Although calcium isn’t classed as one of the main macronutrients (nitrogen, phosphorus, and potassium make up that list) that all plants just have to have, it is still incredibly important for healthy growth and development, and having cannabis deficiencies such as this can be detrimental. That is especially if you are looking to maximize your harvest potential. Poorly managed calcium levels in a cannabis crop can lead to wilting and yellowing of leaves, a loss of overall vigor, and stunted growth – and in the worst cases, a calcium deficiency can even spell the end of your precious cannabis crop.
Let’s take a good, hard look at everything there is to know about calcium and cannabis and see if we can lay out simple answers to all of those burning questions, as well as a couple more.
Calcium is a mineral, and it is an essential element of all plant cell mass (and is imperative for all forms of life, for that matter).
It is a prerequisite to many processes that are crucial for healthy cannabis development throughout the entire lifecycle of the plant, including:
Calcium is mostly absorbed by the roots of the plant and is classed as an immobile nutrient. This means that once it has been absorbed, the mineral will make its way to the leaves and that is where it will stay. This is important to understand when considering all nutrient deficiencies in weed crops, as certain nutrients are mobile, while others (like calcium) are not. This means that if a cannabis plant were to suffer from a calcium deficiency, the symptoms would show up in the leaves of newer growth first, before working their way down the plant to older fan leaves.
Calcium deficiency signs and symptoms come in a mixed bag of indicators ranging from pretty mild to surprisingly severe. They can present differently on different strains, even when grown in the same crop with the exact same substrate with a shared nutrient regime.
Cannabis calcium deficiency issues are much more commonly seen in coco-coir or hydroponic crops when compared to plants grown in soil, but that doesn’t mean they can’t or won’t strike plants grown in organic settings.
The most noticeable symptoms of a calcium deficiency are:
As with most cannabis nutrient deficiency issues, there are a few key reasons why calcium levels may drop to dangerously low levels – or even just become unavailable for the roots to uptake. This is actually the more common cause, as pH issues and nutrient lockout are more common than a lack of calcium in the substrate media.
The first and most common cause is incorrect buffering or pH levels in your grow medium. If you are growing hydroponically or in coco-coir, it is crucial that the pH is kept at the appropriate level (or within the correct range) for the plant to have access to all available nutrients, and this includes calcium. For hydro and coco, this range is usually quoted as 5.5 to 6.5, but for the best results with calcium, you will actually want to be within 6.2 to 6.5 as this is the zone where calcium is most easily absorbed by cannabis. It’s a good idea to vary the pH of your nutrient solution within this 5.5 to 6.5 range between feeds, as this can help to make all the available nutrients that much more accessible.
This is less important for soil-grown crops, especially when grown directly in the ground outdoors. But it is still a good idea to regulate the pH of your water and any nutrient solution you may be supplying the crop. For soil crops, as long as you keep everything between 6.0 to 7.0 pH then you shouldn’t have any pH issues.
If you don’t already have a digital pH meter, then stop whatever you are doing (once you have finished reading this post, of course) and head to your local Walmart, Hydroponic supplier, or online and grab one. They’re cheap, easy to calibrate, and accurate. If you want to grab one online, check out our favorite options right here –
Excessive use of certain organic compounds or synthetic nutrient additives can also interfere with your pH balance and make calcium unavailable to plants. On top of this, some chemical compounds may actually react with calcium and form insoluble compounds that can’t be absorbed by the roots. This is a much less common cause though, with calcium toxicity almost always being the result of incorrect pH.
Another common cause of calcium deficiency is when there are other nutrient deficiencies present. Calcium issues rarely present all on their lonesome. Most often they are accompanied by a magnesium deficiency, which is why you will often see calcium and magnesium supplements combined into an easy-to-dose CalMag additive. The reason for this is that when one of these two essential elements drops below acceptable levels, it can negatively affect the uptake of the other. Again, this is rarely seen in soil-based cultivation (especially if your soil mix has been boosted with dolomite lime, which most cannabis-specific soil blends are), but if you are growing with a hydro or coco setup then you should definitely have a CalMag additive incorporated into your feeding schedule.
Ok, so you’re at the point where you are almost 100% certain that a calcium deficiency is the issue that you are dealing with… What next?
First thing first, let’s get that pH regulated. Grab your digital pH meter, because it is time to run a full flush. Now, this isn’t the same as your pre-harvest flush, where you remove as much of the nutrients left in the substrate as possible and let your plants use up what is left in their leaves.
The point here isn’t to remove all of the nutrients but to get the pH of the root zone back to an optimal range. To do this effectively, you simply take a whole heap of fresh water that has been adjusted to the correct range and run it through the soil, coco-coir, or through a hydroponic setup (check out the link below for a more detailed explanation for flushing a hydro system) to clear everything out and create a good baseline.
To flush soil or coco-coir, you will want to water the crop until you see at least 30% of the water run out of the pots. Wait about 30 minutes or so and then hit the plants with the same amount of freshly regulated water again, and that’s it – flush complete.
For hydroponic crops, the process can be a bit more involved, so it’s important to learn everything you need to know about flushing a hydroponic system.
So, once the flush has been run, you are going to want to add some extra calcium into the growing substrate. The products that you use, and the way that the supplementation is done is dependent on the way you are growing. One thing to keep in mind is that it is pretty difficult to over-supplement with calcium. Unlike most of the other nutrients, overfeeding calcium (which would result in a calcium toxicity issue) isn’t something you need to really worry too much about. But just remember that adding way too much calcium can cause lockout problems for other nutrients.
