Calcium and magnesium for osmosis water and rainwater

Regular price $13.13
Regular price Sale price $13.13
Contenance Price Price per L vs 500 ml
500 ml $13.13 $26.26—
1 L $18.15 $18.15−31 %
5 L $74.08 $14.82−44 %

Plagron's CalMag Pro is a liquid calcium and magnesium supplement for preparing irrigation water or a nutrient solution from reverse-osmosis water, soft water or rainwater. It restores the mineral base this water lacks, at 1 ml per litre at most, before the fertiliser.

Ref. TK-PLAG-CALMAG-500·Engrais
Contenance : 500 ml
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The summary

  • Analysis: CaO 5.7%, MgO 3.3% and 5% nitrate nitrogen, in liquid form.
  • Reverse osmosis water: 0.75 ml per litre, 1 ml at most, added before the fertiliser.
  • Suitable waters: reverse osmosis, soft or rain water, which are low in calcium and magnesium.
  • Fish system: for the separate growing loop and for pots, not for the pond.

Restoring a mineral base to rainwater, soft water or osmosis water

Plagron CalMag Pro is a calcium and magnesium supplement for reverse osmosis, soft or rain water, in 500 ml and 1 litre bottles or a 5 litre container. It is added before the fertiliser, at 0.75 ml per litre of reverse osmosis water. The circuit of an aquaponic system never receives it.

Hard tap water supplies a good share of the calcium and magnesium that plants need all by itself. Reverse osmosis water, very soft water or rainwater contain almost none, whereas a standard fertiliser is formulated on the assumption of moderately mineralised water. CalMag Pro, from the Plagron range, bridges this gap: you add it first, to restore a balanced base of these two minerals, and the fertiliser then completes the solution.

Calcium

It builds the cell walls. A shortage blackens the tips of tomatoes and browns the edges of lettuce leaves.

Magnesium

It sits at the heart of the chlorophyll molecule. A shortage turns the older leaves yellow between the veins.

💧 Water first, fertiliser second

Add 1 ml of CalMag Pro per litre of water at most to your sump tank or watering can, then mix. Next add the growth or flowering fertiliser, and finally check the pH and conductivity of the solution. This order has a chemical reason: concentrated calcium reacts with the phosphates and sulphates of a neat fertiliser to form insoluble salts, which settle at the bottom of the container and become unavailable to the roots. Diluted first in a large volume of water, it stays in solution, all the more so as its formula combines the two elements without sulphate. It is applied at every watering, in soil as in hydroponics.

Measure the hardness of your water before adding calcium

Whether you need a supplement depends entirely on your starting water. A total hardness GH drop test tells you in a few minutes how much calcium and magnesium your water contains. Rainwater or reverse osmosis water shows a value close to zero and calls for the full dose, whereas tap water that is already hard needs none or only a fraction of it. This way you avoid adding salts the plant does not need.

Never mix CalMag Pro with straight fertilisers

If poured directly into one another, the supplement and a concentrated fertiliser form a whitish precipitate that blocks the drip emitters and deprives the plant of some of its calcium. Always dilute each product in water, one after the other, mixing in between, and rinse the syringe or measuring cup between each measurement.

You can use it in soil as well as in hydroponic systems filled with clay pebbles, rockwool or coco fibre. Coco fibre deserves particular attention, because it holds back part of the calcium and magnesium in the solution during the first few weeks, which makes a regular supply all the more useful. Under LED lighting, the plant transpires less than under conventional lamps, which slows the uptake of calcium and magnesium, and a regular supply compensates for this drop. A well-supplied plant forms stronger cell walls and a more developed root system.

The analysis is expressed as oxides, following the regulatory convention for fertilisers: 5.7% calcium oxide, or about 4.1% calcium, and 3.3% magnesium oxide, or about 2% magnesium. The product also supplies 5% nitrogen, which you should bear in mind if you are closely monitoring the nitrogen nutrition of a flowering crop. It is a mineral supplement, with no phosphorus or potassium: it does not replace a complete fertiliser, it prepares the water that will receive it.

