pH: what it is and how to hold it
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pH: what it is and how to hold it

Root-zone pH controls which nutrients your plant can absorb. This paper explains how the 0-14 scale works, why each substrate has a different target range, and how to measure, adjust, and hold pH steadily feed by feed.

Feed8 diagramsEvidence-linked · 6 sources~14 min read
In this guide
01 Purpose and scope02 Definitions03 Evidence and limitations04 pH, nutrient availability and lockout05 Target ranges by substrate: coco, hydro, soil06 pH measurement, calibration and meter care07 pH adjustment and source-water effects08 Runoff pH and stage-specific routine09 Troubleshooting10 Expected results and limitations11 References
Start here

Purpose and scope

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.

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.

The pH scale and your grower targetMost root-zone feeding aims at the narrow green band, not at neutral.lemon ~2grower target 5.5-6.5pure water 7baking soda ~8.50714
Figure 1. Everyday liquids on the pH scale, with the 5.5-6.5 band most growers feed inside. Neutral water (7) is already too high for coco and hydro.
The whole point in one line

You can have perfect nutrients and perfect light and still get deficiencies purely from bad pH. The number gates everything downstream.

Vocabulary

Definitions

pHHow acidic or alkaline the water around the roots is, on a 0-14 scale.
Root zoneThe wet substrate immediately around the roots, where uptake actually happens.
LockoutNutrients are present but chemically unavailable, so the plant shows a deficiency even though it is being fed.
EC (electrical conductivity)How strong or salty the nutrient solution is. A separate dial from pH. Mixing guide →
Substrate / mediumWhat the roots grow in: coco coir, rockwool or water in hydro, or soil.
BufferingA medium or water's resistance to pH change — like a thermostat with a wide deadband, the medium absorbs small shifts before the reading moves. Soil is highly buffered; coco and hydro have almost none, so pH responds immediately to each feed.
AlkalinityThe water's built-in acid-absorbing capacity, mostly bicarbonates. Alkalinity and pH measure different things: a water can have a moderate pH and very high alkalinity, and it will resist acid dosing even when the initial reading looks fine.
RunoffThe solution that drains out the bottom of the pot after watering.
pH and nutrient availability (soilless)most nutrients open up around pH 5.5-6.5lockedokbestoklockedpH 4pH 5pH 6pH 7pH 8Drift out of the green band and nutrients precipitate or stop being taken up, even though they are in the tank: lockout.
Diagram. pH sets which nutrients the plant can actually take up; drift out of the band and you get lockout.
Why EC climbs as the root zone driessalt stays put while water leavesFull + dilute→Dry + concentratedSame number of salt grains, less water to dissolve them: the EC theroots feel rises as water content falls.
Diagram. EC and pH move together as the root zone dries; read both.
Evidence assessment

Evidence and limitations

How sure is this paper?

We've gone to great lengths to keep these guides honest. One of the main ways we do that is self-review: we actively look for claims that are subjective, only lightly backed by literature, or based on grower practice rather than a controlled study — and we call those out instead of dressing them up as settled science.

Often there simply is no paper for the decision you're making. In those cases we're drawing on what other growers report and what has worked in our own rooms. That can still be useful — but it is not a lab proof. Do what works for your plants, your room, and your meters. If a table disagrees with your crop, believe the crop and log the difference.

Solid
Well supported by plant science, standards, or broad multi-source consensus
  • pH gates nutrient availability; lockout can look like deficiency
  • Soilless sweet spots roughly mid-5s to mid-6s; soil higher/wider
Operational
What many growers and rooms actually run — start here, then tune
  • Inflow pH discipline and two-point pen calibration habits
Grain of salt
Subjective, thin literature, single studies, or “this works for us” practice
  • Low-pH chemistry details from basil/lettuce applied without cannabis context
  • Runoff EC always within 10% of feed as a universal law (steering may hold higher root-zone EC on purpose)

See something glaringly wrong? Tell us and we'll fix it. Please open a GitHub issue with the paper name and what looks off (include a source if you have one): Report an accuracy issue. Local law, labels, and licences always override any recipe here. Inline notes labelled grain of salt flag the highest-risk over-trust points in the text.

