing slight signs of magnesium deficiency but I can’t nail down why- everything’s good: temp, humidity, ph in and out, watering and feeding schedule, etc. The info with the seeds said Zkittlez needs lots of nutrients. I’ve been giving them full strength so I guess I’ll try boosting that up for a few feedings and see if that helps.

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