Ok, adding extra calcium into both hydro and coco grows is super simple. All you have to do is grab a bottle of CalMag supplement (remember that we mentioned earlier that calcium and magnesium issues usually go hand in hand for coco and hydro). There are so many options on the market, but we usually suggest sticking with one of the bigger name brands such as General Hydroponics, Canna, House & Garden, etc…
As long as you follow the recommended dosage levels on the side of the bottle, you should see a nice bounceback in a week or so.
Calcium issues are pretty rarely seen in organic soil gardens, but if you do unfortunately run into one then you will want to head out and grab a big old bag of Dolomite Lime.
Dolomite Lime is a common soil additive, and all good soil blends should have been pre-buffered with the stuff. It offers not only high levels of calcium but also magnesium. It also has a natural pH level of 7.0, meaning that the addition will help keep the root zone in the correct range. If you are mixing up your own super soil blend then make sure you add around 40 grams per gallon of soil.
To use it for calcium deficiency treatment, all you have to do is sprinkle a fine layer over the top layer of soil before watering (after the plants have been flushed). Don’t worry too much about how much to add, but just don’t go crazy either.
Can’t get your hands on Dolomite Lime?
No big issue. If you have some good quality compost laying around, simply switch the DOlomite Lime out for that! This trick can work for most nutrient deficiency issues for organic, soil grown crops.
Cannabis crops can suffer from a huge range of issues, and telling these problems apart from one another is maybe the most annoyingly tricky thing for cultivators. Have a quick browse of any growing forum and you will quickly notice the veritable ocean of posts asking for help with identifying the cause of a problem and what the correct remedy might be.
The two problems that we have mentioned, Calcium Deficiency vs Leaf Septoria, present in extremely similar ways and are often confused with each other.
Cannabis Leaf Septoria is a fungal disease that is one of the more commonly seen issues in cannabis crops, especially when the relative humidity in the growing area is above the correct level. Most often hitting crops around the time that flower formation begins, it most often attacks the fan leaves while the buds are left unaffected.
Well, the main symptom of Leaf Septoria is very similar to one of the main signs of a calcium issue – rust spots on the leaves. Yellow/brown spots will form on fan leaves, usually with yellow edges and sometimes with a greyish center.
There are a few obvious differences, actually.
First up, Leaf Septoria will cause multiple spots to form very quickly (sometimes overnight) whereas a calcium deficiency will progress slower and may only affect one leaf at a time. It will also attack the older fan leaves first, whereas calcium deficiency issues will usually present on the newer growth. Leaf Septoria will usually only show rust spots and no other signs, so if you are dealing with a plant that is displaying multiple symptoms then you are likely looking at a nutrient deficiency.
The best way to treat Leaf Septoria is simply to reduce the humidity levels in your growing area and make sure that your plant gets plenty of air circulation. You can also look into using and anti-fungal foliar spray, but since this usually hits plants as the flowers start to form, you will want to be very careful with any foliar application.
While there are some pretty great synthetic options available, we much prefer starting out with an organic fungicide first and only going for the harsher manmade stuff if and when you have to.
On top of controlling the humidity level (which cannot always be done, especially if you are growing in a very humid outdoor area), you will want to also
So there we go, pretty much everything there is to know about calcium deficiencies in cannabis plants. And while we have tried our damndest to be as comprehensive as possible, there will always be some unanswered queries, especially for the newer growers out there.
Let’s quickly run through the more commonly asked questions, and lay out some simple and easy-to-understand answers…
Yes indeed! Unlike some of the more serious nutrient deficiencies that can plague cannabis crops, a lower level of calcium rarely proves deadly. That’s not to say that the signs should be ignored, as a calcium deficiency can heavily negatively the final yield size and the potency of the buds, but with the right supplementation it is usually possible to make a full recovery.
Look, it’s not going to be a super quick fix, no matter if you run CalMag or Dolomite Lime (or even both, although that be just a little bit of overkill). Don’t expect to see full results within the first few days, although some strains may bounce back pretty quickly. For the most part, you should see a return to full vigor within 3 or so weeks, all things being equal.
Cause? No, but the symptoms can be a little different for plants grown under LED panels, even if the light is top-quality. Look for signs of burning on the lower leaves, as well as a purple tinge to leaf yellowing. Remember that plants grown under LED lights need slightly different nutrient levels than those grown under HID rigs. Most growers agree that the nutrient level can be dropped by around 25% for LED-grown plants, but again this is very much strain dependent.
In general terms, no. All cannabis strains can suffer from calcium deficiency, although some are more prone to it than others. Fast-growing Sativas are usually the ones most prone to calcium issues, but again this is very much strain dependent (with the environmental conditions also playing a pretty big role). If you do find that your strain of choice is particularly calcium sensitive, then it might be worth looking into running a CalMag supplement from the get-go.
No, Epsom salts are actually a form of magnesium sulfate. This myth ahs been hanging around for years, and it’s time we put it out to pasture once and for all. If you want to get extra calcium into the mix, then look for a CalMag supplement or use a soluble lime – which is of course the same as dolomite lime. Epsom salts can be great for boosting magnesium levels (which are also very important for cannabis plants), but they won’t do a thing when it comes to calcium.
Calcium deficiency in cannabis crops isn’t a death sentence, although it can certainly cause some pretty serious issues if left untreated. Keep an eye on any signs of yellowing leaves, A loss in overall vigor, weak branches and stems, stunted flower production, or purple patches of discoloration when grown under LED panels. CalMag is the usual go-to solution when it comes to addressing calcium deficiencies, although dolomite lime is a great option if you want to keep things totally organic.
Have you had any issues with calcium levels in your grow? Did the steps above help? Let us know down below in the comments!
Sam N. is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success. About this Author