The core idea

pH, nutrient availability and lockout

Each nutrient stays dissolved, and therefore absorbable, only across a certain pH band. Outside that band it reacts with other ions and converts into chemical forms the roots cannot take up — the same way a supplement tablet that never dissolves in your stomach passes through without helping: the nutrient is present, but in the wrong form for absorption. That failure mode is called lockout. The plant is surrounded by food it cannot use because the root-zone chemistry drifted outside 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 availability can fall while iron and manganese can push toward toxicity 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.

Where each nutrient is available across pHThe green band is where micros and the big cations are available together.all overlap 5.5-6.5Ca / Mg / P lock lowFe / Mn / Zn / B lock high468
Figure 2. Calcium, magnesium and phosphorus fail at low pH; iron, manganese, zinc and boron fail at high pH. The 5.5-6.5 overlap is the only band where all are available.[1]
Lockout looks exactly like a deficiency

Because the symptoms match, growers often add more nutrients and make it worse. Check pH first, before reaching for the bottle.

Your numbers

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]

Coco and hydro respond to pH swings almost instantly because they cannot buffer. Soil is more forgiving but slower to correct.
SubstrateBufferingFull rangeSweet spotWhy
SoilHigh6.0-7.06.2-6.8Microbes and organic matter hold it steady
Coco coirVery low5.5-6.55.8-6.2Nearly inert, swings fast, set it per feed
HydroNone5.5-6.55.8-6.2Water only, moves immediately, watch closely

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.

The tool

pH measurement, calibration and meter 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.

Calibrate and measure, in order1Rinseclean tip withdistilled water→2Cal 7.0set in pH 7.0buffer→3Rinsebetween buffers→4Cal 4.0set in pH 4.0buffer→5Measureread your sample→6Store wetcap in KClsolution
Figure 3. Two-point calibration every time, rinsing between steps, then store the tip wet. Dry storage is the most common way pens die.
Let it settle

Give the reading time to stop moving before you trust it. Temperature and stirring both shift the number, so read at room temperature and wait for it to hold steady.

Mixing and adjusting

pH adjustment and source-water effects

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.

Common pH downs include phosphoric, nitric, sulfuric, or organic acids; common pH ups include KOH or potassium carbonate. Each adds nutrients, so account for them.[6] Your source water matters more than most beginners expect. Tap water carries a built-in reserve of dissolved bicarbonates that absorb acid before the pH reading moves — like antacid neutralising stomach acid without any change on a pH strip until the antacid is used up. That reserve is called alkalinity, reported in ppm CaCO3.[7] High-alkalinity water will drift back up after you set pH, because the remaining bicarbonates keep reacting with the acid you added.

Same pH, very different effort to move itThe suitable alkalinity range changes with container volume, media, crop and fertiliser.061320263 dropsLow alkalinity (soft)8 dropsExample small-container range22 dropsHigh alkalinity (hard)
Figure 4. Alkalinity, not the pH reading, sets how much acid it takes to move the water. Hard, high-alkalinity tap water resists adjustment and drifts back up.[7]
Match alkalinity to the container system

UMass gives different alkalinity ranges for different container volumes. About 40-80 ppm CaCO3 can suit small containers, while larger pots can tolerate more. Use the media, fertiliser, crop and observed pH drift to set the working range. Very hard water may need acid treatment or filtration.[7]

Daily practice

Runoff pH and stage-specific 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. Treat a runaway gap between runoff and feed as salt buildup, though advanced steering may hold root-zone EC higher on purpose.[3]

Coco and hydro figures. Soil runoff is a little more meaningful but still lags and is buffered. Set inflow every feed and do not feed out of range to fix a runoff number.
StageInflow pH targetEC watchCalibrationFlush trigger
Seedling5.8-6.2Low feed EC, gentleMonthlyRunoff EC well above feed
Veg5.8-6.2Rising EC means salt buildupMonthlyRunoff EC running away above feed
Flower5.8-6.2Watch runoff vs feed (beginner: avoid runaway salts)MonthlyRunoff EC climbing day on day
The routine in five habits
  • Calibrate the pen monthly with fresh two-point buffer.
  • Mix nutrients, then set pH, every batch.
  • Set inflow pH inside the band every feed.
  • Log inflow pH and EC so you can see drift.
  • Adjust slowly, in drops, and let buffering work.
Don't do this

Troubleshooting

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.

