Debunking the Cal/Mag Myth in Cannabis Cultivation
Most cannabis growers have been heavily influenced by the legacy growers’ mythology about calcium and magnesium (casually called cal/mag) in cannabis. A quick perusal of any hydroponics store will show that just about every nutrient line includes a separate cal/mag additive. If you have ever read cultivation forums online, you’ve surely heard reports and recommendations about how cal/mag is a cure-all for yellowing leaves or generally low plant vigor.
As a rookie grower back in the 1990s and early 2000s, I fell for the hype just like everyone else. My setup included cal/mag in the mother plant fertilizer blend, even though I was using tap water and a base formula that already contained both minerals. I just took it to be part of a basic nutrient regimen to bump up the cal/mag. There was really no questioning it, although I can’t specifically claim I ever saw it make a positive difference in any plants that I grew. I knew some other growers who swore by it, and I just didn’t have enough nutrient education or scientific evidence to challenge that conventional wisdom.
It’s important to bear in mind that no agricultural testing labs were taking cannabis samples prior to 2010, so where was the science that showed the connection between yellow leaves and a cal/mag deficiency? The only place it could have come from was either anecdotal reports from old-school cannabis growers or agricultural research on other crops. The notion that calcium and magnesium should be supplied in certain ratios dates back to research by Loew and May in 1901, in their research paper entitled, “The Relation of Lime and Magnesia to Plant Growth.” Their idea came from analysis of groundwater and native soils, showing some common tendencies between Ca and Mg, where Ca dominated Mg in a ratio anywhere from between 2:1 to 8:1. If there’s a consistent relationship between calcium and magnesium in nature, then maybe that’s what plants have evolved to prefer, they reasoned. This was never a clear-cut assertion, however. Loew and May knew at the time that the wide variation found in native soil cal/mag ratios created contradictions in their theory, as well as widely varying total quantities of Ca and Mg found in different soil environments, and finally, the varying responses to cal/mag observed in specific crops.
Many articles and research studies have since debunked the notion of ideal cal/mag ratios. One example is this paper by E.E. Schulte and K.A. Kelling of the University of Wisconsin School of Agronomy entitled “Soil Calcium to Magnesium Ratios — Should You be Concerned?” Schulte and Kelling concluded that it works better to simply provide Ca and Mg in sufficient (non-excessive, non-deficient) quantities, in the proper pH range, rather than prioritizing how those minerals relate to each other quantitatively.
The belief in the importance of cal/mag ratios persists among many in the cannabis industry today, so let me share my own personal journey towards discovering not only how excess calcium poses a threat to flowering plants, but also how magnesium is in its own world, entirely independent of calcium in terms of plant response and requirements.
A Personal Cal/Mag Journey
My personal journey with determining cannabis mineral quantities began in 2015, when I first gained access to a lab that would take hemp/cannabis leaf samples and report back the quantities of each essential and secondary mineral. I bought all the basic fertilizer salts (calcium nitrate, magnesium sulfate, potassium nitrate, etc.) and followed a strawberry recipe to set out to deduce the particular mineral needs of cannabis at each stage of growth. I experimented with stonewool, peat, coco fiber, DWC, and aeroponics. All these different media needed to be tested because certain minerals (cations, in particular) can be held in reserve in any soil-like grow media, which requires fortifying the formula to maintain the intended mineral levels in the plants.
I have always grown a wide range of cultivars. This is to ensure my formulas will accommodate the largest possible swath of genetic variations. I have observed that good formulas work well for a large majority of cultivars. Conversely, it’s rarely worth altering a formula to accommodate a finicky cultivar or a recessive genetic trait. This serves as some potentially useful background for the cal/mag findings I’m about to share.
I should offer a couple of caveats before going further. I am a legacy grower turned cultivation consultant and nutrient geek. I have no formal scientific training, but I can run a well-controlled experiment and I certainly know what optimal cannabis plant development looks like. So, take this information for what it’s worth. I’m sure some academic researchers will have their issues with the informal nature of my process, but I feel confident my work is valuable to growers seeking to improve their outcomes.
