Short answer: Calcium deficiency in plants shows first in new growth: young leaves curl, get brown spots or die back at the tips, and growing points stop developing. The most common causes are low-mineral water (soft or RO water), high humidity that slows water transport, and a feed that has no calcium. To fix it, find the cause first, then add a calcium fertilizer such as Ca 15 at the dose from the feeding chart.
Calcium deficiency in plants is easy to mix up with other problems. Brown spots, pale leaves and curled tips can have several causes. This guide shows what calcium does, which signs point to a shortage, how to tell it apart from other deficiencies, and how to correct it step by step. It is written for indoor and outdoor growers, including cannabis growers.
What calcium does in your plant
Calcium is a main building block of cell walls and membranes. It also takes part in reactions inside the plant and helps to control how nutrients and water move around.
Calcium travels with the water stream in the plant (the xylem). This is the key to most calcium problems. If water moves slowly, calcium arrives slowly at the young leaves and growing points, even when there is enough calcium in the root zone.
Calcium deficiency symptoms
A shortage leads to soft cell walls. Soft tissue breaks down more easily and is more sensitive to fungi. The signs you can see are:
- Young leaves that die off or stay small and distorted.
- Growing points that stop growing or die.
- Yellow parts on leaves with round brown dots.
- More problems with fungi on weak tissue.
The Bionova product page for Ca 15 describes the same pattern: yellow areas with brown round dots point to a calcium shortage.
Causes of calcium deficiency, from most to least likely
1. Soft water or reverse osmosis (RO) water
Soft water and RO water contain few minerals, and RO water contains almost none. If your feed does not add enough calcium, the plant runs short. This is the most common cause for growers who use RO water. The Ca 15 product page advises 2 to 4 ml per 10 litres of water added to a Supermix in soft water, and 5 to 15 ml per 10 litres in RO water.
2. High humidity or a very dense crop
In humid air a plant gives off less water through its leaves. Less water flow means less calcium moves to young growth. A compact crop with little airflow has the same effect. Bionova advises to double the Ca 15 dose in humid environments to strengthen the tissue.
3. A feed without enough calcium
Some feeds contain little calcium for your water type. Check your feeding chart and your water.
4. Fast growth or heavy flowering
When a plant grows fast, new tissue needs a lot of calcium in a short time. Demand can go up faster than supply.
5. Too much calcium
The opposite also happens. An excess of calcium can show as brown spots on the leaves too. This is why you should not add calcium before you check the cause.
How to tell calcium deficiency from other problems
Look at where the problem starts and check your water and feed before you change anything.
- New growth affected: this fits calcium, because calcium moves slowly to new tissue.
- Pale leaves with green veins: this points more to magnesium. See the Cal-Mag section below.
- Everything yellow, starting at the bottom: this often points to nitrogen or a pH problem instead.
- Brown spots after you added calcium: check for an excess, and check your EC and pH.
If you feed regularly and the plant still shows deficiencies, pH may block the uptake. In that case fix pH first. The pH Lockout Fix Pack is made for this situation.
How to fix calcium deficiency
- Measure your water. Check EC and pH of your start water. RO water at EC 0.0 has no calcium at all.
- Check humidity and airflow. Better airflow helps water, and so calcium, to move through the plant.
- Add calcium to your feed. Bionova Ca 15 has NPK 8-0-0 + 15% CaO and a concentration of 1:3000. Use the dose from the feeding chart, and the ranges above for soft water and RO water.
- Use a foliar spray for fast help. Ca 15 can also be sprayed on the leaves at 2.5 to 5 ml per litre.
- Check again after a few days. New growth should come in normal. Damaged leaves do not turn green again, so judge the plant by the new leaves.
Ca 15 mixes with all Bionova fertilizers, stimulators, minerals, universals and substrates.
Calcium and magnesium together (Cal-Mag)
Calcium and magnesium are often short at the same time, especially with RO water. Calcium problems show in new growth. Magnesium problems show as pale leaves with green veins. If you see both, the Cal-Mag Deficiency Pack combines Ca 15 and MgO 10 and saves 15% compared to buying them separately. You can find all single products in the minerals collection.
How to prevent calcium deficiency
Test your water before you start. Add calcium from the beginning if you use soft or RO water. Keep humidity in range and keep air moving around the plants. Change doses in small steps, and read the feeding chart for your substrate. For more on how feeding schedules work, see our autoflower feeding guide.
FAQ
What are the first signs of calcium deficiency in plants?
New leaves and growing points are affected first. Look for distorted young leaves, dying tips, or yellow areas with round brown dots.
How do I fix calcium deficiency in cannabis?
Find the cause first: soft or RO water, high humidity or a feed without enough calcium. Then add a calcium fertilizer like Ca 15 at the dose from the feeding chart. You can also use a foliar spray.
Do I need Cal-Mag if I use RO water?
Often yes. RO water has almost no minerals, so calcium and magnesium need to come from your feed. Check the feeding chart and the Ca 15 dose range for RO water.
Can I give too much calcium?
Yes. An excess of calcium can also cause brown spots on the leaves. Measure your water and feed before you add more.