Common mistakeDo this instead
Chasing runoff pH and feeding out of range to correct itSet inflow in range every feed, watch runoff EC not runoff pH
Never calibrating, or storing the probe dry or in plain waterTwo-point calibrate monthly, store wet in KCl
Adjusting pH before mixing nutrientsMix nutrients first, set pH last
Dumping acid then overshootingAdd a few drops, stir, wait, re-measure
Adding more nutrients to fix a deficiencyCheck pH first; it is often lockout, not a shortage
Ignoring source-water alkalinityTest alkalinity; treat hard water before it creeps pH up
Reality check

Expected results and limitations

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.

Root-zone pH over a week: wobble is fine, excursions need actionSmall wobbles inside the band are healthy. The Saturday spike out of band is what you act on.target band56678MonTueWedThuFriSatSunpH
Figure 5. A normal week stays inside the shaded target band with minor wobbles. The one excursion above the band is the signal to check the pen, the water and the feed.
What good looks like
  • A band like 5.8-6.2 is the target, not one exact number.
  • Drift between feeds is expected; coco and hydro need per-feed checks, soil is slower.
  • Pens are consumables: calibrate monthly, replace probes over time.
  • Log every feed: the pattern over weeks tells you more than any single reading.

When a deficiency symptom appears, read the nutrient deficiencies guide and confirm pH before adjusting the feed formula. If source-water alkalinity is the problem, the water quality guide covers how to treat it.

Related papers
Deficiency diagnosis
Read the leaves, fix the feed
Water quality
Source water, RO & alkalinity
Mixing nutrients
Salts, in metric, done right

References

  1. Veazie, P., Cockson, P., Smith, J. T., Schulker, B., Jackson, B., Hicks, K., & Whipker, B. (2025). Impact of substrate pH and micronutrient fertility rates on Cannabis sativa. Agrosystems, Geosciences & Environment, 8(1), e70044. https://doi.org/10.1002/agg2.70044 https://doi.org/10.1002/agg2.70044
  2. Gillespie, D. P., Kubota, C., & Miller, S. A. (2020). Effects of low pH of hydroponic nutrient solution on plant growth, nutrient uptake, and root rot disease incidence of basil (Ocimum basilicum L.). HortScience, 55(8), 1251-1258. https://doi.org/10.21273/HORTSCI14986-20 https://doi.org/10.21273/HORTSCI14986-20
  3. Kpai, P. Y., Adaramola, O., Addo, P. W., MacPherson, S., & Lefsrud, M. (2024). Mineral nutrition for Cannabis sativa in the vegetative stage using response surface analysis. Frontiers in Plant Science, 15, 1501484. https://doi.org/10.3389/fpls.2024.1501484 https://doi.org/10.3389/fpls.2024.1501484
  4. Malik, M., & Tlustos, P. (2025). Soilless growing media for Cannabis cultivation. Agriculture, 15(18), 1955. https://doi.org/10.3390/agriculture15181955 https://doi.org/10.3390/agriculture15181955
  5. Kudirka, G., Virsile, A., Sutuliene, R., Lauzike, K., & Samuoliene, G. (2023). Precise management of hydroponic nutrient solution pH: The effects of minor pH changes and MES buffer molarity on lettuce physiological properties. Horticulturae, 9(7), 837. https://doi.org/10.3390/horticulturae9070837 https://doi.org/10.3390/horticulturae9070837
  6. Saloner, A., & Bernstein, N. (2022). Nitrogen source matters: High NH4/NO3 ratio reduces cannabinoids, terpenoids, and yield in medical cannabis. Frontiers in Plant Science, 13, 830224. https://doi.org/10.3389/fpls.2022.830224 https://doi.org/10.3389/fpls.2022.830224
  7. University of Massachusetts Amherst, Center for Agriculture, Food, and the Environment (Greenhouse & Floriculture Program). Water quality: pH and alkalinity (fact sheet). UMass Extension. (industry/manufacturer or non-journal source) https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/water-quality-ph-alkalinity
  8. University of Nebraska-Lincoln, Plant and Soil Sciences eLibrary (PASSeL). Soils - Part 4: Soil pH - Defining pH. University of Nebraska-Lincoln. (industry/manufacturer or non-journal source) https://passel2.unl.edu/view/lesson/d2b52174b1a7/2

Citations marked in-text as [n] map to this list. Primary literature and official guidance except where noted. Cannabis tissue culture is strongly genotype-dependent, verify dilutions, hormone doses and local regulations against the primary sources before relying on them.