How to Use CalMag: Coco, Soil, and Hydro Doses + Mixing Order
Whether you need CalMag depends on your grow medium and water source. In coco and with reverse osmosis water, it's non-negotiable. In soil with hard tap water, you probably don't need it at all. Add it second in your mixing order, after Armadillo Armour and before Micro. The dose ranges from 1 mL/gal at seedling in hydro up to 10 mL/gal peak flower for Bonnie and Clyde growers in coco, where the base line carries zero calcium or magnesium.
What Is CalMag?
CalMag is a calcium, magnesium, and iron supplement for cannabis and other plants. Cronk CalMag (NPK 2-0-0) delivers 3.2% calcium, 1.2% magnesium, and 0.1% chelated iron. Third-party tested at A&L Canada Laboratories. February 2024. No detectable heavy metals.
Each element does something specific:
- Calcium builds cell walls and supports new growth. Without it, new leaves can't form properly.
- Magnesium sits at the center of every chlorophyll molecule. No magnesium means no photosynthesis.
- Iron helps the plant build chlorophyll. It's a small part of the formula but it earns its spot.
The calcium-to-magnesium ratio in Cronk CalMag is 2.7:1. That ratio is deliberate. Cannabis needs more calcium than magnesium, and this formula is built around that.
Do You Actually Need CalMag?
Here's the decision table. Be honest about your setup.
| Your situation | Need CalMag? |
|---|---|
| Growing in coco coir | Yes, always |
| Using RO or distilled water | Yes, always |
| Soft tap water (under 100 ppm) | Usually yes |
| Any medium under LED lights | Probably yes (LEDs increase calcium and magnesium demand) |
| Hard tap water in soil | Usually not needed |
| Yellowing between leaf veins on older leaves | Check pH first, then yes if pH is fine |
| Using PuurOrganics line | Not this product. Use PuurCalMag instead. |
Here's why coco is non-negotiable. Coco grabs onto calcium from your nutrient solution before your roots can get to it. It doesn't matter how much calcium is in your base nutrients. Coco takes it first. CalMag puts it back.
RO water has had its minerals stripped out, so you're starting with basically zero calcium and magnesium. You have to build that baseline yourself, and CalMag is how you do it.
How Much CalMag to Use
The right dose depends on your grow medium and your base nutrients. Bonnie and Clyde growers run much higher doses than Classic 3-Part growers because Bonnie and Clyde have no calcium or magnesium in them at all. With that base, CalMag is the only source of both nutrients.
Coco coir (Classic 3-Part base)
| Stage | CalMag dose |
|---|---|
| Early veg | 2 mL/gal |
| Pre-flower | 4 mL/gal |
| Early flower (peak demand) | 5 mL/gal |
| Late flower | 2 mL/gal |
| Ripen and Flush | Stop |
Source: Cronk Classic 3-Part Intermediate coco feed chart. pH range for coco: 5.6-6.2.
Hydroponics (Classic 3-Part base, Intermediate program)
| Stage | CalMag dose |
|---|---|
| Seedling | 1 mL/gal |
| Early to mid veg | 4 mL/gal |
| Late veg and pre-flower | 6 mL/gal |
| Early flower (peak demand) | 8 mL/gal |
| Mid to late flower | 4-6 mL/gal |
| Ripen and Flush | Stop |
Source: Cronk Classic 3-Part Intermediate Hydro feed chart. pH range: 5.5-5.8 in veg, 5.8-6.4 in flower. Change your reservoir every 7-10 days. Keep solution below 75F / 24C.
Bonnie and Clyde base (soil or coco)
Bonnie and Clyde have no calcium or magnesium in them. CalMag is the only source of both from seedling through late flower. Doses start at 5 mL/gal at seedling and go up to 10 mL/gal at peak flower, then taper before Ripen and Flush.
For the full week-by-week schedule, see the Cronk feed charts page.
Soil with RO or soft tap water
Start at 2-3 mL/gal and watch for deficiency signs. Soil holds onto some calcium on its own, but RO or soft water (under 100 ppm) starts with almost nothing. If you're on hard tap water in soil, you probably don't need CalMag unless you're seeing symptoms.
CalMag in the Mixing Order
The mixing order keeps your nutrient solution clean before it ever hits your plants. Follow it every feed.
- Fill with water first. Always add nutrients to water, never the other way around.
- Armadillo Armour first, if using. Add and stir. Wait 2-5 minutes before adding anything else.
- CalMag second. Stir well. It has to go in before Micro, Grow, or Bloom.
- Micro next. Always before Grow or Bloom.
- Grow or Bloom, then remaining additives. Bud Booster, Sticky Bandit, anything else. Adjust pH after everything is in.
