pH, EC and Water Quality: Targets, Meters and the Cal-Mag Question

What pH and EC should you run, and does water quality matter? In coco and hydroponics, target a root-zone pH of 5.5–6.2; in soil, aim for 6.0–6.8. EC — electrical conductivity, the measure of dissolved nutrient salts — should climb from roughly 0.4–0.8 mS/cm for seedlings to a peak of 1.6–2.0 in flower. Coco binds calcium and magnesium and RO water contains none, so both need Cal-Mag at every feed.

At a glance	Target
Soil pH	6.0–6.8
Coco pH	5.5–6.2
Hydro pH	5.5–6.2 (most hold 5.8–6.0)
Seedling EC	0.4–0.8 mS/cm
Peak flower EC	1.6–2.0 mS/cm

pH is the 0–14 scale of how acidic or alkaline a solution is, and it decides which nutrients roots can absorb. Get it wrong and you starve a plant in a full tank of feed — the classic "nutrient lockout" (yellow leaves and pH shows the symptoms).

Who this guide is for: UK home and hobby growers who want pH, EC and water quality right without a lab — soil, coco or full hydro.

What pH should you target, and does it change with your growing medium?

Yes — the ideal pH depends on what your roots are sitting in (choosing a medium is covered in our soil vs coco vs hydro guide).

Soil is buffered: organic matter, clay and microbes absorb pH swings, so the window is wider and most nutrients stay available around 6.5. Coco has only a slight buffer and hydro none, so swings act fast and the target is tighter. Manufacturer CANNA puts the best overall nutrient-availability window at pH 5.2–6.2 across its substrates, narrowing to 5.5–6.2 for coco and 5.8–6.2 for its Terra potting mix (CANNA FAQ).

In coco and hydro, let pH drift gently across the range rather than pinning one number — calcium and magnesium absorb best above 6.0, some micronutrients lower. Rockwool and clay pebbles are inert too, so prep inert media properly. Check feed pH before it goes in, and in a recirculating system check the reservoir too (see our reservoir management guide). A hot room warms the reservoir too: cool your grow tent first. New to hydro? Try our beginner's guide.

EC, CF and PPM — how do the scales convert?

EC (electrical conductivity) is the true, international measurement of dissolved nutrient, read in millisiemens per centimetre (mS/cm). Those salts are the nutrients in our NPK guide. CF and PPM are just EC dressed up differently, so one tank can show three "different" numbers.

• CF (Conductivity Factor) removes the decimal point: CF = EC × 10.
• PPM 500 (the "500" or US scale, used by most North American meters) is EC × 500.
• PPM 700 (the "700" or European/Australian scale, used by Bluelab) is EC × 700.

EC 1.5 shows as 750 ppm on a 500-scale meter but 1,050 ppm on a 700-scale meter. That gap is why Bluelab recommends sticking to EC, the one scale that never needs converting.

EC (mS/cm)	CF	PPM 500 (US)	PPM 700 (EU/AUS)
0.5	5	250	350
1.0	10	500	700
1.5	15	750	1,050
2.0	20	1,000	1,400
2.5	25	1,250	1,750

What EC should you feed at each stage?

Feed strength rises with growth, then falls at the end. These are broad targets — begin at your nutrient line's recommended EC and adjust to the plant (our full-cycle feeding schedule charts them week by week). On hard UK tap water, final EC is nutrient EC plus the water's own EC, so measure raw water first.

Stage	Typical EC (mS/cm)	Notes
Seedlings / clones	0.4–0.8	Tender roots burn easily
Early vegetative	1.0–1.4	Build up gradually
Late veg / early flower	1.4–1.8	Peak feeding demand
Mid–late flower	1.6–2.0	Hold steady; watch for tip burn
Flush	0.0–0.4	Plain or very light feed

Cuttings? See the propagation masterclass. PK boosters add to total EC, so check the PK booster EC ceilings before using one.

If runoff or reservoir EC climbs well above what you feed, salts are building up towards lockout and stresses roots (root-zone health) — the fix is in our deficiency and flushing guide.

How do you calibrate a pH meter, and how often?

Calibrate a pH meter with two fresh buffer solutions, pH 7 first and then pH 4, at least monthly and after every clean. The glass probe drifts, so calibration resets it against solutions of known pH. These steps follow Bluelab's pH Pen guidance and suit most probes:

1. Clean and hydrate the probe first; a dirty or dry probe will not calibrate.
2. Calibrate in order: fresh pH 7 buffer first, then rinse and repeat in fresh pH 4.
3. Use fresh pH calibration solutions every time. Pour a little into a clean cup, never pour used buffer back and never calibrate in tap water. Growth Technology pH Buffer comes as separate pH 4 and pH 7 solutions.
4. Check the result. If your meter shows slope, around 95–105% suggests a healthy probe; outside it usually points to a fouled or damaged probe, or old buffer (Commercial Water Lab).
5. Never store the probe dry or in RO or distilled water. Use KCl storage solution, such as Growth Technology pH Probe Refill, and top up the cap weekly.
6. Replace a worn probe. Bluelab puts a pH pen's life at about 18 months; replace it if it will not calibrate after cleaning and hydrating. Bluelab meters, monitors and controllers take a replacement Bluelab pH Probe.

