Grow Guides · Feeding & Nutrients

What Is Cal-Mag? Calcium and Magnesium for Plants

October 15, 2026· 8 min read· 0 comments

Cal-Mag is a supplement that supplies calcium and magnesium, two nutrients plants need in larger amounts than trace minerals but that many feeding programs leave a little short. The Harvest Company treats it as a targeted tool, because the signs of a shortage are easy to read once you know where on the plant to look.

Calcium shows up in new growth and fruit, and magnesium shows up in older leaves. Where the symptom appears tells you which one to look at first.

Quick answer
Cal-Mag is a liquid or powder supplement that adds calcium and magnesium to a feeding program. Calcium builds strong cell walls and healthy new growth and fruit, and magnesium sits at the center of chlorophyll. Growers in coco coir, hydroponics, or on filtered or soft water use it most, because those setups often supply less of both.
What Is Cal-Mag? Calcium and Magnesium for Plants
Steady water carries calcium

Calcium travels to new growth and fruit with the water the plant draws up. Even, consistent watering is often as important as the amount of calcium in the feed.

Coco coir and soft water need more

Coco coir tends to hold onto calcium and magnesium, and filtered or soft water carries very little of either. Growers in these setups are the ones who reach for Cal-Mag most often.

Two nutrients, two patterns

Calcium shows up high. Magnesium shows up low.

The two move through the plant differently, so their shortages appear in different places.

Calcium
Magnesium
What it does
Builds strong cell walls, sturdy stems, and healthy new leaves, roots, and fruit.
Sits at the center of the chlorophyll molecule and helps the plant turn light into energy.
How it moves
Stays where it lands, so the plant cannot pull it from old leaves to feed new ones.
Moves freely, so the plant pulls it from older leaves when supply runs short.
Where signs appear
New growth, growing tips, and developing fruit.
Older, lower leaves first, then working upward.
Classic sign
A dark, sunken patch on the bottom of tomatoes and peppers, or curled, scorched new leaves.
Yellowing between the veins of older leaves while the veins stay green.
Common cause
Uneven watering, soft water, or coco coir holding calcium back from the roots.
Soft or filtered water, coco coir, or a feed with plenty of potassium and little magnesium.
Symptom checker

I see this on my plants.

Match the sign to the most likely cause and the first thing to change. Work from the highest-severity match down.

Symptom
Likely cause
Quick fix
Severity
Dark, sunken patch on the bottom of tomatoes or peppers
Blossom end rot, where calcium does not reach the developing fruit fast enough, most often because watering swings between wet and dry.
Water deeply and evenly, mulch to hold moisture, and add Cal-Mag in coco coir or on soft water. Remove affected fruit so the plant puts energy into new ones.
High
Curled, distorted, or scorched new leaves
Calcium is not reaching the newest growth, which is common in fast growth, high heat, or a feed short on calcium.
Keep watering steady, check that the root zone is not too wet or too dry, and add Cal-Mag to the feed at the label rate.
High
Yellowing between the veins on older, lower leaves
A magnesium shortage, where the plant moves magnesium from old leaves to new ones and the old leaves lose color between the green veins.
Add Cal-Mag to the feed, and check that pH is in range so the roots can take up what is already there.
Medium
Rusty spots and yellowing in coco coir
Coco coir can hold onto calcium and magnesium, leaving less for the roots, especially in coir that was not buffered.
Use buffered coir, and include Cal-Mag in every feeding while growing in coir.
Medium
Problems began after switching to filtered water
Reverse osmosis and many filtered or soft waters carry almost no calcium or magnesium, so the feed has to supply all of it.
Add Cal-Mag to the water before the base nutrients, then measure and adjust pH.
Medium
Brown tips on lettuce and other leafy greens
Tip burn, where calcium does not reach the fast-growing leaf edges, often in warm weather or still air.
Keep the root zone evenly moist, improve airflow, and harvest a little earlier during hot spells.
Low
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For calcium and magnesium
Drip Hydro CaMg 2-0-0

A concentrated liquid calcium and magnesium supplement with a 4:1 calcium-to-magnesium ratio, fortified with nitrogen and iron. It dissolves completely for use in soil, coco coir, hydroponic, and drip systems.

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Drip Hydro Powder A 15-0-0
For a calcium-rich base
Drip Hydro Powder A 15-0-0

A fully water-soluble nitrogen and calcium base nutrient for strong stems, healthy foliage, and root growth. The 25 lb bag is part of the Drip Hydro powder program for larger gardens.

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Why Cal-Mag does not seem to work

Three habits that keep the problem going.

Each one is about how the plant receives calcium and magnesium, not how much you add.

Common beginner mistakes
No.
Mistake
What goes wrong
The fix
Severity
01
Adding Cal-Mag to fix uneven watering
✕Pouring in more Cal-Mag for blossom end rot while the soil still swings between soaked and bone-dry, which keeps calcium from reaching the fruit.
✓Water deeply and evenly first, and mulch the soil surface.
Most common
02
Skipping it in coco coir or on filtered water
✕Feeding coir or filtered-water plants with a base nutrient alone and missing the calcium and magnesium those setups lack.
✓Include Cal-Mag in every feeding in these setups.
Common
03
Ignoring pH
✕Adding more supplement when pH is out of range, so the roots cannot take up the calcium and magnesium already there.
✓Measure pH after mixing, and bring it into range before you feed.
Underrated
Key takeaways

Five things to remember.

  1. 01Cal-Mag supplies calcium and magnesium, two nutrients many feeding programs leave a little short.
  2. 02Calcium problems show up in new growth and fruit, and magnesium problems show up in older leaves.
  3. 03Blossom end rot is most often a watering problem, so steady moisture comes before more supplement.
  4. 04Coco coir and filtered or soft water are the setups that need Cal-Mag most.
  5. 05Check pH so the roots can take up the calcium and magnesium you provide.
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FAQ

Common questions about Cal-Mag.

It adds calcium and magnesium to a feeding program. Calcium supports strong cell walls, new growth, and fruit, and magnesium supports chlorophyll and the plant's use of light.
Often not. Many garden soils supply enough calcium and magnesium. It matters most in coco coir, in hydroponics, and when you water with filtered or soft water that carries very little of either.
It can help, but blossom end rot is most often caused by uneven watering that keeps calcium from reaching the fruit. Steady, deep watering and mulch come first.
Look at where the symptom appears. Calcium problems show up in new leaves, growing tips, and fruit, while magnesium problems start as yellowing between the veins of older, lower leaves.
A common practice is to add it to the water first, before the base nutrients, then measure and adjust pH. Follow the order and rate on your product's label.
The promise

Grow better. Eat better. Every day.

Your partner in every harvest. Read where the sign appears, give the plant what it is missing, and the next fruit comes in clean.

View Drip Hydro CaMg Browse all guides
Posted October 15, 2026 · Updated October 15, 2026 · 8 min read
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