Cal-Mag Guide: Calcium and Magnesium Deficiency in Plants
Cal-mag is a calcium and magnesium supplement, and it's the bottle growers reach for first when leaves turn yellow or spotty. Sometimes it's the right fix; often the real fault is pH. This guide covers what the two elements do, who runs short, how to read the common signs, and the cal-mag additives we sell. The CalMag bottles are under CalMag additives.
What calcium and magnesium do
- Magnesium sits at the centre of chlorophyll, the green in the leaf. The plant can move it, so a shortage shows on the old leaves first.
- Calcium builds cell walls. It travels up with the water the plant drinks and then stays put, so a shortage shows in the newest growth.
Base nutrients contain both, and some expect your tap water to add a little more: Canna say their UK nutrients are designed for water with a base EC of 0.4-0.7 mS/cm. Cal-mag is the top-up.
Who runs short
- Soft tap water. Hard water brings calcium and magnesium with it; soft water brings little. Canna count water with an EC under 0.4 as soft (an EC meter such as the Bluelab Truncheon will tell you). Canna's Aqua chart assumes softer water (EC 0.2), so check the chart for the feed you use.
- RO water. A reverse osmosis filter takes most of the dissolved minerals out, so with most feeds you have to put some back; the Canna CALMAG Agent page says that without that buffer the pH is very hard to control. See our reverse osmosis guide.
- Coco. With soft or RO water coco growers usually add cal-mag, and the Athena CaMg page says inert media such as coco or rockwool need more calcium and magnesium.
- LED lights. Growers moving from HPS to LED commonly find they need more.
Check pH first
The wrong pH can lock out calcium and magnesium that are already in the feed. As a rule, both get harder to take up as the pH drops, and iron as it climbs.
Before blaming a shortage, check the feed's pH and EC against your feed chart and put them right: our pH and EC meters guide and the pH lock-out section of our yellow leaves guide show how. Check the watering too: cold, waterlogged roots take up less of everything.
Reading the signs
| Short of | Shows first on | What it typically looks like | Usual cause |
|---|---|---|---|
| Magnesium | Older, lower leaves | Yellowing between the veins while the veins stay green; rusty spots later | Soft or RO water, coco, low pH |
| Calcium | New growth and growing tips | Young leaves small, twisted or crinkled, with brown dead patches; blossom end rot on tomatoes | Same as magnesium, plus humid, still air, which slows the plant's drinking (see our VPD chart) |
| Nitrogen | Oldest leaves, then upwards | Leaves turn pale, then yellow all over, veins included | Feed too weak. Some yellowing low down is normal late in flowering |
| Iron | Newest leaves at the top | Yellowing between the veins while the veins stay green | pH too high, more often than a feed short of iron |
| Phosphorus | Older leaves | Dark, dull, purple-tinged; slow growth | Cold roots, weak feed |
| Potassium | Older leaves | Edges yellow, then brown and scorched | Feed too weak. Overfeeding also browns edges, so check the EC |
| Sulphur | Newest leaves | Pale yellow all over, veins included | Rare with a complete feed |
| Trace elements (zinc, manganese, boron) | New growth | Small, misshapen or mottled; dying growing tips | pH too high |
| One element, from too much of another | Varies | That element's signs above; excess potassium can block magnesium | Overfeeding, salt build-up |
Marked leaves rarely recover, so judge any fix by whether the damage stops spreading and the leaves that follow come through clean.
The cal-mag products we sell
| Product | What its page says | Dose |
|---|---|---|
| Canna CALMAG Agent | Buffers soft or RO water up to the base level Canna design their nutrients for | See the label |
| Mills CalMag | Raises soft or RO water to the correct base EC before you blend the feed | See the label |
| Athena CaMg | Calcium 2.1%, magnesium 1.1%, nitrogen 2%, iron 0.06%; for RO water, coco and rockwool | See the Athena feed chart |
| Biobizz CalMag | Organic-based; soil, coco or hydro; watered in or sprayed on the leaves | See the label |
| Plant Magic Magne-Cal+ | Adds trace elements; developed to use alongside a PK booster; hard or soft water; best pH range 5.5-6.2 | 0.5-2 ml per litre |
| Dutchpro Calmag | Soil, hydroponics and coco | 1 ml per litre, depending on the plants and the EC you want; 0.1-1 on the Dutchpro feed chart |
| Advanced Nutrients Sensi CalMag Xtra | All growing media and systems, seedling to harvest | See the label |
| Intense Mag Cal | For deficiencies and conditioning RO water | See the label |
How to add cal-mag
- Cal-mag first when buffering soft or RO water. Stir it into the plain water and measure the EC as you go. The Athena CaMg page recommends bringing RO water up to an EC of around 0.3 or 0.4 before adding any other products. This is an exception to the usual order in our pH and EC guide (base feed, then additives). Athena's feed chart mixes in row order, with CaMg after the base feed and PK; the buffer-first line is the CaMg page's advice for RO water.
- Then the feed, and pH last.
- Start low. As a rule of thumb, begin at the bottom of the label rate and see how the next leaves come through before adding more.
More is not better. The Intense Mag Cal page warns against over-use late in flowering because of its nitrogen, and points to Canna's single-element additives for a shortage that late. It adds that it is all too easy to cause toxicity, or throw out the balance of the whole feed, when treating deficiencies. Athena's feed chart leaves CaMg out of the last two weeks of flowering.
Canna Elemental Additives (nitrogen, phosphorus, potassium, calcium, magnesium sulphate, iron chelate and a trace mix) go in at the label rate, and their page suggests extreme caution: an apparent shortage is frequently a feeding or environmental issue. Where salts have built up, Athena Cleanse and T.A. FlashClean target that build-up, which their pages tie to lock-out.
Do you need it, and which one?
- Hard tap water and healthy plants: usually no. Don't add it just in case.
- Hard tap water but signs showing: check pH first. Hard water usually carries more calcium than magnesium, so if magnesium is short, ask us about adding magnesium on its own.
- Soft or RO water: usually yes. Start with the one from the maker of your base feed. If they do not make one, the Biobizz, Plant Magic and Athena pages say theirs work with other brands.
- Check your base feed: the pages for FloraMax Veg 1 and FloraMax VegaFlora A & B say no extra cal-mag is needed when used as directed, even in RO water. T.A. TriPart Micro comes in a hard water version (more than 70 mg/L of calcium) and a soft water one (less, or RO).
Quick answers
What is cal-mag?
A liquid calcium and magnesium supplement, sometimes with nitrogen, iron or trace elements. It is used to top up soft or RO water and to correct shortages.
What does magnesium deficiency look like in plants?
Typically yellowing between the veins of the older, lower leaves while the veins stay green, with rusty spots later. Check the pH first; if it is in range, cal-mag at the label rate is the usual fix.
What does calcium deficiency look like in plants?
Typically small, twisted new leaves with brown dead patches, and blossom end rot on tomatoes. Calcium moves with the water the plant drinks, so check pH, watering and airflow before adding more.
More Growers' Guides
- Yellow leaves and other leaf problems
- pH and EC meters
- Reverse osmosis water for growing
- Coco coir guide
- Hydroponics: the systems explained
Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.