🐟 Calcium and magnesium in aquaponics

In a coupled aquaponic system, where the same water circulates between the fish and the plants, calcium is among the elements most often in short supply, along with potassium and iron. The New Mexico State University aquaponics guide recommends adding it as calcium hydroxide when the pH drops, and the FAO manual shows a submerged bag of shells that slowly releases carbonate into the water. CalMag Pro must never be used in this circuit: it adds salts and nitrogen dosed for a solution without livestock, whereas the fish gain no benefit from them.

It remains very useful around the system, though. In a decoupled installation, the fish water is sent to a separate growing loop that never returns to the pond, and the plant solution can be topped up freely there, calcium and magnesium included. It is also used to prepare the rainwater for your seedlings and young plants before they enter the system, as well as to water vegetables grown in pots alongside. The article on rainwater in aquaponics details the precautions to take when this water is also used to top up the pond.

🧴 Sizes and storage

At the maximum dose, each millilitre treats one litre of water. The 500 ml bottle therefore makes 500 litres of solution, which is ten changes of a 50-litre hydroponic reservoir emptied every week. The 1 litre bottle doubles that, and the 5 litre container, which treats 5,000 litres, suits a greenhouse watered with rainwater or a large hydroponic system. If your water already contains a little calcium, the reduced dose stretches each container even further.

Close the cap after each use and store the bottle away from frost, heat and light, out of the reach of children. Shake it before dosing if a slight sediment has formed at the bottom. A solution that has already been prepared, with supplement and fertiliser mixed, is best used within the day or within the normal lifespan of the sump tank, without storing it for several weeks.

🔬 Dosing according to your starting water

For reverse-osmosis water, the starting point is 0.75 ml per litre, which gives a conductivity of about 0.4 mS/cm before any fertiliser, the value that growing schedules assume for average tap water. The maximum dose remains 1 ml per litre. Rainwater is treated in the same way, after a measurement, as its mineral content varies with the roof and the tank that collect it. For soft tap water, a conductivity tester or a total hardness test shows you what is missing, and the dose is reduced in proportion. The concentrate is classified as hazardous to the eyes: pour it wearing goggles and gloves, and rinse thoroughly in case of splashes. It also contains a preservative that may cause an allergic reaction in sensitive people.

Technical specifications

The analysis

Type
Liquid mineral supplement
Calcium oxide
CaO 5.7%That is about 4.1 % calcium
Magnesium oxide
MgO 3.3%That is about 2 % magnesium
Nitrogen
5%To be counted in the total nitrogen nutrition

Usage

Dose
At most 1 ml per litreReduced if the starting water is already hard
Order
Before the fertiliser, in the waterPrevents calcium precipitation
Waters concerned
Reverse osmosis, soft or rain water
Grow media
Soil, coco fibre, clay pebbles, rockwool

Depending on the starting water

RO water
0.75 ml per litreStarting conductivity of around 0.4 mS/cm
Soft tap water
Reduced dose after measuringHardness or conductivity test

Safety

Concentrate
Danger, causes serious eye damageWear goggles and gloves when measuring out
Storage
4 years unopened, 0 to 35 °C

4 further specifications are available in the detailed view.

In the box

  • 1 bottle or 1 container of CalMag Pro, from 500 ml to 5 L depending on the size chosen

Analysis, dose and reference for osmosis water taken from plagron.com (CalMag Pro page, technical data sheet of September 2024); classification according to the 2024 safety data sheet.

Manufacturer and product safety

Manufacturer
Bertels B.V.

Information provided under Regulation (EU) 2023/988 on general product safety.

Estimate your
delivery time

Bottles and cans are dispatched as parcels from a distributor's warehouse in France, with sealed caps and wedged against leaks. The exact shipping cost is shown in your basket.

Frequently asked questions

6 questions

6 key questions

CalMag Pro according to your water volume

Enter the volume of osmosis or rain water to prepare. The calculation gives the starting point, 0.75 ml per litre.

L
1 L500 L

0ml

of CalMag Pro, before the fertiliser

at most, at 1 ml per litre
0 ml

This guideline applies to reverse osmosis or rainwater; with soft tap water, reduce it according to the hardness reading. Goggles are recommended, as the concentrate is classified as hazardous to the eyes.

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