- Monkey Juice last, after pH. The beneficial bacteria in Monkey Juice are pH-sensitive. They go in after you've set your pH, every time.
Why does CalMag have to go before Micro, Grow, and Bloom? Calcium and phosphorus will lock together into white chunks in your reservoir if they meet in a concentrated solution. Once that happens, both nutrients are gone. Adding CalMag before any phosphorus-bearing product stops this from happening.
What CalMag Deficiency Looks Like
Calcium and magnesium deficiencies look different because the plant moves them differently.
Calcium deficiency
Calcium can't move around once it's inside the plant. So when there isn't enough, new growth shows the problem first.
- Brown or rusty spots on young leaves
- Leaf tips curling up or under
- Distorted, twisted, or cupped new growth
- Affected tissue can die off in bad cases
Magnesium deficiency
Magnesium can move. The plant pulls it out of older leaves to feed new growth. So older, lower leaves show the problem first.
- Yellowing between the veins on older leaves, while the veins themselves stay green
- Moves upward through the plant as it gets worse
Check pH before you add more CalMag
Both deficiencies can show up even when your dose is right, if pH is off. Calcium absorbs best at pH 6.2-7.0 in soil and 5.5-6.5 in coco and hydro. Magnesium shuts down below pH 5.5 and above 7.0.
If you're seeing symptoms and the dose looks right, check pH first. Adding more CalMag to a pH problem won't fix anything.
Common Mistakes
Adding CalMag after base nutrients. Calcium and phosphorus will lock together into solid chunks in your container before the feed reaches your plants. CalMag goes in second, before Micro. Every time.
Skipping it in coco. Doesn't matter how much calcium is in your Micro. Coco strips it from the solution before your roots get a shot. There's no workaround.
Chasing symptoms without checking pH. pH controls what your plant can actually pull out of the solution. If it's off, more CalMag won't fix anything. Check your meter first.
Adding more without checking EC. If your EC is already at the top of range, pushing in more CalMag will tip your plant into lockout. Check your meter before increasing any dose.
Using Classic CalMag in a PuurOrganics feed. They don't work together. Use PuurCalMag. Different system, different product.
PuurOrganics Growers: Use PuurCalMag, Not This Product
Cronk CalMag (2-0-0) is a synthetic formula. It is not compatible with the PuurOrganics line.
PuurOrganics growers use PuurCalMag instead. It's OMRI-certified, delivers 4% calcium and 1.65% magnesium with no nitrogen, and is built specifically for the organic system.
Do not mix CalMag 2-0-0 into a PuurOrganics feed.
Frequently Asked Questions
Do I need CalMag in coco coir?
Yes, always. Coco grabs onto calcium from your solution before your roots can get it. It doesn't matter what's in your base nutrients.
How much CalMag should I use in coco?
Classic 3-Part growers: 2 mL/gal in early veg, up to 5 mL/gal at peak flower, then back down. Bonnie and Clyde growers run 5-10 mL/gal because that base has zero calcium or magnesium. Check your feed chart for exact week-by-week numbers.
When do I add CalMag in my mixing order?
Second. After Armadillo Armour (if you use it) and before Micro, Grow, or Bloom. If it goes in after your phosphorus-bearing nutrients, calcium and phosphorus will lock together in the container.
What does CalMag deficiency look like?
Calcium deficiency shows on new growth first: brown spots, curling leaf tips, twisted leaves. Magnesium deficiency shows on older leaves first: the tissue between the veins turns yellow while the veins stay green. Always check pH before adding more of either.
Can I use CalMag with PuurOrganics?
No. Use PuurCalMag instead. It's the version made for the organic system.
Key Takeaways
- Coco and RO water growers always need CalMag. Soil with hard tap water usually doesn't.
- Classic 3-Part coco: 2-5 mL/gal by stage. Bonnie and Clyde: 5-10 mL/gal because the base has no calcium or magnesium.
- CalMag goes second in the mixing order, after Armadillo Armour and before Micro. Adding it after Micro causes calcium and phosphorus to lock together.
- Calcium deficiency shows on new growth (brown spots, curled tips). Magnesium deficiency shows on older leaves (yellowing between the veins). Always check pH before adding more of either.
- PuurOrganics growers use PuurCalMag, not Classic CalMag. They're separate products for separate systems.
Related Resources
- Cronk feed charts by line and medium
- Nutrient deficiency diagnosis guide
- Growing in coco coir: complete guide
Cronk CalMag is available in 500mL, 1L, and 4L on the site. Need larger volumes? 10L and up (including 20L, 200L, 55-gallon drum, and 1,000L tote) are available by special order. Shop CalMag or get in touch for bulk pricing.