pH Explained: The Right pH for Soil, Coco and Hydro

You can buy the best nutrients on the shelf, mix them at exactly the right strength, and still watch your plants go pale and stall — because at the wrong pH they simply cannot take the food up. pH is the single cheapest thing to get right and the most expensive thing to ignore, and it costs about twenty pounds to start measuring it properly. This guide covers what the number actually means, where to aim it in soil, coco and hydroponics, how to measure it without being lied to, and how to move it when it drifts.

What pH actually is, and why plants care

pH is a measure of how acidic or alkaline a solution is, on a scale from 0 to 14. Seven is neutral, below seven is acidic, above seven is alkaline. The important thing — and the bit that catches people out — is that the scale is logarithmic. Each whole point is a tenfold change. A feed at pH 5.0 is ten times more acidic than one at 6.0 and a hundred times more acidic than one at 7.0. That is why "it's only half a point out" is not the small problem it sounds like.

Plants don't have an opinion about pH itself. What they care about is that pH controls which nutrients are chemically available for the roots to absorb. Every element in your feed — nitrogen, phosphorus, potassium, calcium, magnesium, iron, manganese, zinc — has a band of pH within which it stays in a form the plant can take up. Move outside that band and the element is still sitting in the pot or the tank, it just may as well not be there.

This is what people mean by nutrient lockout, and it is the reason pH problems disguise themselves so well. A plant that cannot absorb iron looks exactly like a plant that has run out of iron: pale new growth, yellowing between the veins. The instinctive response is to feed it more — which makes the solution stronger, does nothing to fix availability, and often makes things worse. A deficiency that appears while you are feeding normally is a pH problem until proven otherwise.

Where to aim: soil, coco and hydroponics

There is no single correct pH, because the medium does half the work. Soil is a buffer — it holds a reserve of chemistry that resists change, and it hosts microbial life that shifts availability in your favour. That forgiveness means soil growers can run higher and worry less. Coco and hydroponics have almost no buffering at all, so what you put in is what the roots get, and the target band is both lower and tighter.

In soil, aim for roughly 6.0 to 7.0, with most growers settling around 6.3 to 6.8. The soil's own buffering pulls whatever you pour in back toward its natural point over a few hours, so precision matters less than consistency. If you are growing in a good pre-mixed compost and using tap water in the sevens, you may genuinely never need to adjust anything.

In coco coir, aim for 5.8 to 6.2. Coco behaves like an inert sponge that happens to hold onto calcium and magnesium, which is why coco feeds need CalMag and why the band sits lower than soil. Our coco feeding guide covers the whole schedule if that's your medium.

In hydroponics — DWC, NFT, flood and drain, clay pebbles — aim for 5.5 to 6.0. There is no medium to buffer anything, so the tank is the whole story and it needs checking daily rather than weekly. Many hydro growers deliberately let the number wander slowly across the band rather than pinning it, on the reasoning that different elements peak at slightly different points and a gentle drift gives everything a turn.

How to measure it without being lied to

There are two honest ways to read pH at home, and one that people try first and regret. A digital pH pen is the standard: dip the probe, read the number, rinse it. It's fast, it's precise enough to work to a tenth of a point, and it will keep working for years provided you look after the probe. A liquid drop-test kit is the low-tech alternative — a few drops of reagent into a sample of your feed, then match the colour against a chart. It's slower and coarser, but it needs no calibration, no batteries, and it never quietly goes wrong.

The route to avoid is paper test strips. They're cheap and everyone has a pack somewhere, but the colours are ambiguous in the exact range you care about and they age badly in a damp shed. If you're deciding between a strip pack and a proper drop kit for the sake of a couple of pounds, buy the drop kit.

Pens are worth spending on, and the price ladder is mostly about how stable the reading is and how long the probe lasts. An entry-level pen tells you the truth for a season and needs calibrating often. A mid-range waterproof unit holds calibration longer and survives being dropped in the tank. At the top, a lab-grade pen with a replaceable probe reads steadily for years and is the one to buy if you'll still be growing in five years' time.

If you'd rather not own an electronic thing at all, or you want a second opinion when a pen starts reading oddly, a drop kit is the honest backup. It costs a few pounds, lives in a drawer for years, and settles the argument when you suspect the pen has drifted. Plenty of experienced growers keep one for exactly that reason.

Calibration, and the probe care nobody does

Here is the part that quietly ruins more grows than bad nutrients ever have. A pH pen does not stay accurate on its own. The glass probe changes as it ages and as it sits in solutions, and the drift is gradual and invisible — the pen keeps giving you a confident number to one decimal place while being half a point wrong. If you have never calibrated yours, you do not currently know your pH. You know what your pen says about it.

Calibration means showing the pen two solutions of known pH and letting it correct itself. Those solutions are Buffer 4 and Buffer 7, and a two-point calibration using both is far better than one, because it fixes the slope of the reading and not just its offset. The routine takes about two minutes: rinse the probe, sit it in Buffer 7 until the number settles, confirm, rinse again, repeat in Buffer 4, confirm. Never pour used buffer back into the bottle — decant what you need into a small pot and throw it away afterwards.

How often depends on use. Monthly is a sensible default for a home grower; weekly if you're reading a reservoir every day or the pen is a cheap one. Always calibrate after a new probe, after a battery change, and any time a reading makes you frown.

Cleaning matters too. Nutrient salts and biofilm build up on the glass and slow the response until readings take a minute to settle and then land in the wrong place. A dedicated probe cleaner used every month or two takes that off without scratching the glass — which is what happens if you attack it with a cloth.

