Mills Nutrients - CalMag - Calcium and Magnesium for Soft Water and RO Growers
Best for: Growers using reverse osmosis water or growing in soft-water areas whose feed EC needs a lift in calcium and magnesium.
Our thoughts
If you're on RO water or your tap water runs soft, CalMag isn't optional, it's the thing that stops the classic RO deficiency symptoms before they show up on the leaves. Mills' version does the job without fuss. Simple, reliable feeding that delivers results.
Mills CalMag is a concentrated calcium and magnesium supplement, containing 13.1% calcium, 2.5% magnesium and 8.3% nitrogen by weight. It's aimed specifically at growers whose water starts with a low EC, below around 0.3, which is typical of reverse osmosis-filtered water or naturally soft tap water. Both scenarios strip out the calcium and magnesium plants would normally get from the water itself, and a shortfall shows up as stunted new growth, curled leaf tips and weak stems.
Dose at roughly 1ml per litre to lift a low starting EC up to around 0.3-0.4 before adding your main base nutrient, or run it at 1ml per litre once a week on its own, at a pH of 6.5, as ongoing deficiency prevention alongside a full feeding programme.
How it compares
CalMag supplements are a standard line-up across every major nutrient brand, and the products are functionally similar.
- Versus General Hydroponics CaliMagic, both target the same RO/soft-water calcium-magnesium gap; Mills is formulated to sit alongside its own Basis A/B and coco ranges specifically.
- Versus Botanicare CalMag, dosing and purpose are near-identical, so the choice often comes down to which base nutrient range you're already running.
- Versus skipping CalMag altogether on RO water, the difference shows up within a week or two as visible deficiency symptoms if you don't supplement.
It's a straightforward top-up rather than a specialist product, worth having on the shelf if your water supply is soft or RO-filtered.
Usage guidance
Dosage depends on why you're using it, correction or ongoing prevention.
Raising a low starting EC
Add around 1ml per litre to your reservoir before other nutrients, then check EC and add more in 0.5ml/litre increments if you need it higher, up to about 0.4 EC from CalMag alone.
Weekly prevention
If you're already running a full feed but want to guard against deficiency, dose 1ml per litre on its own, once a week, at pH 6.5.
Coco growers
Coco naturally locks up some calcium and magnesium, so CalMag is worth running as a standing addition to your base feed rather than just as an occasional top-up.
Technical specifications
- Nitrogen (N): 8.3% w/w
- Calcium (Ca): 13.1% w/w
- Magnesium (Mg): 2.5% w/w
- Typical dose: 1ml per litre
- Target use case: Soft water or RO-filtered water with starting EC below 0.3
- Recommended pH: 6.5 for standalone weekly dosing
- Sizes available: 1 Litre, 5 Litre
- Manufacturer: Mills Nutrients, Netherlands
Who this is for
- Growers using reverse osmosis or deionised water
- Anyone in a soft-water area with a naturally low tap water EC
- Coco coir growers who need a standing calcium/magnesium top-up
- Growers seeing early signs of Ca/Mg deficiency, such as curled tips or stunted new growth
Not needed if your tap water already runs a healthy EC with decent mineral content, hard water areas usually don't need it.
Our take
It's a cheap insurance policy against one of the most common RO and soft-water feeding mistakes, and the dosing is simple enough that there's no excuse to skip it if you're on filtered water.
Works consistently without overcomplicating your feeding schedule. Does exactly what it should, day in, day out.
