Plagron Calmag Pro — Calcium Magnesium Supplement for RO Water & Soft Water Growing
Best for: Growers using RO water, soft water areas, or LED systems who need a reliable calcium and magnesium base without guessing dosages
Our thoughts
If you're running RO water or live in a soft water area, this isn't optional — it's foundational. Plagron Calmag Pro does exactly what it should: gives your plants the calcium and magnesium they need to build strong cell walls and absorb nutrients properly. Twenty-five years in the game, and it still performs. One we sell a lot of for growers upgrading from tap water to filtered systems.
Plagron Calmag Pro is a liquid calcium and magnesium supplement designed to mineralise reverse osmosis water and soft water, creating a stable foundation for your nutrient solution. RO and demineralised water strip out naturally occurring minerals — calcium and magnesium included — which plants need as secondary macronutrients. Without them, you get deficiencies that show up as yellowing leaves and weak growth. This additive puts them back in, in the exact ratio your plants need.
How it compares
Calmag products sit in a crowded space, but Plagron's formula has real distinctions.
- Includes 5% nitrogen (NPK 5-0-0) — competitors like GH CALiMAGic don't. Gives you a small nutrient boost alongside the Ca and Mg
- Balanced Ca:Mg ratio without sulphate interference — no mixing issues with other additives
- More concentrated than most — you only need 0.75–1 ml per litre, where competitors ask for 1.5–2 ml
- Proven 25+ year track record. Not a new-to-market product trying to prove itself
If you're comparing to brands that add iron (Humboldts Secret, Botanicare), note that Plagron keeps it simple. Iron deficiency is less common than Ca/Mg deficiency, and you can add it separately if needed.
Usage guidance
Calmag Pro works across all growing systems, substrates, and lighting setups.
RO and demineralised water
Add 0.75 ml per litre to establish a baseline EC of 0.4 before adding your main nutrients. Use this every feed. Maximum dosage is 1 ml per litre — don't overshoot.
Soft water areas
If you're in Wales, Scotland, or the North, your tap water is already low in minerals. Start at 0.75 ml/L and adjust based on your water hardness. Test your source water first if possible.
LED growing systems
LED lights reduce plant transpiration compared to traditional lamps. This means reduced calcium and magnesium uptake. Calmag Pro compensates for this biological shift — use it even if you're not on RO water.
Coco coir substrates
Works perfectly in coco. Apply with every watering as part of your base solution.
Storage
Unopened, it lasts 4 years. Once opened, use within 6 months.
Technical specifications
- Type: Liquid calcium and magnesium supplement
- NPK Analysis: 5-0-0
- Calcium oxide (CaO): 5.7%
- Magnesium oxide (MgO): 3.3%
- Nitrogen (N): 5.1% (as nitrate)
- pH of product: 4.3 (adjusts solution pH to 6.0–6.5)
- Appearance: Blue liquid, odourless
- Dosage: 0.75–1 ml per litre of water
- Available sizes: 250 ml, 500 ml, 1 L, 5 L, 10 L, 20 L
- Shelf life (unopened): 4 years
- Shelf life (opened): 6 months
Who this is for
- Growers using reverse osmosis or demineralised water systems who need a proper mineraliser base
- Anyone in soft water regions (particularly UK) wanting to avoid secondary nutrient deficiencies
- LED growers compensating for reduced mineral uptake under modern lighting
- Coco coir growers building a reliable feeding schedule without guesswork
- Experienced growers who want professional-grade simplicity without unnecessary additives
Our take
This is genuinely foundational kit if you're serious about water quality. Calcium and magnesium deficiencies aren't flashy problems — they're insidious ones that quietly tank your results. Calmag Pro prevents that before it starts.
The nitrogen content is a bonus that sets it apart. The 25-year pedigree isn't marketing fluff — it means the formula actually works. Good time to pick up while stock lasts, especially if you're switching to RO or upgrading your lighting system.
