Short answer: Cal mag (calcium and magnesium) is only needed when your water or feed does not supply enough of both. It is most often needed with reverse osmosis (RO) or soft water, and when you see signs of a shortage. If you use a Bionova main fertilizer with tap water that has minerals, you usually do not need extra. Check your water first, then add Ca 15 and MgO 10 only for the element that is short.
Many growers add cal mag to every watering "to be safe". That is not always a good idea. Cal mag for plants solves a real problem in some set-ups and creates a new one in others. This guide explains what calcium and magnesium do, when a cal mag supplement makes sense, how to recognise a magnesium deficiency, and how to correct it. It is written for indoor and outdoor growers, including cannabis growers.
What cal mag is and why plants need both
Cal mag is a short name for a calcium and magnesium supplement. Both are secondary nutrients and both are needed in every grow.
Calcium is a building block of cell walls. It moves with the water stream, so it reaches young growth slowly. Magnesium sits at the centre of chlorophyll, the molecule that turns light into energy. Without magnesium, photosynthesis slows down and leaves lose colour.
The two also compete. A large excess of one can make it harder for the plant to take up the other. This is the main reason not to add cal mag by habit.
When you need cal mag
You need extra calcium and magnesium in a few clear situations:
- RO or very soft water. RO water has almost no minerals, so both elements must come from your feed. This is the most common reason for a shortage.
- Visible signs of a deficiency. See the symptom section below.
- Fast growth or heavy flowering. Demand can rise faster than supply.
- Coco or other substrates that hold on to calcium and magnesium. Read the feeding chart for your substrate.
Bionova states that its main fertilizers and stimulators already contain the right amount of magnesium, except when you grow with RO water. So with mineral-rich tap water and a main fertilizer dosed from the feeding chart, extra magnesium is rarely needed.
When you do not need cal mag
Skip it if your tap water has a normal mineral content, your plants look healthy, and you already follow the feeding chart. Extra calcium and magnesium will not make healthy plants grow faster. An excess of magnesium can show as slow growth and small leaves, and too much calcium can cause brown spots.
Also check pH before you add anything. If pH is outside the range for your substrate, the plant cannot take up calcium or magnesium even when both are present. In that case fix pH first. The pH Lockout Fix Pack is made for this situation.
Magnesium deficiency: how to recognise it
Magnesium moves inside the plant, so the plant takes it from old leaves to feed new growth. This is why the first signs appear on older leaves.
- Pale or yellow areas between the veins, while the veins stay green (chlorosis).
- Leaves that dry out and die when the shortage continues.
- In flowering, smaller and lighter coloured flowers.
How to tell magnesium and calcium problems apart
Look at which leaves are affected first. This is the quickest way to separate the two.
- Old leaves, pale between green veins: this points to magnesium.
- New growth, distorted leaves, dying tips or brown round dots: this points to calcium. See our guide on calcium deficiency in plants.
- Whole plant pale, starting at the bottom: this often points to nitrogen or a pH problem.
- Both signs at once: this is common with RO water and is the case where cal mag helps.
How to fix a magnesium shortage
- Measure your water. Check EC and pH of your start water. RO water at EC 0.0 contains no calcium or magnesium.
- Check pH in the root zone. Correct it first if it is out of range.
- Add magnesium to your feed. Bionova MgO 10 has NPK 6-0-0 + 10% MgO and a concentration of 1:3000. For a deficiency, add 2.5 to 10 ml per 10 litres of water. With RO water, add 10 to 20 ml per 10 litres.
- Use a foliar spray for fast help. MgO 10 can be sprayed at 2.5 to 5 ml per litre.
- Judge by the new leaves. Damaged leaves do not turn green again. Healthy new growth shows that the fix works.
For calcium, Bionova Ca 15 (NPK 8-0-0 + 15% CaO, 1:3000) is used in the same way, with 2 to 4 ml per 10 litres in soft water and 5 to 15 ml per 10 litres in RO water. Both products mix with all Bionova fertilizers, stimulators, minerals, universals and substrates. Always use the dose from the feeding chart for your substrate and change in small steps.
A simple way to cover both: the Cal-Mag Deficiency Pack
If you grow with RO water or you see both signs, the Cal-Mag Deficiency Pack combines Ca 15 and MgO 10 and saves 15% compared to buying them separately. You can also pick single products from the minerals collection. For more on how to set doses, see our guide on how to read a feeding chart.
FAQ
Do I need cal mag with tap water?
Often not. Tap water usually contains some calcium and magnesium, and Bionova main fertilizers already contain magnesium. Test your water and add cal mag only if you see signs of a shortage.
What are the signs of magnesium deficiency in cannabis?
Older leaves turn pale between the veins while the veins stay green. When the shortage continues, leaves dry out. In flowering, flowers can be smaller and lighter.
Can I use calcium and magnesium together?
Yes. Ca 15 and MgO 10 can be used together, and the Cal-Mag Deficiency Pack contains both. Do not give more than the feeding chart advises, because an excess of one can block the other.
Why do plants show a magnesium deficiency when I feed enough?
The most common reasons are RO water, a pH that is out of range, or a root zone that is stressed. Check pH and water first before you add more magnesium.
