pH: what it is and how to hold it

What this is (and why one number matters so much)

pH is a 0-14 scale for how acidic or alkaline a liquid is. 7 is neutral, lower is acidic, higher is alkaline. For a grower it is the single setting that decides whether the nutrients you already paid for can actually enter the roots. Get it wrong and a fully fed plant can still starve.

This guide assumes you know nothing about chemistry and builds up from the scale itself to a daily routine you can run. Pure water sits at 7. Lemon juice is around 2 (strongly acidic). Baking soda solution is around 8.5 (mildly alkaline).

One thing trips people up: the scale is logarithmic, so each whole number is a tenfold change in acidity. pH 5 is ten times more acidic than pH 6, and a hundred times more acidic than pH 7.^[8] That is why a reading that looks ‘close enough’ can still be far outside the window your roots need.

Key terms in plain English

These words come back through the rest of the guide. Read them once and the troubleshooting section will read cleanly.

Why pH controls availability (and causes lockout)

Each nutrient stays dissolved, and therefore absorbable, only across a certain pH band. Outside that band it binds into forms the roots cannot take up. That is lockout: the plant is surrounded by food it cannot eat because the root-zone chemistry drifted out of the window.

Push pH too high, above about 6.5 in inert media like coco or hydro, and the micronutrients drop out of solution first: iron, manganese, zinc and boron.^[1] Drop it too low, below about 5.5, and calcium, magnesium and phosphorus lock out instead.^[2]

Phosphorus is the clearest example. It is most available around pH 6.0-7.0, binds with iron and aluminium below 5.5, and binds with calcium above 7.5.^[3] The ‘sweet spot’ exists because it is the pH where the most nutrients overlap as available at once.

Target ranges by substrate: coco, hydro, soil

There is no single correct pH, because the right target depends on what the roots are sitting in. The number you control is the inflow, what you pour in, not the runoff.

In soil, organic matter and microbes buffer the root zone, so aim for inflow water at roughly 6.0-7.0 with a sweet spot of 6.2-6.8.^[8] In coco coir, which is nearly inert with almost no buffering, set the inflow nutrient solution to 5.5-6.5, and many growers run 5.8-6.2.^[4] In hydroponics, target 5.5-6.5 with 5.8-6.2 as the all-nutrient sweet spot.^[5]

Some growers nudge the target slightly within range across the week to favour specific nutrients. As a beginner, pick one number in the sweet spot and hold it.

Measuring it: pen, calibration and care

A pH pen is only as honest as its last calibration. An uncalibrated or dried-out probe is worse than no reading, because it lies with confidence.

Calibrate with fresh two-point buffers, pH 7.0 first then pH 4.0, about once a month. A single-point calibration is not enough to trust across your whole working range.^[7] Store the probe tip wet in KCl storage solution, never dry and never in plain water, which strips the reference electrolyte and permanently kills accuracy. Retire the probe when drift exceeds about 0.2 pH between calibrations or it cannot settle within about 30 seconds.

Adjusting with pH up/down, and the water you start with

Mix your nutrients first, then adjust pH last. Adding nutrients shifts pH on its own, so if you set pH before mixing you will have to redo it.

1. 1
   Mix nutrients

   Add and stir all your feed into the water first.
2. 2
   Measure

   Take a settled pH reading of the mixed solution.
3. 3
   Adjust small

   Add pH Down or pH Up a few drops at a time.
4. 4
   Stir and wait

   Mix it in and give it a moment to react.
5. 5
   Re-measure

   Read again. Repeat in small steps, never dump and chase.

pH Down is usually phosphoric acid and pH Up is usually potassium hydroxide.^[6] Your starting water matters more than beginners expect. Alkalinity is the water's built-in acid-absorbing capacity, mostly bicarbonates, reported as ppm CaCO3, and it is distinct from a high pH reading.^[7] High-alkalinity water fights your acid and creeps the pH back up after you set it.

Runoff pH and a by-stage routine

Runoff is the solution that drains from the pot, and beginners over-rely on it. In inert media like coco it is a momentary, indirect sample distorted by salt buildup and what the roots have done locally. It is not a soil test.^[4]

The reliable lever is the inflow pH you set going in. For the root zone itself, watch runoff EC for salt accumulation rather than runoff pH: a flush is due when runoff EC climbs well above your feed EC. Aim to keep feed and runoff EC within about 10 percent of each other.^[3]

Common mistakes and how to avoid them

Most pH problems are self-inflicted. The classic error is feeding nutrient solution outside the safe range to fix a runoff reading, which causes the very lockout the grower fears.^[4] The rest are about tools and patience.

Realistic expectations

pH will drift between feeds, and that is normal, not a crisis. The goal is to keep the root zone inside a band, not to pin a single decimal. Soil buffers and corrects slowly. Coco and hydro move fast and need checking every feed.

Hold the inflow steadily in range over weeks and most ‘mystery’ deficiencies never appear. When a symptom does show, read the nutrient deficiencies guide and check pH before you change the feed, and sort your source water with the water quality guide first.