pH: the definition and how to keep it stable
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pH: the definition and how to keep it stable

The pH of the root zone controls which nutrients your plant can absorb. This paper shows how the scale from 0 to 14 operates. It shows the cause of the different target ranges for each substrate. It also shows how to measure and adjust the pH, and how to keep it stable for each feed.

Feed8 diagrams6 sources~14 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 pH, nutrient availability and lockout05 Target ranges for each substrate: coco, hydroponics, soil06 pH measurement, calibration and meter maintenance07 pH adjustment and the effects of source water08 Runoff pH and the procedure for each stage09 Troubleshooting10 Expected results and limitations11 References
Start here

Purpose and scope

pH is a scale from 0 to 14. It shows how acidic or alkaline a liquid is. A pH of 7 is neutral. A lower pH is acidic and a higher pH is alkaline. For a grower, the pH is the one setting that controls if the nutrients that you give to the plant can go into the roots. If the pH is incorrect, a plant that has all the nutrients can have a deficiency.

Pure water has a pH of 7. Lemon juice has a pH of approximately 2 (strongly acidic). A solution of baking soda has a pH of approximately 8.5 (weakly alkaline).

A grower frequently makes errors with the scale, because the scale is logarithmic. Each whole number is a change of ten times in acidity. A pH of 5 is ten times more acidic than a pH of 6, and a hundred times more acidic than a pH of 7.[8] Thus a reading that is almost correct can be far from the range that is necessary for your roots.

The pH scale and the target for a growerThe target of most root-zone feed is the small green area, not neutral.lemon ~2grower target 5.5-6.5pure water 7baking soda ~8.50714
Figure 1. The figure shows liquids on the pH scale. It also shows the range of 5.5 to 6.5, in which most growers apply feed. Neutral water (7) is too high for coco and hydroponics.
The most important information

Your nutrients and your light can be correct, and the plant can have deficiencies because of an incorrect pH. The pH value controls all the next steps.

Terms

Definitions

pHA value on a scale of 0 to 14 that shows how acidic or alkaline the water around the roots is.
Root zoneThe wet substrate around the roots, where uptake occurs.
LockoutThe nutrients are in the root zone, but they are not available chemically. Thus the plant shows a deficiency, but it receives feed.
EC (electrical conductivity)The quantity of salts in the nutrient solution. EC is a different setting from pH. Guide to mix nutrients →
Substrate or mediumThe material that holds the roots: coco coir, rockwool, soil, or water in hydroponics.
BufferingThe resistance of a medium or of water to a change of pH. A medium with buffering absorbs small changes before the reading changes. Soil has high buffering. Coco and hydroponics have almost none. Thus the pH changes immediately at each feed.
AlkalinityThe capacity of the water to absorb acid, caused mostly by bicarbonates. Alkalinity and pH are different measurements. Water can have a usual pH and a very high alkalinity. The first reading is correct, but acid that you add to this water does not change the pH easily.
RunoffThe solution that drains from the bottom of the pot after you apply water.
pH and nutrient availability (soilless)most nutrients are available at approximately pH 5.5-6.5lockoutgoodbestgoodlockoutpH 4pH 5pH 6pH 7pH 8When the pH is not in the green band, nutrients precipitate or the roots cannot absorb them. The nutrients are in the tank: lockout.
Diagram. The pH controls which nutrients the plant can absorb. If the pH is not in the band, lockout occurs.
The EC increases when the root zone driesthe salt stays, the water decreasesFull and low EC→Dry and high ECThe salt quantity is the same, but there is less water. Thus the EC atthe roots increases when the water content decreases.
Diagram. The EC and the pH change together when the root zone dries. Read the two values.
Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid
Tests of plants, specifications or many sources that agree show that the information is correct
  • The pH controls which nutrients are available to the plant. A lockout can show the same symptoms as a deficiency.
  • In soilless media, the best pH values are approximately from the middle of the 5s to the middle of the 6s. In soil, the range is higher and wider.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • Control of the feed pH, and the usual procedure for a two-point calibration of the pH meter.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • Chemical information at low pH from basil or lettuce for cannabis, with no cannabis data.
  • A runoff EC that is always a maximum of 10% different from the feed EC. In steering, the operator can select a higher root-zone EC.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

Basic information

pH, nutrient availability and lockout

Each nutrient stays dissolved, and the roots can absorb it, only in a specified pH range. If the pH is not in this range, the nutrient reacts with other ions and changes to chemical compounds that the roots cannot absorb. The nutrient is in the solution, but the roots cannot absorb it. Lockout is the name of this condition. The plant has nutrients around it that it cannot use, because the pH of the root zone is not in the correct range.

If the pH is too high, more than approximately 6.5 in inert media such as coco or hydroponics, the micronutrients precipitate first. The micronutrients are iron, manganese, zinc and boron.[1] If the pH is too low, less than approximately 5.5, the availability of calcium, magnesium and phosphorus can decrease. At the same time, iron and manganese can cause toxicity.[2]

Phosphorus is a clear example. It is most available at a pH of 6.0 to 7.0. At a pH of less than 5.5, it binds with iron and aluminum. At a pH of more than 7.5, it binds with calcium.[3] The ‘sweet spot’ is the pH at which the largest number of nutrients are available at the same time.