An EC meter is different. The Bluelab Truncheon Nutrient Meter is the shop-floor default: factory calibrated and needs no calibration, ever — no buttons, switches or knobs. It reads EC, CF and both PPM scales across EC 0.2–3.6, is temperature-compensated and waterproof, and carries a 5-year limited warranty. That is why a robust EC-only tool plus a well-kept pH pen beats a cheap all-in-one.

pH Up vs pH Down — do you need both, and what are they?

You'll reach for pH Down far more, because most feeds and hard UK tap water sit too alkaline once mixed — but keep both to correct an overshoot.

• pH Down is an acid that lowers pH. Dutch Pro pH Down Bloom is phosphoric acid; Canna Organic pH- is citric acid, which suits organic grows. Add a little at a time to mixed feed, stir, wait, re-test.
• pH Up is an alkali — Growth Technology pH Up is a 25% potassium hydroxide solution — that raises pH the same careful way.

Add nutrients and any Cal-Mag first, mix, then adjust pH, since feed shifts pH again. A grow- or bloom-specific pH Down is mostly convenience; the acid does the same job.

UK tap water vs RO water for plants — what's the difference for pH and EC?

UK tap water is deliberately kept slightly alkaline. The statutory pH band is 6.5–9.5 (Water Supply (Water Quality) Regulations 2016), and companies aim high in it to protect pipework — so your tap often starts near pH 7.5–8 with minerals already dissolved.

Reverse osmosis (RO) water is the opposite: a healthy membrane rejects 95–99% of dissolved solids, leaving near-zero EC and a neutral pH — a blank canvas for coco, hydro and precise recipes, as long as you add Cal-Mag. Our water treatment range covers RO and filtration.

In a hard-water area — the Drinking Water Inspectorate classes supplies above 200 mg/L of calcium carbonate as "hard", and much of southern and eastern England sits well into that band — your tap already carries background EC, eating into your headroom. That is when RO, or a hard-water nutrient such as Dutch Pro Hydro/Coco Bloom A&B Hard Water, earns its place (see our nutrient line comparison).

Why do coco and RO water need Cal-Mag every feed?

Because both remove calcium and magnesium before your plant can use them — coco by binding, RO by having none.

Coco coir has a high cation exchange capacity: its fibres grab calcium and magnesium out of solution and hold them, releasing sodium and potassium instead. Even pre-buffered coco keeps binding calcium, so without a top-up every feed the plant runs short — rusty spots and yellowing between veins on older leaves. RO water starts with none, so you add everything.

The fix is the same for both: add a Cal-Mag supplement at every feed, mixed in before you adjust pH, and follow the dose on the bottle. If your base nutrient is a coco-specific line (like Canna Coco A&B), it already accounts for some of this — check the label before doubling up. Some other ranges build calcium and magnesium in, so see the FloraMax nutrient guide before adding a separate Cal-Mag.

FAQ

What pH should hydroponic water be? For hydroponics and coco, target a root-zone pH of 5.5–6.2, with most growers holding around 5.8–6.0. In soil, aim higher at 6.0–6.8, because soil buffers pH while water and coco do not.

How often should you calibrate a pH meter? Calibrate a pH meter at least once a month and after every clean, using fresh pH 7 then pH 4 buffer solutions in that order. A conductivity meter such as the Bluelab Truncheon is factory calibrated and never needs calibrating.

Do you need Cal-Mag with tap water? It depends on hardness. Hard UK tap water already carries calcium and magnesium, so you may need little or none. Soft tap water, all RO water and all coco grows need Cal-Mag every feed, because coco binds calcium and RO contains none.

What's the difference between EC and ppm? EC (electrical conductivity, in mS/cm) is the true measurement of dissolved nutrient. PPM is EC multiplied by 500 (US scale) or 700 (European/Bluelab scale), so one tank reads two different ppm numbers. Match your meter's scale to your nutrient bottle, or just use EC.

Can you use tap water for hydroponics? Yes, with care. UK mains water sits within a statutory pH band of 6.5–9.5, but it is usually alkaline and may be hard, so you'll pH it down and account for its starting EC. In hard-water areas, many switch to RO water.

Recommended kit and further reading

Here's the kit we point UK growers to:

• Bluelab Truncheon Nutrient Meter — the no-calibration EC/CF/PPM workhorse; drop it in and read the lights.
• Bluelab pH Pen and Bluelab Combo Meter — accurate pH, or combined pH/EC/temp.
• Bluelab Guardian Monitor WiFi — continuous monitoring. For automatic pH dosing you want a controller such as the Bluelab pH Controller; our monitors and controllers guide covers the options.
• Growth Technology pH Up, Dutch Pro pH Down Bloom and Canna Organic pH- — the adjusters that get you into range.
• Cal-Mag: BioBizz CalMag, Canna CalMag Agent, Dutch Pro CalMag and Shogun CalMag — essential for coco and RO grows.

Most relevant collection: if you only click one link, start with pH/EC Testers & pH Up/Down.

Related guides: plant nutrients explained · choosing your growing medium · reservoir management and pH · nutrient deficiencies and flushing · hydroponic systems compared · first hydroponics shopping list · grow room setup checklist.

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Why trust The Grow Superstore

We've been helping UK indoor growers since 2009, both online and from our physical store in Peterborough (6 Wulfric Square, Bretton, PE3 8RF). Setting up meters, dialling in feeds and diagnosing "mystery" deficiencies is advice we give in store, and more often than not in coco or RO grows the answer is the same: check your pH, then check your Cal-Mag. Pop in for a free in-store consultation, or get in touch and we'll match a meter and pH kit to your local water.