CalMag supplies calcium and magnesium, the two secondary nutrients most often missing from an indoor grow. It is the single most commonly needed additive, and also the one most often added when it is not needed at all.
When you definitely need it
Growing in coco. Coco binds calcium and magnesium and releases potassium in exchange. Unless your base feed is coco-specific, you will run short by around week three. See coco coir.
Using RO or filtered water. Reverse osmosis strips everything, including the calcium your feed chart assumed was already there.
Running LEDs hard. High light intensity drives faster growth and higher transpiration, which raises calcium demand. This is why cal-mag deficiency became common the same year everyone switched to LED.
When you probably do not
If you are on hard tap water – which covers most of London and the south-east, Croydon included – and growing in soil with a soil-specific feed, you likely have enough already. Adding CalMag on top pushes EC up for no benefit and can lock out other nutrients. Check your water first.
Spotting a real deficiency
Symptom
Likely cause
Rust-coloured spots on newer leaves, tips curling
Calcium
Yellowing between the veins on older, lower leaves
Magnesium
Both at once, in coco, around week three
Classic coco cal-mag lockout
Worth ruling out pH first – calcium locks out below about 5.5 even when there is plenty present, so a meter tells you more than another bottle will. See pH and EC management.
What we stock
CalMag from Canna, Athena, Plant Magic, Buddhas Tree, Dutch Pro and Advanced Nutrients, in concentrate and ready-to-use. Related: silica and silicon supplements for cell wall strength, and base nutrients with calcium already included.
CalMag FAQs
How much do I add? Start at half the label rate and watch. Most charts assume RO water; on hard tap you need far less.
Can I add it to my nutrient mix? Add it to the water first, before your base feed, and never mix concentrates together undiluted – calcium and phosphates precipitate out and you get sludge in the bottom of the tank.
Does it raise EC? Yes, noticeably. Account for it in your total EC rather than adding it on top of a full-strength feed.
Why buy CalMag from The Hydro Bros?
We are a hydroponics shop in Croydon, we know the local water, and we will tell you if you do not need it. Discreet UK and European delivery, and excellent customer reviews on Trustpilot.
