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What Is Cal-Mag and Why Gardeners Use It
Cal-Mag is a supplement that provides extra calcium and magnesium when your plants canโt get enough from soil, potting mix, or water. Itโs used to prevent common growth issues like weak new leaves, tip burn, blossom end rot on fruit, and stubborn yellowing that doesnโt improve even when you keep feeding.
In gardening, Cal-Mag is most often needed in pots, coco coir, hydroponics, and in situations where watering or fast growth causes nutrients to wash through quickly. Itโs not about โfeeding moreโ. Itโs about filling a specific gap that stops plants from performing properly.
Calcium: structure + new growth
Calcium helps build firm plant tissue and strong root tips. When itโs low, the newest growth shows problems first: twisted leaves, crispy tips, weak shoots, and fruit disorders like blossom end rot.
Magnesium: green colour + energy
Magnesium supports chlorophyll and photosynthesis. When itโs low, older leaves often fade or yellow between veins, and plants lose vigour even if theyโre getting enough NPK.
The reason Cal-Mag is so popular is simple: many gardens have enough NPK, but growth still stalls because calcium and magnesium are missing or unavailable. Once those two nutrients are stable, plants usually respond more predictably to whatever fertiliser program youโre already using.
When Cal-Mag makes the biggest difference:
Fast growth, heavy fruiting, container plants, coco coir, hydroponics, and soft or purified water where calcium and magnesium are naturally low.
Fast growth, heavy fruiting, container plants, coco coir, hydroponics, and soft or purified water where calcium and magnesium are naturally low.
When Should You Use Cal-Mag in Gardening?
Cal-Mag is most useful when calcium and magnesium are not staying available in the root zone. This usually happens because your growing setup leaches nutrients quickly, your water contains very little Ca/Mg, or the plant is growing so fast that demand outpaces supply.
If you are feeding consistently but still seeing weak new growth, fruit disorders, or stubborn yellowing, Cal-Mag is often the missing support nutrient.
1) Growing media that โruns outโ of Ca & Mg
Cal-Mag is commonly needed in pots, coco coir, and hydroponics because these systems do not hold minerals the way a rich garden soil can. With frequent watering, calcium and magnesium can wash through quickly, leaving plants under-supplied even when you are feeding regularly.
Pots Coco Hydro Fast-draining mixes
2) Water type that contains little to no minerals
If you use reverse osmosis, distilled, or very soft water, you are often starting with almost no calcium or magnesium. In that situation, Cal-Mag helps create a stable baseline so plants can maintain strong growth and predictable feeding response.
RO water Distilled Soft water
3) High-demand growth stages
Demand increases during rapid vegetative growth and especially during flowering and fruit development. Calcium supports firm new tissue and fruit integrity, while magnesium supports photosynthesis to power that growth. If either is limiting, you can see tip burn, poor setting, or uneven fruit development.
Fast growth Flowering Fruiting
4) Symptoms that donโt improve with normal feeding
A classic sign you need Cal-Mag is when you keep feeding your usual fertiliser but the plant still looks โoffโ. Adding more NPK can increase growth demand and make problems worse. Cal-Mag is used here to stabilise growth, strengthen new tissue, and restore predictable nutrient uptake.
Tip burn Blossom end rot Interveinal yellowing Stalled growth
Practical rule:
If your setup is coco, hydro, or pots with frequent watering, Cal-Mag is often a โbaseline nutrientโ. If youโre in rich soil with slow, steady growth, you may not need it at all.
If your setup is coco, hydro, or pots with frequent watering, Cal-Mag is often a โbaseline nutrientโ. If youโre in rich soil with slow, steady growth, you may not need it at all.
Some CompleteGrow fertilisers already include calcium and magnesium as part of their formulation. This is intentional for high-demand feeding stages where additional structural support is needed.
Advanced Formula NPK 4-18-38 + Calcium & Magnesium and our Hydroponic Nutrients (A & B) both supply calcium and magnesium within the base feed. When using these products, a separate Cal-Mag supplement is often unnecessary unless deficiency symptoms appear or water quality is very low in minerals.
Cal-Mag is most useful as a support supplement when growth demand temporarily exceeds supply. This commonly occurs in coco, hydroponics, and fast-draining potting mixes where calcium and magnesium can be depleted quickly.