The pH range in which each nutrient is availableThe green area is where the micronutrients and the primary cations are available at the same time.all overlap 5.5-6.5Ca / Mg / P lockout, low pHFe / Mn / Zn / B lockout, high pH468
Figure 2. Calcium, magnesium and phosphorus are not available at a low pH. Iron, manganese, zinc and boron are not available at a high pH. The overlap of 5.5 to 6.5 is the only range in which all are available.[1]
Lockout and a deficiency show the same symptoms

Before you add more nutrients, examine the pH. The symptoms of lockout and of a deficiency are the same. Thus growers frequently add more nutrients and the problem becomes worse.

Your numbers

Target ranges for each substrate: coco, hydroponics, soil

There is not one correct pH. The correct target is different for each substrate. You set the pH of the inflow, which is the solution that you apply. You do not set the pH of the runoff.

In soil, organic matter and microbes give buffering to the root zone. Thus the target for the inflow water is approximately 6.0 to 7.0, and the sweet spot is 6.2 to 6.8.[8]

Coco coir is almost inert and has almost no buffering. Set the pH of the inflow nutrient solution to 5.5 to 6.5. Many growers use 5.8 to 6.2.[4]

In hydroponics, the target is 5.5 to 6.5. The sweet spot for all nutrients is 5.8 to 6.2.[5]

In coco and hydroponics, the pH changes almost immediately because they have no buffering. Soil has more buffering, but you correct it more slowly.
SubstrateBufferingFull rangeSweet spotCause
SoilHigh6.0-7.06.2-6.8Microbes and organic matter keep the pH stable
Coco coirVery low5.5-6.55.8-6.2Almost inert. The pH changes fast. Set it for each feed.
HydroponicsNone5.5-6.55.8-6.2Only water. The pH changes immediately. Monitor it carefully.

Some growers change the target by a small quantity in the range during the week, to increase the availability of specified nutrients. If you are a new grower, select one value in the sweet spot and keep it.

The instrument

pH measurement, calibration and meter maintenance

A pH pen is correct only if the last calibration is correct. A probe that you did not calibrate, or that is dry, is worse than no reading. It shows an incorrect value and gives no sign of the problem.

Calibrate the pen with new two-point buffer solutions: first pH 7.0 and then pH 4.0. Do this approximately one time each month. A calibration with one point is not sufficient for the complete range that you use.[7]

Keep the tip of the probe wet in KCl storage solution. Do not keep it dry. Do not keep it in plain water, because plain water removes the reference electrolyte. This decreases the accuracy permanently. Replace the probe when the drift between calibrations is more than approximately 0.2 pH. Also replace it when the reading is not stable in approximately 30 seconds.

Calibrate and measure, in sequence1Cleanclean tip withdistilled water→2Calibrate 7.0put the tip inpH 7.0 buffer→3Cleanbetween buffers→4Calibrate 4.0put the tip inpH 4.0 buffer→5Measureread your sample→6Keep wetput a cap withKCl solution
Figure 3. Do a two-point calibration each time. Clean the tip between the steps. Then keep the tip wet. Dry storage is the most frequent cause of a defective pen.
Wait until the reading is stable

Wait until the reading stops before you use it. A change of temperature changes the value. If you mix the solution, the value also changes. Thus read the pH at room temperature and wait until the reading is stable.

Mixing and adjusting

pH adjustment and the effects of source water

First mix your nutrients, then adjust the pH. When you add nutrients, the pH changes. Thus if you set the pH before you mix the nutrients, you must set it again.

  1. 1
    Mix nutrients
    First add all your feed to the water and mix it.
  2. 2
    Measure
    Read the pH of the mixed solution when the reading is stable.
  3. 3
    Adjust in small steps
    Add a small quantity of pH Down or pH Up, some drops each time.
  4. 4
    Mix and wait
    Mix the solution and wait a short time.
  5. 5
    Measure again
    Read the pH again. Do the steps again with small quantities. Do not add a large quantity at one time.

The usual pH Down products contain phosphoric acid, nitric acid, sulfuric acid or organic acids. The usual pH Up products contain KOH or potassium carbonate. Each product adds nutrients. Include these nutrients when you calculate the feed.[6]

Your source water is more important than many new growers think. Tap water has a quantity of dissolved bicarbonates that absorb acid before the pH reading changes. This quantity is the alkalinity. The unit is ppm CaCO3.[7] Water with high alkalinity will increase in pH again after you set the pH. The remaining bicarbonates continue to react with the acid that you added.

