Why Calgary Growers Still Use Cal-Mag Even Though Our Water Is Hard

If you grow in Calgary, you have probably heard that you do not need Cal-Mag because Calgary has hard water. That sounds reasonable at first, but it leaves out an important detail. Hard water means your water contains calcium and magnesium, but it does not automatically mean your plants are getting the right amount, the right ratio, or a form that works smoothly in your growing medium and feeding program.

The City of Calgary states that Calgary’s water is considered hard because of the amount of calcium and magnesium carried from the Bow and Elbow River systems. The Government of Alberta also explains that hardness is mainly a measure of calcium and magnesium in water, not a guarantee that the water is ideal for every horticultural use.

What Hard Water Actually Means

In water testing, hardness is usually reported as calcium carbonate equivalent. That is useful for describing general water character, but it is not the same as saying your tap water provides a perfect calcium and magnesium program for container plants. The City of Calgary’s published water data shows hardness values that are firmly in the hard range, and the annual treatment summaries also show measurable calcium, magnesium, and alkalinity in the water supply. You can review the city’s hardness data here and one of the treatment summaries here.

That sounds encouraging, and in some situations it is. Calgary tap water often does contribute useful calcium and magnesium. The problem is that plant nutrition is about availability, balance, and root-zone chemistry, not just whether those minerals exist somewhere in the water.

Why Plants in Calgary Can Still Need Cal-Mag

The biggest reason is that plant nutrition is about usable supply, not just raw presence. A municipal water report can show that calcium and magnesium are in the water, but it does not mean the ratio is ideal for your crop, your nutrient line, your root-zone pH, your medium, or your irrigation practices.

Another major issue is alkalinity. High-alkalinity water can gradually push the pH of a growing medium upward over time, especially in containers and soilless systems. Purdue Extension explains that irrigation water alkalinity can drive substrate pH problems in container production. Penn State Extension also notes that high alkalinity can lead to elevated media pH and nutrient issues. Calgary’s own treatment data includes meaningful alkalinity and bicarbonate values, which means Calgary growers are not just dealing with hardness, but also with water that can influence root-zone chemistry over time.

This matters because once root-zone pH drifts upward, plants can begin to struggle with nutrient uptake even when the minerals are technically present in the irrigation water. In plain English, your water can contain calcium and magnesium while your plants still behave like they are not getting enough of one, the other, or both.

Coco coir makes this issue much more obvious. Oklahoma State University notes that calcium and magnesium often need to be added when using coconut coir. Coco is not an inert medium in the way many people assume. It has cation exchange properties, which means it can interact with nutrient ions and affect how calcium and magnesium behave in the root zone. The University of Florida also discusses coco coir as a hydroponic medium with important chemical properties. This is one of the main reasons growers using coco often still need Cal-Mag even when their tap water is considered hard.

Fast-growing plants can also outpace what hard water alone can provide. Calcium is not very mobile inside the plant, so deficiency tends to show up in newer growth first. Iowa State University and North Carolina State Extension both describe calcium deficiency as affecting young tissue first because calcium does not move easily from older leaves to new growth. Under strong lighting and vigorous growth, plants can run into calcium delivery issues surprisingly quickly.

Magnesium behaves differently. Magnesium is central to chlorophyll, so when plants do not get enough, photosynthesis suffers and older leaves often show interveinal yellowing. The University of Minnesota explains magnesium’s role in chlorophyll and crop function clearly. A plant can therefore show magnesium deficiency even when the local water supply is technically considered hard.

Balance also matters. Calcium, magnesium, and potassium can compete with one another. If one is pushed too far relative to the others, uptake problems can follow. e-GRO discusses this calcium, magnesium, and potassium relationship, and Michigan State University also notes how excess levels of some nutrients can interfere with others. This means a grower can be using hard water and still see symptoms that improve when calcium and magnesium are supplemented properly.

What Cal-Mag Is Actually Doing

Cal-Mag is not some magical secret ingredient. In most cases it is simply a way to make a feeding program more predictable. In Calgary, that often means it is helping offset variation in source water, coco coir’s tendency to interfere with calcium and magnesium availability, aggressive plant growth, nutrient imbalance, or pH-related uptake issues.

That is why the statement “Calgary has hard water, so you do not need Cal-Mag” is too simplistic to be useful. A better statement would be this: Calgary hard water may reduce how much Cal-Mag you need, but it does not guarantee that you never need it.

When Calgary Growers Are Most Likely to Need It

Cal-Mag is most commonly useful in Calgary when growing in coco coir, when using reverse osmosis water, when feeding fast-growing crops heavily, when running strong LED lighting, or when symptoms suggest calcium or magnesium uptake is not keeping up with plant demand.

Coco is the clearest example. If you are growing in coco and assuming Calgary tap water alone will handle calcium and magnesium, you are taking a gamble that often does not pay off. With reverse osmosis water, the situation is even more obvious because the mineral content has been largely stripped out, which means calcium and magnesium usually need to be rebuilt intentionally.

Container size matters too. Purdue Extension points out that small substrate volumes are affected more quickly by alkalinity. In other words, the smaller and more controlled the root zone, the less forgiving the system becomes.

When You May Need Less

Not every plant in Calgary needs Cal-Mag at every watering. Slow-growing houseplants in a decent potting mix may do perfectly well on Calgary tap water and a complete fertilizer. Some soils and compost-based mixes already provide a buffer that makes supplementation less urgent. Hard water can absolutely contribute useful calcium and magnesium. It just should not be treated as proof that supplementation is never necessary.

What Deficiency Symptoms Often Look Like

Calcium deficiency usually affects newer growth first. You may see distorted new leaves, weak tips, browning along new tissue, stalled growth points, or other damage that seems to hit the top of the plant first. Iowa State and NC State both describe this pattern because calcium is not very mobile within the plant.

Magnesium deficiency more often shows on older leaves first because the plant can move magnesium from old tissue into new growth. That is why the classic symptom is yellowing between the veins on older leaves while the veins remain greener for a time. The University of Minnesota and the University of Maryland Extension both describe this general pattern.

Conclusion

Calgary water is hard. That part is true. It contains real calcium and magnesium, and the city publishes data showing exactly that. But hard water is not the same thing as a crop-specific Cal-Mag program, and it is definitely not the same as perfect calcium and magnesium availability in every medium.

If you are growing in coco, feeding aggressively, using small containers, running bright lighting, or dealing with root-zone pH drift, Calgary hard water does not make you immune to calcium and magnesium problems. In many cases it just changes how much supplemental Cal-Mag you need to use.

So yes, Calgary growers often still need Cal-Mag. Not because Calgary water contains none, but because plant nutrition is more complicated than a municipal hardness rating.

Sources

City of Calgary: Water quality, hardness and data
City of Calgary: Glenmore Water Treatment Plant Summary
Government of Alberta: Common water quality terms
Purdue Extension: Alkalinity Management for Container Substrates
Penn State Extension: Interpreting Irrigation Water Tests
Oklahoma State University: Soilless Growing Mediums
University of Florida: Common Media Used in Hydroponics
University of Minnesota Extension: Magnesium for Crop Production
Iowa State University: Nutrient Deficiencies and Application Injuries in Field Crops
North Carolina State Extension: Calcium Deficiency
e-GRO: Calcium, Magnesium, and Potassium Interactions
Michigan State University: More Reasons for Soil Testing
University of Maryland Extension: Nutrient Deficiency of Trees and Shrubs

This article is for general educational purposes only. Water chemistry, medium chemistry, fertilizer composition, and crop demand all interact, so individual results can vary.