Adjusting it: order of operations, and a light hand

The single most common mistake is adjusting the water before adding the nutrients. Base feeds are acidic and will pull the number down substantially on their own — sometimes a full point — so anything you did to the plain water is undone the moment you mix. Fill, add every nutrient and additive you intend to use, stir it properly, and only then read and adjust. Doing it in that order usually means you need far less adjuster than you expect, and on some feed and water combinations you need none at all.

The adjusters themselves are straightforward. pH Down is typically a phosphoric or nitric acid solution; pH Up is usually potassium hydroxide. Both are strong and both are extremely concentrated — on a 10-litre mix you are looking at drops or millilitres, not capfuls. Add a small amount, stir it right through, wait a couple of minutes, then read again. The commonest way to waste an evening is to add more before the last dose has finished working, overshoot, correct back the other way and end up with a tank full of salts you never wanted.

Your water source matters more than most people realise. Hard tap water — most of southern England — carries dissolved carbonates that act as a buffer, resisting your adjustment and then dragging the number back up over the following day. That's why some growers find their reservoir climbs overnight no matter what they do. Soft water does the opposite: it barely resists at all, so a small dose swings it wildly and it's easy to overshoot. Neither is a problem once you know which you have — it just changes whether you dose in millilitres or in drops.

Which brand you choose matters much less than being consistent. If you're growing organically, use an organic regulator — a citric-acid-based pH down won't feed the soil life the way mineral acids can disrupt it. If you're running mineral nutrients in coco or hydro, the standard phosphoric acid products are what everything else is formulated around.

When it drifts, and how to read the symptoms

A reservoir does not hold still. As roots take nutrients out of solution the balance shifts, water evaporates and concentrates what's left, and the buffering in your tap water keeps pulling in its own direction. In a recirculating system, expect the number to move every day and check it daily. A drift of a tenth or two overnight is normal and needs nothing. A drift of half a point or more, repeatedly, usually means the reservoir is too small for the plants drinking from it, or it's overdue a full change.

In coco and soil the equivalent check is run-off — catching what comes out the bottom and measuring it. The run-off tells you what is actually happening at the roots, which can be some way from what you poured in. If you're feeding at 6.0 and run-off comes back at 5.2, something in the pot is driving it down and no amount of adjusting the input will fix it; that's a flush-and-reset situation rather than a dosing one.

One more thing worth knowing: temperature affects the reading. Measuring an ice-cold reservoir in an unheated garage and a warm tank in a tent will not give you comparable numbers from the same solution. Most decent pens compensate automatically, and cheap ones often don't — which is one of the real differences the price ladder buys you. Measure at roughly the temperature you feed at and you sidestep the issue entirely.

If you're running a large system, or several, the job can be automated — a controller sits in the tank, reads continuously and doses acid or alkali to hold a set point. It is a serious piece of kit at a serious price, and it earns its keep only when the manual version has become a daily chore you resent. For one tent and one reservoir, a pen and two bottles is the right answer.

Quick answers

What pH should I feed at?

It depends on your medium. In soil, aim for about 6.0 to 7.0 — soil buffers, so it forgives a lot. In coco coir, aim for 5.8 to 6.2. In hydroponics such as DWC or NFT, aim for 5.5 to 6.0. Anywhere inside the band is fine, and chasing decimal places is not worth the effort. The tighter targets for coco and hydro exist because neither medium buffers the way soil does.

Should I pH the water before or after adding nutrients?

Always after. Base nutrients are acidic and will pull the pH down significantly on their own — sometimes by a full point — so anything you adjust beforehand gets undone the moment you mix. Fill the container, add every nutrient and additive you plan to use, stir it thoroughly, then read and adjust. Doing it in that order usually means you need much less adjuster than expected.

How often should I calibrate a pH pen?

Monthly is a sensible default for a home grower, or weekly if you read a reservoir daily or own a budget pen. Always calibrate after fitting a new probe, after a battery change, and any time a reading looks wrong. Use a two-point calibration with Buffer 7 and Buffer 4 rather than one point — it corrects the slope of the reading, not just its offset. An uncalibrated pen gives you a confident number that can be half a point out.

Why do my plants look deficient when I'm feeding them properly?

That is the classic signature of nutrient lockout. pH controls which elements are chemically available to the roots, so at the wrong number the nutrients are present but unusable, and the plant shows the same symptoms as a genuine shortage. Above roughly 6.5, iron, manganese and zinc lock away and new growth goes pale. Below about 5.5, calcium and magnesium struggle. Adding more feed makes it worse — check the pH first.

How should I store a pH probe?

Wet, in proper KCl storage solution, with the cap on. Never store a probe dry, and never store it in deionised or RO water — pure water leaches the probe and damages it quickly. Letting a probe dry out is the single most common way growers kill a pen that was working fine. If yours has dried out, an overnight soak in storage solution sometimes revives it, though not always.

Can I use pH test strips instead of a pen?

You can, but they are the weakest option. The colour differences are hard to read in exactly the range that matters, and strips degrade in damp storage. A liquid drop-test kit costs only a little more, is far easier to read against its chart, and needs no calibration or batteries. Many experienced growers keep a drop kit alongside a pen specifically as a second opinion when the pen starts looking suspect.

Why does my reservoir pH keep rising overnight?

Usually hard water. Dissolved carbonates act as a buffer, resisting your adjustment and pulling the number back up over the following hours. It is very common across much of the UK. The fix is smaller, more frequent corrections rather than one large one, and if it is a constant battle, switching to RO or filtered water removes the buffering entirely. Plants drinking heavily from an undersized reservoir will also shift pH faster.