The same pH, but very different work to change itThe correct alkalinity range changes with the container volume, the media, the crop and the fertilizer.061320263 dropsLow alkalinity (soft water)8 dropsExample range for a small container22 dropsHigh alkalinity (hard water)
Figure 4. The alkalinity, and not the pH reading, gives the quantity of acid that is necessary to change the pH of the water. Hard water, which has high alkalinity, does not change its pH easily when you adjust it, and the pH increases again.[7]
The alkalinity must agree with the container system

UMass gives different alkalinity ranges for different volumes of containers. Approximately 40 to 80 ppm CaCO3 can be correct for small containers, and larger pots can have a higher alkalinity. You set the range for your system with the media, the fertilizer, the crop and the pH drift that you monitor. For very hard water, an acid treatment or filtration can be necessary.[7]

Procedure for each day

Runoff pH and the procedure for each stage

Runoff is the solution that drains from the pot. New growers use it too much. In inert media such as coco, the runoff does not measure the root zone directly. It is a sample from one time only. Salt buildup and the local changes that the roots make change the value. The runoff is not a soil test.[4]

The correct lever is the pH of the inflow that you set. For the root zone, monitor the runoff EC for salt buildup and not the runoff pH. When the runoff EC is much higher than your feed EC, a flush is necessary. Think that a difference between the runoff EC and the feed EC that becomes larger is salt buildup. But in crop steering, you can select a higher root-zone EC.[3]

The values are for coco and hydroponics. In soil, the runoff gives some more information, but it is slower and it has buffering. Set the inflow at each feed. Do not apply feed with a pH that is not in the range to correct a runoff value.
StageInflow pH targetEC to monitorCalibrationFlush trigger
Seedling5.8-6.2Low feed EC, careful feedEach monthRunoff EC much higher than the feed EC
Vegetative stage5.8-6.2If the EC increases, there is salt buildupEach monthRunoff EC that becomes much higher than the feed EC
Flowering stage5.8-6.2Monitor the runoff EC and the feed EC (new grower: prevent a large increase of salts)Each monthRunoff EC that increases each day
The procedure in five tasks
  • Calibrate the pen each month with new two-point buffer.
  • Mix the nutrients, then set the pH, for each batch.
  • Set the pH of the inflow in the range at each feed.
  • Record the inflow pH and EC. Then you can see the drift.
  • Adjust slowly, with drops. Thus the buffering has time to operate.
Do not do this

Troubleshooting

The grower causes most pH problems. The usual error is to apply nutrient solution that is not in the safe range, to correct a runoff reading. This error causes the lockout that the grower wants to prevent.[4] The other problems are about tools and about the time that you wait.

Frequent errorCorrect procedure
You try to correct the runoff pH and you apply feed that is not in the rangeSet the inflow in the range at each feed. Monitor the runoff EC and not the runoff pH.
You do not calibrate the probe, or you keep it dry or in plain waterDo a two-point calibration each month. Keep the probe wet in KCl.
You adjust the pH before you mix the nutrientsFirst mix the nutrients, then set the pH.
You add a large quantity of acid at one time, and the pH becomes lower than the targetAdd some drops, mix, wait, and measure again.
You add more nutrients to correct a deficiencyFirst examine the pH. The cause is frequently lockout and not a low quantity of nutrients.
You do not examine the alkalinity of the source waterDo a test of the alkalinity. Apply a treatment to hard water before the pH increases slowly.
Usual results

Expected results and limitations

The pH changes between feeds. This change is usual, and it is not a problem. The target is to keep the root zone in a range. It is not necessary to keep one accurate decimal value. Soil has buffering, thus the pH corrects slowly. In coco and hydroponics, the pH changes quickly and you must monitor it at each feed.

Root-zone pH in one week: small changes are usual, large changes are notSmall changes in the range are correct. On Saturday the value is not in the range: correct it.target range56678MondayTuesdayWednesdayThursdayFridaySaturdaySundaypH
Figure 5. In a usual week, the pH stays in the target range (the green area) with small changes. The one value that is higher than the range is the signal to examine the pen, the water and the feed.
The correct result
  • The target is a range such as 5.8 to 6.2, and not one accurate value.
  • A change of the pH between feeds is usual. In coco and hydroponics, you must do a check at each feed. Soil is slower.
  • A pen has a short life. Calibrate it each month. Replace the probe after some time.
  • Record each feed. The data for some weeks show more than one reading.

When a deficiency symptom shows, read the nutrient deficiencies guide. Make sure that the pH is correct before you change the feed formula. If the alkalinity of the source water is the problem, the water quality guide shows how to apply a treatment to it.

Related papers
Nutrient deficiencies
Read the leaves and change the feed
Water quality
Source water, RO and alkalinity
Mix nutrients
Mix salts correctly, in liters and kilograms

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. (source not from a journal, for example from a manufacturer) 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. (source not from a journal, for example from a manufacturer) https://passel2.unl.edu/view/lesson/d2b52174b1a7/2

Each citation in the paper, for example [1], refers to an item in this list. Each item is a primary source or an instruction from an authority, unless the item shows a different type. The results of cannabis tissue culture change much with the genotype. Before you use the dilutions, hormone doses and local regulations, make sure that they agree with the primary sources.