The second caveat is about my physical setting. I only grow indoor cannabis, with temperature and humidity settings that never stray from the ranges of 65-85°F and 45-65 percent RH and light levels that never exceed 1,500 PPFD. For those growing in more extreme conditions, calcium uptake patterns can be affected, thus I can’t contribute confidently to the calcium conversation when crops are in outdoor or greenhouse settings.
Designing and Testing a Veg Formula
As I set out to design my first veg formula, I remained only mildly cognizant of cal/mag ratio as I proceeded to try different recipes. I didn’t let it guide me. I was guided primarily by three key goals:
- Finding a formula that remained pH-stable after being applied to plants.
- Keeping all essential minerals within a certain range in the leaf tissue analysis lab results, such that flushing would rarely be necessary.
- Settling on a formula that empirically resulted in healthy-looking, fast-growing plants.
The lab I used for leaf tissue analysis reported “Cannabis Average Values” of each mineral, based on collected data from its clients, year over year. These values didn’t tell me what ranges were ideal for cannabis minerals, but, rather, told me the commonly found quantities in all previously submitted samples. It did not distinguish between stages of growth. It was not an ideal data set, but it was the best option I had at the time, so I used it as a point of departure to determine my own ideal sufficiency ranges.
Anyone who has experimented with raw fertilizer salts knows minerals tend to come in pairs, defined by a chemical and electrical bond. Calcium and magnesium are entirely independent of each other in this context. The most common calcium source is calcium nitrate, because it’s pH-neutral, affordable, easily soluble, and contributes about as much nitrogen to the formula as it does calcium. The most common magnesium source is magnesium sulfate. It has the similar benefits as calcium nitrate, all in all, but with sulfate as an adjunct ingredient. Since nitrate and sulfate levels both need to be balanced in a formula as much as calcium and magnesium do, a fertilizer designer must take all four minerals into account (not to mention the other ingredients) when finalizing a formula. Fortunately, plants can handle high amounts of all four of these minerals in the veg stage. It’s not until about day 30 of flower that the situation gets more sensitive.
Once I had settled on a basic veg formula (no small task) that showed reasonably consistent leaf tissue levels of each mineral over the course of time, the next step was to apply it to flowering plants and see what happens in the leaf tissue analysis as the plants develop. This is the process by which I discovered something important about cannabis and its calcium needs.
I had already noticed that leaf tissue levels of calcium in veg could vary anywhere from 20,000ppm (two percent) to 60,000ppm (six percent) without any noticeable change in the health of the plants. Magnesium could vary between 0.3 percent and 2.5 percent. These are huge sufficiency ranges. Maybe this is why most cannabis growers commonly think adding cal/mag all the time is a great idea; if it helps some of the time, maybe it will help all of the time?
But what many growers have been missing, presumably due to the industry still being in the early days of leaf tissue analysis, is that by Day 30 of the flower cycle, everything changes in terms of mineral uptake patterns.
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Calcium: The Cannabis Yield Killer?
Experienced growers know Day 30 is right around the time when plants stop getting taller and begin to focus on flower formation. It’s a big shift in the morphology of the plant and mineral behaviors change accordingly. I’m here to contend this is the point in the cycle when pumping cal/mag into the plants becomes a fool’s errand. Leaf tissue analysis will repeatedly show that the same levels of calcium and magnesium that yielded leaf tissue levels at a steady four percent of Ca and 0.8 percent of Mg during veg will end up yielding around nine percent and 2.5 percent at harvest time if left unadjusted.
Several nutrient geeks out there might be thinking at this point, “But nine percent and 2.5 percent isn’t too much. The plants are fine with it.” I had several colleagues tell me exactly that when I began to sound the alarm about calcium excess a few years ago. Besides the lack of historical data for cannabis mineral uptake, maybe growers and nutrient designers believed high cal/mag in late flower was okay because they’d seen some cultivars perform well with it.
