Cal-Mag Supplements -- Calcium & Magnesium for Plants
Calcium and magnesium deficiencies are among the most common nutrient problems in hydroponic, coco coir, and soft-water soil grows. Calcium plays a structural role in cell walls and drives new tissue development -- deficiency first appears as brown or necrotic spots on new growth, tip burn on leaves, and stunted growing tips. Magnesium is the central atom in chlorophyll molecules and is essential for photosynthesis -- deficiency causes interveinal chlorosis (yellowing between leaf veins) starting on older lower leaves. Cal-Mag supplements deliver concentrated calcium and magnesium in a form immediately available for plant uptake, correcting deficiencies quickly and preventing them from recurring across the entire growing cycle.
When to Use Cal-Mag: RO Water, Coco Coir & High-EC Grows
Cal-Mag supplementation is most critical in three situations. First, when using reverse osmosis (RO) water -- RO filtration removes 94-99% of dissolved minerals including calcium and magnesium, leaving your starting water with essentially zero baseline Ca-Mg. Second, in coco coir grows -- coco fiber has a natural chemical affinity for calcium and magnesium ions, pulling them out of solution and making them temporarily unavailable to plant roots. This buffering effect is most pronounced in new, unbuffered coco. Third, in high-intensity grows with rapid plant growth rates, where calcium demand for new cell wall development outpaces what base nutrients provide. Add Cal-Mag at 1-5 ml/gallon to your nutrient mix before pH adjustment, with exact rate depending on your water baseline EC and crop type.
Cal-Mag Products: Botanicare, General Hydroponics, Fox Farm & More
Hydrobuilder carries Cal-Mag supplements from qualified brands including Botanicare Cal-Mag Plus (one of the most widely used liquid Ca-Mg supplements in hobby and commercial growing), General Hydroponics CaliMagic, and Fox Farm Bush Doctor Cal-Mag. Botanicare Cal-Mag Plus also includes iron and a small amount of nitrogen, making it a useful baseline supplement for soft-water and coco grows throughout the vegetative and early flowering stages. General Hydroponics CaliMagic provides calcium and magnesium without the iron addition -- suitable for growers who want clean Ca-Mg supplementation without the nitrogen contribution. Both are available in quarts, gallons, and case quantities.
Foliar Application for Fast Deficiency Correction
In addition to reservoir supplementation, Cal-Mag can be applied as a foliar spray (diluted to 1-2 ml per liter) for fast correction of visible deficiency symptoms. Foliar application bypasses root uptake entirely, delivering calcium and magnesium directly to leaf tissue within hours -- useful when deficiency symptoms are actively progressing and you need to stop damage quickly while correcting the root-zone supply simultaneously. Apply foliar Cal-Mag during lights-off or in low-light conditions to avoid potential leaf burn from the concentrated solution on wet leaves.
Pair Cal-Mag with a complete nutrient program from General Hydroponics or Athena Nutrients, and browse our RO water filtration systems if you are not already filtering your source water. Fast shipping.
Cal-Mag FAQ
What is Cal-Mag and what does it do for plants?
Cal-Mag is a concentrated calcium and magnesium supplement for plants. Calcium is essential for cell wall structure and new tissue development -- without adequate calcium, growing tips become stunted, leaves develop tip burn, and new growth shows necrotic spotting. Magnesium is the central element in chlorophyll and is required for photosynthesis -- without it, leaves lose green color between veins (interveinal chlorosis), starting with older lower leaves. Cal-Mag supplements deliver both elements in plant-available form to prevent and correct these deficiencies, which are especially common in hydroponics, coco coir, and soft or RO water grows where baseline Ca-Mg is low.
How much Cal-Mag should I add to my nutrient solution?
Standard dosing is 1-5 ml per gallon depending on your baseline water quality and growing medium. Starting point: 2-3 ml/gallon for soft tap water or RO water in hydroponic systems; 3-5 ml/gallon for coco coir grows where the medium buffers Ca-Mg from solution. Add Cal-Mag to your reservoir before your base nutrients and before adjusting pH. Always check for deficiency or toxicity signs and adjust the rate accordingly -- too much Ca-Mg raises the solution EC and can contribute to nutrient lockout at high concentrations. When in doubt, start at the lower end of the recommended range and increase if deficiency symptoms appear.
Do I need Cal-Mag with RO water?
Almost always yes. Reverse osmosis filtration removes 94-99% of dissolved minerals including calcium and magnesium from your source water. Starting with near-zero baseline Ca-Mg, most base nutrient programs do not provide enough calcium and magnesium to fully meet plant requirements -- especially in fast-growing flowering crops with high calcium demand for new cell walls. Add Cal-Mag at 2-3 ml/gallon before your base nutrients when working with RO water. If you run Athena Pro Line or other programs that include Ca-Mg in the core product, check the label before adding additional Cal-Mag -- some all-in-one programs already include adequate Ca-Mg at standard rates.
What is the difference between Botanicare Cal-Mag Plus and General Hydroponics CaliMagic?
Both are liquid calcium and magnesium supplements used for the same purpose, but with slightly different formulation choices. Botanicare Cal-Mag Plus includes a small amount of iron (0.1%) and 2% nitrogen (as amino-acid nitrogen) in addition to calcium (3.2%) and magnesium (1.2%). The nitrogen addition makes it particularly useful during vegetative growth but means it contributes to overall nutrient EC. General Hydroponics CaliMagic contains calcium (5%) and magnesium (1.5%) without the nitrogen or iron addition -- a cleaner Ca-Mg source for growers who do not want the additional nitrogen contribution, particularly during flowering when nitrogen requirements decrease. Both are effective; the choice typically comes down to your full nutrient program and whether additional nitrogen in the Cal-Mag product is beneficial or unwanted.
How do I know if my plants have a calcium or magnesium deficiency?
Calcium deficiency symptoms appear on new growth first: brown, necrotic spots on young leaves, tip burn on the margins of new leaves, and stunted or malformed growing tips. This is because calcium is not mobile in plant tissue -- plants cannot relocate it from old leaves to new growth, so deficiency shows up where new tissue is being built. Magnesium deficiency appears on older lower leaves first: yellowing between the leaf veins (interveinal chlorosis) while the veins remain green, progressing upward if uncorrected. This is because magnesium is mobile and plants will relocate it from lower leaves to supply new growth. If you see both patterns simultaneously, a general Ca-Mg deficiency across the system is likely -- check your water baseline, medium buffering, and supplement dosing.
