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.

pH is the 0–14 scale of how acidic or alkaline a solution is, and it decides which nutrients your plant's roots can actually absorb. Get it wrong and you starve a plant in a full tank of feed — the classic "nutrient lockout".

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 alive and buffered: organic matter, clay and microbes absorb pH swings, so the window is wider. Coco and hydro are nearly inert, so pH acts immediately 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.

Growing medium	Target root-zone pH	Why
Soil / potting mix	6.0–6.8	Buffered and forgiving; most nutrients stay available around 6.5
Coco coir	5.5–6.2	Slight buffer only; swings act fast, so keep it tight
Hydroponics (DWC, NFT, drip)	5.5–6.2	No buffer; most growers hold 5.8–6.0

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 slightly lower. Check feed pH before it goes in, and in a recirculating system check the reservoir too (see our reservoir management 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). 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.

Because the multipliers differ, the same solution reads very differently: 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 starting targets — begin at your nutrient line's recommended EC and adjust to the plant. 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

If runoff or reservoir EC climbs well above what you feed, salts are building up towards lockout — the fix is covered in our deficiency and flushing guide.

How do you calibrate and care for a pH meter?

A pH meter needs regular calibration; a conductivity meter like the Bluelab Truncheon does not.

The Bluelab Truncheon Nutrient Meter is the shop-floor default: factory calibrated, it needs no calibration, ever — no buttons, switches or knobs. It reads EC, CF, PPM 500 and PPM 700 across a range of EC 0.2–3.6, is temperature-compensated and waterproof, and carries a 5-year limited warranty.

A pH meter is the opposite — the glass probe drifts, so it must be calibrated and kept hydrated. Bluelab's guidance for its pH Pen:

1. Clean and hydrate the probe first — a dirty probe calibrates wrong.
2. Two-point calibration, in order: fresh pH 7 buffer first, then rinse and repeat in fresh pH 4.
3. Calibrate monthly and after every clean.
4. Never store the probe dry or in RO/distilled water — store it in KCl storage solution and top up the cap weekly.

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 without stressing roots.

• pH Down is an acid — commonly phosphoric acid — that lowers pH. Add a little at a time to fully 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 (like the Dutch Pro pair) is mostly convenience; the acid does the same job.

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

UK tap water is deliberately kept slightly alkaline. The Drinking Water Inspectorate sets a statutory pH band of 6.5–9.5, and companies aim high in it to protect pipework — so your tap often starts near pH 7.5–8 and already carries dissolved minerals before you add feed.

Reverse osmosis (RO) water is the opposite: a healthy membrane rejects 95–99% of dissolved solids, leaving near-zero EC — a blank canvas you build up exactly as you choose.

Water source	Typical starting EC	pH tendency	Best for
UK tap (soft area)	Low–moderate	Alkaline (~7.5–8)	Soil and most feeding, with pH Down
UK tap (hard area)	Moderate–high	Alkaline	Usable; watch total EC and Cal-Mag
RO / filtered	Near zero	Neutral	Coco, hydro, precise recipes — add Cal-Mag

In a hard-water area — the UK 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 a background EC before you add feed, eating into your headroom. That is when RO, or a nutrient line's hard-water version, 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 chemically, RO by not containing any.

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 unless you top up every feed the plant runs short — rusty spots and yellowing between veins on older leaves. RO water starts with none at all, so anything the plant needs, you add.

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), it already accounts for some of this — check the label before doubling up.

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, direct measurement of dissolved nutrient. PPM is EC converted using a multiplier — 500 on the US scale, 700 on the European/Bluelab scale — so the same 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 is safe and 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 and build the feed from scratch.

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 (and combined pH/EC/temp).
• Bluelab Guardian Monitor and Bluelab pH Controller — continuous monitoring and automatic pH dosing.
• Growth Technology pH Up, Dutch Pro pH Down Grow and Dutch Pro pH Down Bloom — 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 · Soil vs Coco vs Hydro · Automatic Watering and Reservoir Management: AutoPot, Drippers, Anti-Siphon Valves · Nutrient Deficiencies, Lockout and Flushing: A Visual Triage Guide

All UK orders over £50 ship free (UK mainland, exclusions apply).

---

Why trust The Grow Superstore

We've been helping UK indoor growers since 2009, both online and from our physical store in Peterborough. Setting up meters, dialling in feeds and diagnosing "mystery" deficiencies is advice we give across the counter every week — 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.