It is true that the problems caused by high calcium are not noticeable in all cultivars. It occurred to me that maybe the most user-friendly cultivars out there, such as Blue Dream or Gorilla Glue, could have become so legendary, in part, because they aren’t sensitive to high calcium. If most growers overdose plants on calcium, then the least sensitive cultivars would surely become industry favorites, right?
For 50-plus percent of all cultivars, however, high calcium after Day 30 is a yield killer. I’m throwing down the gauntlet on this one. And I’ll go one step further with my assertions — that high calcium in dried flower material is a leading cause of bad smokeability (harshness) in the final product. This second claim of mine is complicated by the fact other minerals excessively present at harvest time can also cause smokeability problems, but I believe calcium is the primary cause of this problem because of how commonly it ends up very high in plants at harvest time relative to other minerals, based on what I’ve seen in hundreds of leaf tissue analysis results over the years.
Here are some images of plants that tested negative for bugs and pathogens, and tested high for calcium and magnesium while testing moderate for levels of the other essential minerals. In other words, calcium and magnesium were the only known inputs or environmental factors that were out of range. In the plant shown here, after eight weeks of flowering, the leaf margins are yellow and getting crunchy, the leaves are sagging downward, and the flower clusters aren’t swelling towards a mature, hardened state of completion.
Yellow leaf margins are a key indicator of high calcium levels.
In this second image (below), the flower clusters developed more fully, but the yellow leaf margins and chlorotic tissue in the background indicate that some quality and yield were likely lost due to excess calcium.
Yellow leaf margins and chlorotic tissue in ripe flowers indicate that quality and yield were affected due to excess calcium levels.
Common Sources of Calcium
In the Water — All tap water has some calcium and some magnesium in it, and it’s often at a ratio around 5:1. This is because the groundwater that supplies reservoirs picks up minerals from the land as it flows, and land releases calcium and magnesium in about that ratio, with some variations depending on geographic region. The concentration of the cal/mag in tap water can vary from low levels that would not meet minimum plant nutrition levels to very high levels that could damage yields.
I prefer not to use tap water for plant irrigation unless it is very low EC. Otherwise, it adds variables to the equation that are inconsistent and difficult to compensate for. Certainly, if you are using tap water with high Ca, you should ensure that any additional Ca in your media and/or nutrient formula doesn’t push the total beyond the acceptable range, especially after the first month of flowering.
In the Nutrient Formula — Almost all cannabis nutrient formulas contain calcium and magnesium. These formulas are often designed to work hydroponically, which means they must contain every essential nutrient cannabis will need to meet its genetic potential. The main reason, in fact, that liquid nutrients come in multi-part formulas, is because calcium must be separated from most of the other minerals when in concentrated form. If it were combined with the other minerals, it would bind with them and drop out of solution, causing a layer of off-white powder to settle at the bottom of the container. So, you know you’ve got calcium in your formula if it’s a multi-part liquid formula. Of course, you can also look at the Guaranteed Analysis on the label, being aware that labels aren’t always super accurate. Batch variation in commercial nutrients is a real thing, with some brands being bigger offenders than others.
Reputable nutrient companies must make a judgment call over how to handle calcium and magnesium quantities in their formulas because they don’t know if you are using tap water which contains calcium, or if your soil contains it, or if you are also supplementing with it. For the most part, I haven’t seen much concern or awareness over ensuring properly controlled Ca levels throughout the life cycle of the cannabis plant. It’s as if the industry as a whole is still blindly assuming there’s no upper limit.
In the Soil/Media — Whether you are buying an amended organic soil that’s designed for cannabis, or you are buying bulk peat or coco with the intention of using hydroponic nutrients with it, you are most likely getting quite a bit of calcium in your new bags of media. Soil companies use it as one of their most common additives because it brings up and stabilizes the pH of the media. If you’ve ever heard of “agricultural liming,” you’re aware of how calcium carbonate and magnesium carbonate (the ingredients contained in dolomitic lime) have been used as a critical tool in balancing out the acidity caused in agricultural soil from the use of modern mineral-based nutrients. Many farmers will lime their fields on a yearly basis, and most crops do not react negatively to the high quantities of calcium and magnesium that come along with the alkalizing effect.