Cal-Mag also pairs well with All Purpose NPK 20-20-20 during periods of fast vegetative growth, helping stabilise structure and nutrient uptake. For citrus grown in pots or sandy soils, it can be used alongside Citrus Fertiliser (NPK 6-2-12 + Trace Elements) to support leaf strength and fruit integrity where nutrients leach rapidly.
Simple takeaway:
If your base fertiliser already contains calcium and magnesium, Cal-Mag is usually optional. If your growing setup or water source limits availability, Cal-Mag becomes a targeted support rather than a constant additive.
If your base fertiliser already contains calcium and magnesium, Cal-Mag is usually optional. If your growing setup or water source limits availability, Cal-Mag becomes a targeted support rather than a constant additive.
How to Use Cal-Mag Correctly
Cal-Mag works best as a support nutrient, not a replacement for your main fertiliser. Its job is to keep calcium and magnesium reliably available during fast growth, heavy feeding, and fruiting, especially in coco, hydro, and containers.
For most growers, the โbestโ way to use Cal-Mag is simple: apply it consistently at a sensible dose, watch plant response, and avoid overcorrecting. Calcium and magnesium issues often show up when plants are already growing hard, so stability matters more than strength.
1) Start with the correct dose
Use 0.5โ1 ml per litre of water. This suits both vegetative and flowering stages.
2) Apply it the right way
Water first if the media is dry. In pots and coco, aim for consistent moisture to reduce nutrient swings.
3) Look for the right improvements
Stronger new growth, fewer leaf-edge issues, steadier colour, and better fruit integrity over the next 1โ2 weeks.
Tip:
If youโre using very soft or reverse-osmosis water, Cal-Mag is often best treated as a baseline nutrient rather than a โfix onceโ supplement.
If youโre using very soft or reverse-osmosis water, Cal-Mag is often best treated as a baseline nutrient rather than a โfix onceโ supplement.
Why CompleteGrow Cal-Mag Performs Better
Cal-Mag Hydroponic Nutrients Fertiliser is a high-strength, chelated calcium and magnesium solution designed for fast absorption and predictable results. Chelation helps keep nutrients soluble and available, which is especially important in hydroponics, coco, and fast-draining mixes.
It is formulated as NPK 4-0-0 with a focused mineral profile: Calcium 6%, Magnesium 2%, and Iron EDTA 0.1%, plus trace minerals. This makes it ideal for preventing and correcting issues linked to calcium and magnesium limitation without overloading your main feeding program.
Because it is double the strength of many standard Cal-Mag formulas, you can achieve effective results with lower volumes, which helps keep your nutrient routine simple and consistent.
Pairs well with your main feeding:
Use Cal-Mag as the stabiliser alongside your core fertiliser, particularly in high-demand setups or growth stages where calcium and magnesium are easily depleted.
Use Cal-Mag as the stabiliser alongside your core fertiliser, particularly in high-demand setups or growth stages where calcium and magnesium are easily depleted.
Cal-Mag FAQ
What is Cal-Mag used for?
Cal-Mag is used to prevent or correct calcium and magnesium deficiencies that cause weak new growth, tip burn, blossom end rot, poor setting, and leaf discolouration that doesnโt improve with normal feeding.
When should I add Cal-Mag to my feeding routine?
Add Cal-Mag when growing in coco, hydroponics, or containers with frequent watering, when using reverse-osmosis or very soft water, or when plants are growing fast and showing early calcium or magnesium stress.
Can I use Cal-Mag in soil as well as hydroponics?
Yes. Cal-Mag can be used in soil, potting mix, coco, and hydroponics. It is most useful where calcium and magnesium are not staying available in the root zone.
How much Cal-Mag should I use per litre?
A typical rate is 0.5โ1 ml per litre of water. Start at the lower end if you are unsure, then adjust based on plant response and your growing setup.
Does Cal-Mag replace my normal fertiliser?
No. Cal-Mag is a supplement. It supports structure and photosynthesis so plants can properly use your main fertiliser, especially during high demand.
Why does Cal-Mag help prevent blossom end rot and tip burn?
Blossom end rot and tip burn are commonly linked to calcium not reaching new tissue consistently. Cal-Mag supports steady calcium availability, which improves tissue strength and reduces these disorders when combined with consistent watering.
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