Calcium and magnesium: the nutrients nobody talks about
Cal-mag issues cause more hydro problems than NPK ever will. When your tap water provides enough, when it doesn't, and what deficiency looks like.
Most hydroponic growers obsess over nitrogen, phosphorus, and potassium. These are the numbers on the bottle, the macros you measure, the values you target. Meanwhile, calcium and magnesium quietly cause more deficiency problems than NPK combined, and the fix (or the prevention) depends almost entirely on what's already in your tap water.
Why calcium and magnesium matter
Calcium is a structural component of cell walls. Without enough of it, new growth comes in distorted: cupped leaves, brown edges on young tissue, blossom end rot on tomatoes and peppers. Calcium is immobile in the plant, meaning the plant can't pull it from older leaves to feed new ones. Once a calcium deficiency appears, it shows up at the growing tips first.
Magnesium sits at the center of every chlorophyll molecule. It's literally the atom that makes photosynthesis work. Without enough magnesium, the plant loses the ability to harvest light energy. Magnesium is mobile, so the plant pulls it from older leaves to support new growth. Deficiency shows as interveinal chlorosis (yellowing between the veins) on lower leaves first.
The relationship between calcium, magnesium, and potassium also matters. These three cations compete for uptake through root membranes. Too much potassium can block magnesium uptake even when magnesium is present in the solution. The general target ratio is roughly 3:1 calcium to magnesium by weight, though this varies by crop.
Your tap water changes everything
Here's where hydroponics diverges from soil growing. In soil, calcium and magnesium are released slowly from minerals and organic matter. In hydro, your water source is the baseline.
Hard tap water (above 150 parts per million total dissolved solids, EC above ~0.3 mS/cm) usually contains meaningful calcium and magnesium from dissolved limestone, chalk, or dolomite. If your water report shows 40+ parts per million calcium and 10+ parts per million magnesium, you may already have enough of both before adding any nutrients. Many commercial nutrient lines sell "hard water" formulations that account for this by reducing the calcium and magnesium in the concentrate.
Soft tap water (below 70 parts per million TDS) has minimal calcium and magnesium. You'll need to supplement. This is where cal-mag products come in, or you can add calcium nitrate and magnesium sulfate (Epsom salt) separately for more control.
RO water has almost zero calcium and magnesium. Everything must be added. This gives you the most control but also the most responsibility. Many growers using RO add back a small amount of tap water (15-25%) to provide a calcium and magnesium base without needing heavy supplementation.
If you don't know your water's hardness, your municipal water utility publishes an annual water quality report. Look for calcium, magnesium, and total hardness (often reported as mg/L of calcium carbonate equivalent). If you're on well water, get a test kit or send a sample to a lab.
Deficiency symptoms
Calcium deficiency
New growth is affected first because calcium is immobile. Look for:
Leaf tips and edges turning brown and crispy on young leaves while older leaves look fine. Distorted, cupped, or curling new leaves. Growing tips that die back or become stunted. Blossom end rot on tomatoes, peppers, and squash (the bottom of the fruit turns black and mushy).
Calcium deficiency can also be caused by humidity extremes. Calcium moves into the plant through transpiration, so high humidity (above 85%) reduces transpiration and limits calcium delivery to leaf tips and fruit, even when the solution has plenty of calcium.
Magnesium deficiency
Older leaves are affected first because magnesium is mobile. Look for:
Yellowing between the veins (interveinal chlorosis) starting on lower leaves. Veins stay green while the tissue between them turns pale yellow. In advanced cases, the yellow areas brown and die. Leaves may curl upward along the edges.
The most common cause of magnesium deficiency isn't missing magnesium. It's potassium excess. High potassium in the nutrient solution antagonizes magnesium uptake. If you see magnesium deficiency symptoms, check your K levels before dumping in more mag.
How to supplement
Cal-mag products are the simple approach. Commercial cal-mag solutions typically provide calcium nitrate and magnesium nitrate (or magnesium sulfate) in a pre-mixed ratio. Add it at roughly one to three millilitres per litre depending on your water source and the product concentration. Add cal-mag to your reservoir before adding the main nutrient solution, because calcium can precipitate out if it meets concentrated sulfates or phosphates directly.
DIY approach using dry salts gives you finer control. Calcium nitrate (Ca(NO3)2) provides both calcium and nitrogen. Magnesium sulfate, commonly sold as Epsom salt, provides magnesium and sulfur. You can adjust each independently. When mixing, always dissolve calcium nitrate in water separately before combining it with the rest of the nutrient solution. Calcium nitrate reacts with sulfates and phosphates in concentrated form, creating insoluble precipitates.
Target concentrations vary by crop, but a reasonable starting point for most leafy and fruiting crops: 150 to 200 parts per million calcium and 50 to 75 parts per million magnesium in the final solution. Use the nutrient mixing calculator to dial in the exact amounts for your target EC and crop.
The lockout trap
Adding more cal-mag isn't always the answer. If your EC is already high (above 2.5-3.0 mS/cm), adding more salts can push total EC beyond what the plant can handle, causing nutrient lockout across the board. In hard water areas, the calcium and magnesium from tap water may already be taking up a significant portion of your EC budget, leaving less room for the nutrients you're intentionally adding.
This is the main argument for RO water in hard water areas: starting from zero lets you control the full composition of the solution. If your tap water starts at EC 0.6 and your target is 2.0, you only have 1.4 mS/cm of headroom for nutrients. With RO, you get the full 2.0.
Use the EC to PPM converter to verify your measurements are consistent across meters and scales.
Reading your water report
Municipal water reports list dozens of parameters. For cal-mag decisions, focus on these:
Calcium (Ca): Reported in mg/L (same as ppm). Below 40 parts per million, you need supplementation. 40 to 70 parts per million, you have a partial base that reduces how much you add. Above 70 parts per million, use a hard-water nutrient formula that accounts for this existing calcium.
Magnesium (Mg): Reported in mg/L. Below 10 parts per million, supplement with magnesium sulfate or a cal-mag product. 10 to 25 parts per million, adequate for most crops. Above 25 parts per million, already generous; additional magnesium from nutrients may push the total higher than needed.
Total hardness: Often reported as parts per million of calcium-carbonate equivalent. This combines calcium and magnesium into a single number. Below 60 parts per million (as calcium carbonate) is soft water (supplement cal-mag). 60 to 180 is moderate (may need partial supplementation). Above 180 is hard water (use hard-water nutrient formula, and consider whether RO is worth it to reclaim EC headroom).
Calcium-to-magnesium ratio: Calculate this from the individual calcium and magnesium readings. If calcium is 60 parts per million and magnesium is 8 parts per million, the ratio is about 7.5:1, which is calcium-heavy relative to the 3:1 target. This water provides plenty of calcium but not enough magnesium. In hard water areas, this imbalance is common because limestone aquifers are high in calcium but relatively low in magnesium. The fix: supplement magnesium only (Epsom salt) rather than using a combined cal-mag product that would push calcium even higher.
Use the EC to PPM converter to verify your measurements are consistent across meters and scales.