pH: The Bouncer at the Nutrient Door

What You Need to Know

You can do everything right at the feeding bench — perfect ratios, premium nutrients, measured to the millilitre — and still watch the plant starve in front of you. When that happens, the culprit usually isn’t the food. It’s the bouncer at the door deciding what gets let in. That bouncer is pH.

pH is a measure of how acidic or alkaline your root zone is, on a scale where lower is more acidic and higher is more alkaline. Here’s why it matters more than almost any other number: every nutrient has a pH range where the plant can actually absorb it. Drift outside that range and the nutrient binds to other elements and goes chemically unavailable. It’s still physically there in the medium — you paid for it, you mixed it — but the roots can’t touch it. That’s lockout, and it’s behind a huge share of the “deficiencies” beginners chase.

The Ranges By Medium

The right range depends on what your roots are sitting in — which is exactly why the last lesson came first.

• Soil: pH 6.0–7.0, sweet spot 6.2–6.8. This is where the major and minor nutrients overlap in availability. Drift below 6.0 and calcium, magnesium and phosphorus start to lock out. Drift above 7.0 and iron, manganese and zinc disappear.
• Coco and hydro: pH 5.5–6.5, sweet spot 5.8–6.2. Lower, because the medium is inert and the chemistry of the nutrient solution behaves differently without soil’s buffering.

You’re not chasing one exact number. You’re staying in a band, and a gentle drift within it is actually helpful — different nutrients peak in availability at slightly different points, so bouncing between, say, 6.2 and 6.8 in soil means everything gets its turn at the door.

Lockout — The Deficiency That Isn’t One

Picture the grower who does everything by the book. His plant shows magnesium deficiency, so he adds magnesium. It gets worse. Then calcium symptoms appear too, so he adds CalMag. Worse again. He’s feeding a balanced diet and the plant is going downhill — because he never checked pH. The food was always there. The door was just locked.

Here’s the cruel mechanism: feed more of a locked-out nutrient and it piles up unused, often locking out another nutrient in turn. So the harder you feed, the worse it looks. That’s the tell that separates lockout from real hunger — a genuine single deficiency creeps up the plant cleanly and improves when you feed; lockout shows several symptoms at once and gets worse the more you add.

Seb’s Corner — why pH controls availability. Nutrients live in the root zone as charged ions. Whether an ion stays free for the roots to absorb, or bonds with something else into an insoluble compound the roots can’t use, depends heavily on the acidity around it. Phosphate, for instance, stays available in a fairly narrow band — too acidic and it ties up with iron and aluminium, too alkaline and it locks with calcium. Iron does the reverse: freely available when acidic, precipitates out as the pH climbs. There’s no single pH where every nutrient is maximally available — the soil sweet spot of 6.2–6.8 is simply the range where the most elements overlap with the fewest locked out. That’s why the chart looks like a set of overlapping bands rather than one clean line, and why staying in range beats hitting any single number.

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How To Apply This

1. Get a pH pen and calibrate it monthly with buffer solutions, or it starts lying to you. A reliable pen is the one tool that separates growers who fix real problems from growers who chase phantom ones.
2. Test the input — your water after you’ve added nutrients, before it goes in the pot. Aim for your medium’s band: 6.0–7.0 in soil, 5.5–6.5 in coco/hydro.
3. Test the runoff — the water that comes out the bottom. The gap between input and runoff tells you what’s happening in the root zone. Input 6.5, runoff 5.2 means the root zone has drifted acidic — that’s your lockout.
4. Adjust gently with pH up or pH down, a few drops at a time, stir, retest. You’re staying in the band, not hitting a bullseye.
5. Fix pH first, then wait — don’t feed. Correct the pH and leave the nutrients alone. More often than not the “deficiency” corrects itself over the following week, because the food was always present.
6. If salts have built up, the lockout can sit on top of a salt buildup — flush with plain pH’d water until the runoff settles, then resume at reduced strength.

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Watch Out For

This is the one that fools everyone, so don’t take it personally when it fools you.

Reaching for the bottle before the pen. The instinct when you see yellowing is to feed. But pH sits at level three of the diagnostic order — environment, water, pH, nutrients — and most growers never get there because they jump straight to nutrients. Before you diagnose any single deficiency, check pH. Nine times out of ten the food was never the problem.

Feeding into a lockout. Adding more of a nutrient the plant can’t absorb makes it worse, not better — the surplus builds up and locks out something else. If symptoms worsen as you feed, stop feeding and check the pen.

Skipping calibration. An uncalibrated pen reads confidently and wrongly. You’ll adjust to a number that doesn’t exist and push the root zone out of range while believing you fixed it. Calibrate monthly, every time, no exceptions.

Chasing a perfect decimal. You don’t need 6.5 exactly. A gentle drift across the band does the plant good. Get in range and stop fiddling.

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Quiz

1. What is nutrient lockout, and how does it differ from a genuine deficiency?
2. Give the target pH band for soil and the separate band for coco/hydro.
3. Why does feeding more of a locked-out nutrient often make the plant look worse?
4. You measure input water at pH 6.5 and runoff at pH 5.2. What does that gap tell you?
5. Why is there no single pH at which every nutrient is maximally available?

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Sources

Chapters 9 and 12, The Grower’s Guide (book draft) — pH ranges by medium, the lockout mechanism, the diagnostic order, and measuring input vs runoff. Ion-availability chemistry (phosphate and iron behaviour across pH) is general soil-science knowledge; no paywalled sources used. Aligned with the GGB pH-lockout grow-guide and the Nutrient Calculator.

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Answer Key

1. Lockout is when a nutrient is physically present in the root zone but the pH is wrong, so it bonds into a form the roots can’t absorb — the plant starves with food in front of it. A genuine deficiency means the nutrient is actually missing. The tell: lockout shows several symptoms at once and worsens as you feed; a real deficiency is cleaner and improves with feeding.
2. Soil: 6.0–7.0 (sweet spot 6.2–6.8). Coco/hydro: 5.5–6.5 (sweet spot 5.8–6.2).
3. Because the locked-out nutrient can’t be taken up, so the surplus accumulates unused and can lock out another nutrient in turn — making the overall picture worse.
4. The root zone has drifted more acidic than your input, which points to a developing lockout — the medium is pulling the pH down out of range.
5. Because different nutrients become available and unavailable at different pH points (phosphate locks at both extremes, iron precipitates as pH rises) — so the best you can do is sit in the band where the most elements overlap.

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Next lesson: Feeding for Real: EC and the Trend — pH decides whether food gets in; now we deal with how much food to send, and why the trend in your runoff matters more than any number on the bottle.