You could be feeding your plants perfectly and still starving them
If your plants are showing yellowing leaves, slow growth, or deficiency symptoms despite regular feeding, pH is often the reason. It is one of those factors that feels technical until you understand what it is actually doing — after that, it becomes straightforward to manage.
What pH is actually doing
pH is a measure of how acidic or alkaline your water is, on a scale from 0 to 14. Seven is neutral. Below 7 is acidic, above 7 is alkaline.
The reason it matters so much for plants is that each nutrient — nitrogen, phosphorus, calcium, iron, and so on — becomes chemically available to roots only within a certain pH range. Outside that range, the nutrient is still present in the water or soil, but the plant cannot absorb it. This is sometimes called nutrient lockout. You can pour in a perfectly balanced feed and have almost none of it reach the plant, simply because the pH is off.
This is why pH is not an optional detail. It is the thing that determines whether everything else you are doing actually works.
Target ranges
The right pH depends on your growing method.
Soil and coco
In soil, aim for 6.0 to 7.0, with the sweet spot being around 6.2 to 6.8. Soil has some natural buffering capacity — meaning it resists sudden swings — so you have a little more room for error. Coco coir behaves more like hydroponics and is better kept at the lower end of this range, around 5.8 to 6.3.
Hydroponics and water culture
In any system where roots sit directly in water or mist — DWC, NFT, aeroponics — the target is tighter: 5.5 to 6.5, ideally 5.8 to 6.2. Without soil to buffer anything, small fluctuations have a more immediate effect, so checking pH every day or two is worth the habit.
Where UK tap water usually sits
UK tap water is generally alkaline, typically falling somewhere between 7.0 and 8.5 depending on your area. Hard water regions — much of the South East and Midlands — tend to sit toward the higher end of that range. Scotland and parts of Wales often have softer, slightly lower-pH water, but it still usually comes in above 7.0.
This means most UK growers using tap water need to bring their pH down before watering. Left uncorrected, alkaline water will gradually push the root zone pH upward, locking out nutrients over time even when your feed schedule looks correct.
How to measure it
A digital pH pen gives you a reading in seconds. Mix your water and nutrients first, then dip the probe, wait for the reading to settle, and compare it to your target range. The 5-in-1 Digital Water Quality Tester Pen covers pH alongside TDS, EC, and temperature — useful if you also want to keep an eye on nutrient concentration in one go.
Whichever meter you use, rinse the probe with clean water after each use and calibrate it periodically using a known reference solution. A drifting meter gives false confidence, which is worse than no meter at all.
How to adjust pH
pH adjusters come as liquid solutions labelled pH Up and pH Down, and are widely available from grow shops. They are concentrated, so small amounts go a long way.
To lower pH (most common for UK tap water)
Add a few drops of pH Down — typically phosphoric or citric acid — to your mixed water, stir well, and test again. Add gradually. It is easy to overshoot, and chasing the reading back and forth with Up then Down wastes both solutions and time.
To raise pH
pH Up is usually potassium hydroxide. The same approach applies — add a small amount, mix thoroughly, test, and repeat if needed.
- Always add nutrients to your water before adjusting pH. Nutrients change the pH of the water, so testing before you add them gives you a meaningless number.
- Mix thoroughly after each addition before testing — stratified water reads inconsistently.
- Aim for the middle of your target range rather than the edges, so you have room for drift between watering.
How often to check
In soil, testing every watering is sensible until you have a feel for how stable your source water is. Once you know, for example, that your tap water reliably sits at 7.6 and needs four drops of pH Down per litre to reach 6.5, you can work more quickly. In hydroponics, check the reservoir pH every one to two days. Roots, beneficial microbes, and nutrient uptake all shift the pH of standing water over time.
What to do next
Test your tap water before your next watering and write the reading down. That single number will tell you immediately whether pH correction is something you need to build into your routine — and for most people in the UK, it is.
Read next
- Is UK tap water OK for plants? What you need to know
- Am I Overwatering My Plants? How to Tell for Certain
- The Order You Wish You'd Bought Things In
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