When you think about calcium, you’re probably thinking about your mom telling you to drink your milk so that your bones grow strong. While that’s true for animals, plants need calcium for different reasons. A lack of calcium can cause major problems for cannabis plants, luckily, spotting and fixing the problem is easy.
Why do cannabis plants need calcium?
Calcium might not be the first nutrient that comes to mind when growing weed, after all, it isn’t present in any standard nutrient mix (those are primarily nitrogen, phosphorous, and potassium). However, calcium is one of the more important micronutrients for a healthy cannabis plant.
Calcium is used by plants to create root hairs (a key component of a healthy root system) and to strengthen the cell walls of the plant. This second part is key as cannabis plants can be as much as 95% water by volume; a lack of calcium can cause the plant to break down. Additionally, calcium plays a role in nearly every part of a cannabis plant’s life as it is a necessary component for the enzymes that cause photosynthesis, respiration, and cell signalling.
Lastly, calcium helps regulate the uptake and transportation of other nutrients within the plant. It helps nutrients move from the roots to the shoots and vice versa, ensuring an even distribution of nutrients around the plant.
Fortunately, soils (both outdoor dirt and pre-packaged growing soils) are quite high in calcium, meaning that most growers won’t have to deal with this. However, if you’re growing weed in hydroponics, using already-spent soil, soil analogues (coco coir), or in a place with very acidic dirt, you might need to keep an eye on things.
What causes a calcium deficiency in weed plants?
The obvious answer is “not enough calcium” but that hardly covers everything. If you’re concerned about a calcium deficiency in cannabis plants, these are the things to keep an eye on.
- Soil pH
As mentioned above, highly acidic soils tend to lock out certain nutrients. Cannabis plants flourish best in a soil pH of 6.0-7.0 (5.5-6.5 if you’re growing in hydroponics). Keeping a close eye on this from the beginning can save you a lot of hassle.
- Treated water
In much of the world, tap water contains enough calcium to keep a plant going. However, many growers make use of reverse osmosis or distilled water. In this case, you’ll likely need to add in calcium (as well as a few other micronutrients) to offset what you removed in the filtration process.
- Soft water
If you’re in an area with soft water, then you’ll likely need to add in calcium. Soft water is much lower in minerals than standard water and can cause deficiencies in plants.
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How to spot a calcium deficiency in weed plants
Like any other nutrient, calcium is absorbed by a plant’s roots where it is transported to the leaves. Whereas most nutrients can then travel from leaf to leaf as needed, calcium is stuck in one spot until it gets used up, meaning that deficiencies tend to appear in localised areas—at first.
If your cannabis plant is running low on calcium, the first thing you’re likely to notice is yellowing on the newest leaves. If the youngest leaves (at the top of the plant) are turning yellow along the veins—especially if this is accompanied by brown spots—then you might be dealing with low calcium.
If left untreated, this yellowing will begin to appear on lower leaves, followed by brown spots. The leaves will also begin to curl downward at the tips before the entire plant begins to slowly break down from the inside. Eventually, the stems will become softer making the plant slump.
If a calcium deficiency occurs during flowering, the results can be disastrous. Low calcium during flowering will severely affect the formation of buds and may even cause them to rot while still on the plant. This is one of those situations where the best treatment is prevention. But that doesn’t mean that you’re out of luck yet.
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Confusing calcium deficiency with other problems
The symptoms of low calcium can easily be confused for a few other problems and knowing which thing you’re trying to fix is a must.
- Nitrogen deficiency
Like low calcium, a nitrogen deficiency can cause yellowing of the cannabis leaves. However, this typically occurs in older, lower leaves. Additionally, low nitrogen tends to cause the entire leaf to turn yellow, not just the veins.
- Zinc deficiency
Again, this can cause yellowing and curling cannabis leaves but with a more pronounced curling at the tip. Further, this effect will not necessarily be confined to the newest growth.
How to fix a calcium deficiency in cannabis
Now that you’ve made certain that you’re dealing with low calcium, it’s time to fix the problem. Fortunately, this is a fairly simple fix most of the time. If treated properly, a calcium deficiency can be fixed in 10-15 days.
- Check your soil pH
If it’s too low, you’ll need to flush the entire thing. Using (ideally) distilled or RO water, pour water through the soil until you see runoff. Clear the runoff, wait a few minutes, and repeat several times.
- Add additional calcium
There’s no shortage of calcium additives on the market. Many of these are so-called “CalMag” mixtures as they contain both calcium and magnesium. These two elements are very interconnected in plant development and an imbalance of one can often affect the other.
Simply follow the instructions on whatever product you’re using and add the CalMag to your normal feeding routine.
- Use foliar sprays
Since calcium is stored in the cannabis leaves, many calcium additives can be applied directly where they’re needed most. Unless otherwise stated, you can use the same CalMag mixture that you’ve been using for feeding. When spraying the leaves, you’ll want to make sure that the pH of the water is between 5.5 and 6.0 for best results.
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Calcium toxicity: Can I give weed plants too much calcium?
As with anything, you can overdo it. Unfortunately, in this case, the symptoms of calcium toxicity are nearly identical to those of a deficiency. Yellowing leaves, brown spots, curling, and stunted growth are all signs that your plant has too much (or not enough!) calcium. Fortunately, there are a few ways to spot the difference.
If you want to know if you’re dealing with toxicity or deficiency, the best thing to do is check your soil pH. If it’s too acidic, you’re likely dealing with deficiency while a pH above 7.0 is likely a sign of toxicity. In either case, flushing your soil is the best first step.
Finishing strong
By paying close attention, you should be able to spot a calcium deficiency in your weed before it becomes a major problem. Keeping an eye on the soil pH and rigorously monitoring new growth should be enough most of the time. But if you find yourself in a tight spot, the solutions are at least easy enough that a calcium deficiency shouldn’t become a problem for most growers.