What About Magnesium?
In my efforts to cut back on calcium, compensating for the loss in nitrogen (because calcium nitrate contains both) was a higher priority for me than adjusting magnesium, so my initial reformulations reduced calcium but left magnesium at levels 5-6x higher than necessary. At this point, any remaining vestiges of consideration for cal/mag ratios was officially out the window. What I found was that leaf tissue levels of magnesium could get as high as 2.5 percent and flower quality and yields seemed unaffected. It’s not that I want high Mg but having luxury levels has not appeared to negatively affect quality and yield the way high calcium has. I’ll admit, more research is warranted on the upper limits of Mg in cannabis. I haven’t found the limit.
Because I’m a perfectionist about balancing mineral uptake, though, I experimented to determine how low I could go on sulfur in late flower, because if sulfur could go low, then I could drop the magnesium sulfate as low as necessary to prevent accumulation of Mg in leaf tissue at harvest time. I determined that sulfur can be as low as 10ppm in late flower without causing a deficiency (yes, really), which made it easy to pull back magnesium levels exactly where I wanted them to be, even if higher levels seemed benign.
Conclusions
After a few years of experimentation with different media and Ca levels, I determined it’s hard to overdo Ca or Mg in veg and for the first month of flower, but it’s easy to overdo Ca thereafter, and can have catastrophic consequences if you do, depending on the cultivars. I also determined there is absolutely no connection between Ca and Mg from the plant’s perspective. You can raise and lower each mineral independently in a nutrient formula, just like you can with any other mineral ingredient. There’s no magic ratio to be found, and there is significant variation between cultivars in terms of ideal quantities.
As for ideal Ca levels, in water culture settings, assuming the use of distilled or RO water, I recommend about 40ppm of Ca after Day 30 of the flowering cycle. For any soil-like media that is calcium buffered, I recommend 0ppm of additional Ca after Day 30 of flower. As for leaf tissue quantities, the goal is to end up with 4-5 percent Ca in the leaf tissue during the last few weeks of harvest. If you have more than that, you’ve likely dented your yields and smoke quality in most cultivars.
In cases where I can’t remove calcium from fortified soil, I’ve still sometimes ended up with far too much Ca in the plants at harvest time, even using RO water and a fertilizer formula containing zero calcium. I have found that soil companies are generally not consistent or careful with the amount of calcium they add to their soils, not even the big well known national and international manufacturers. I can only assume it’s because they aren’t aware of the potentially negative consequences of high Ca, and their main priority is stabilizing pH more-so than supplying an ideal amount of calcium for a particular crop type.
Unbuffered coco fiber (coir) is one commonly used media type that requires special attention. When you buy coco fiber, it is usually calcium buffered, but sometimes it’s not. This is important to find out from the manufacturer because unbuffered coco fiber can act as a sink for supplemented calcium and magnesium (a phenomenon known as Cation Exchange Capacity [CEC]), so this is one exception where a cal/mag supplement could be required, especially at the time of transplant, in order to avoid a deficiency.
All in all, I’m making a case for cutting way back on calcium inputs to flowering plants. If you cut back too far, it’s easy to quickly see and correct the problem. This is what it looks like. It shows up in between the margins of leaves and is more common to see in the upper canopy:
Yellowing between the margins of the leaves in the upper canopy indicates low calcium during flowering and is easy to see.
This is just one of maybe a couple dozen key components of growing massive high-quality cannabis flowers. If your other factors are out of range, you could still come up short. Don’t underestimate for example, how much weight cannabis can gain from increased root zone oxygen levels or ideal VPD settings. All the factors are important if you are aiming for the stars as a master grower.
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I’d like to wrap up this article by acknowledging two of my mentors. Without them, I would be several years behind where I am now. Special thanks to Daniel Fernandez of Science in Hydroponics and Grant Mahy of Manic Botanix for being there when I needed them.
