pH and EC are the two numbers you check most often when you feed cannabis. pH shows how acidic or alkaline your water or nutrient solution is. EC shows how much fertiliser salt is dissolved in it. If either is off target, the plant cannot use food that is already in the pot.
Grower communities often say that most apparent “nutrient deficiency” problems are really caused by wrong pH. No published study gives a reliable figure for this, and none of the studies in the reference list below supports one. The lesson still holds: measure pH before you add more fertiliser.
This part explains what pH and EC measure and which targets to aim for. It covers the right pH for each growing medium and which nutrients become locked out at which pH. It also gives the EC for each growth stage. Part 2 covers meters, calibration, adjusting pH, runoff testing, common mistakes and a step-by-step lockout check.
What pH measures
pH is a scale from 0 to 14 that shows how acidic or alkaline a liquid is. 0 is the strongest acid, 14 is the strongest alkali and 7.0 is neutral. The name stands for “potential of hydrogen”, because the scale tracks hydrogen ions (H⁺) in the water.
The scale is logarithmic. Each step of 1.0 means a tenfold change in hydrogen ions. So pH 5.0 is 10 times more acidic than pH 6.0, and 100 times more acidic than pH 7.0. That is why a drift of only half a point already matters.
Why pH decides what roots can absorb
Roots take up nutrients as ions. Ions are tiny particles that carry an electrical charge. The root releases some of its own ions and takes in nutrient ions in exchange. This swap is called cation and anion exchange.
When pH is too high or too low, two things change. The electrical charge on the root surface shifts. The nutrients also change into chemical forms that cannot pass through the root cell membranes. The result is nutrient lockout. The nutrient is in the pot, but the plant cannot take it in.
Three examples show how this works:
- Below pH 5.5, phosphorus binds with iron and aluminium and stops dissolving.
- Above pH 7.5, phosphorus binds with calcium instead.
- Iron (Fe), zinc (Zn) and manganese (Mn) dissolve well at low pH. If pH drops too far, uptake rises, and with heavy micronutrient feeding they can reach toxic levels.
What a 2025 study found
In 2025, Veazie and colleagues published a study in the journal Agrosystems, Geosciences & Environment. They tested how substrate pH and micronutrient feeding rates affect cannabis. “Substrate” means the growing medium. Shoot and root growth fell when substrate pH dropped below 5.0. Leaves held the most micronutrients at the lowest pH, but at normal feeding rates they did not reach toxic levels. Sensitivity to micronutrient toxicity differed between cultivars.
What to do: when leaves turn yellow or get spots, measure pH before you add any fertiliser. Measure both the solution you give and the water that drains from the pot. This shows whether the cause is lockout or a real shortage. Part 2 explains how to collect and read the drainage water.
Target pH for each growing medium
Different media need different pH targets because of two properties. Buffering capacity is how strongly the medium resists a change in pH. Cation exchange capacity (CEC) is how many nutrient ions the medium can hold on its particles. A medium with high CEC and strong buffering keeps pH steady. A medium with no buffering lets pH swing at every watering.
| Medium | Target pH | Buffering |
|---|---|---|
| Soil | 6.0–7.0 (aim for 6.3–6.8) | High |
| Coco coir | 5.8–6.3 | Medium |
| Hydroponics | 5.5–6.5 (best 5.8–6.0) | None |
| Rockwool | 5.5–6.5 | None |
| Living soil | 6.2–7.0 | Very high |
| Peat | 5.7–6.1 | High at first, then runs down |
What each medium needs from you:
- Soil: its high CEC holds nutrients on the surface of soil particles. pH stays stable, so you rarely need to adjust it.
- Coco coir: Published CEC values vary widely, from about 30 to over 100 meq/100 g. This lab unit (milliequivalents per 100 grams) counts how many charged particles the medium can hold. Fresh coco carries too much sodium (Na⁺) and potassium (K⁺). Before use, soak it in a calcium–magnesium supplement (CalMag). This step is called buffering, and Part 2 gives the dose. Coco pH changes faster than soil pH.
- Hydroponics: roots sit directly in the nutrient solution. With no medium to buffer it, pH changes quickly. Check pH every day.
- Rockwool: it is alkaline when new. Before use, soak it in nutrient solution adjusted to pH 5.5, as Grodan’s wetting instructions say. It has no buffering at all, so manage it like hydroponics.
- Living soil: humus and soil microbes keep pH stable by themselves. Mycorrhizal fungi live in partnership with roots and help the plant absorb nutrients. You usually do not need to adjust the pH of your water. Dolomite lime works as a natural buffer.
- Peat: its natural pH is 3.0–4.0. Before use, mix in lime. Published recipes use about 5–15 lb of limestone per cubic yard of peat mix, about 3–9 kg per cubic metre. Test the pH of the mix and adjust. Its buffering is good but runs down over time.
Alternating between two pH values
Many experienced growers do not hold one fixed pH. Instead, they switch between two values from one watering to the next. For coco or hydroponics they use 5.8 and 6.3. For soil they use 6.0 and 6.5. The aim is for every nutrient to meet its best pH at some point in the watering cycle.
Translator’s note: your source water has its own pH and EC before you add anything. Tap water, well water and rainwater differ from area to area in Thailand and Southeast Asia. They may also change between the dry and rainy seasons. Measure your own source water before you mix nutrients. This shows whether the water itself pushes pH up and how much EC it already carries.
What to do: write your medium and its target pH range on a label next to your nutrient tank. That way, everyone who waters the plants works to the same number.
Which nutrients each pH range locks out
Macronutrients are the elements a plant needs in large amounts. Micronutrients, also called trace elements, are needed in tiny amounts but are just as essential. The macro and micronutrient guide explains what each one does. The tables below show where each nutrient is available and where it gets locked out.
Macronutrients
| Nutrient | Best pH | Locked out when | Note |
|---|---|---|---|
| Nitrogen (N) | 6.0–8.0 | Below 5.5 | Available over a wide range. |
| Phosphorus (P) | 6.0–7.0 | Below 5.5 or above 7.5 | Very sensitive to pH. The element most often locked out. |
| Potassium (K) | 5.5–8.0 | Below 5.0 | Little affected by pH. Competes with Ca and Mg for uptake. |
| Calcium (Ca) | 6.2–7.5 | Below 5.0 | More available as pH rises. Fully locked out in hydroponics below pH 5.0. |
| Magnesium (Mg) | 6.0–7.5 | Below 5.5 | More available as pH rises. Often short in coco because coco’s CEC holds on to Mg. |
| Sulfur (S) | 5.5–8.0 | Rarely | Available at most pH levels. Rarely a problem. |
Nitrogen comes in two forms, and they push pH in opposite directions. Nitrate (NO₃⁻) raises the pH in the root zone. The root zone, or rhizosphere, is the thin layer of medium around the roots. Ammonium (NH₄⁺) lowers it. Phosphorus gets locked out at both ends of the scale. In acid conditions it binds with iron and aluminium, and in alkaline conditions it binds with calcium.
Micronutrients
| Nutrient | Best pH | Locked out or toxic when | Note |
|---|---|---|---|
| Iron (Fe) | 5.0–6.5 | Growth suffers below 5.0; locked out above 7.0 | Dissolves well at low pH. Uptake is highest at low pH, but at normal feeding rates it did not reach toxic levels in cannabis (Veazie et al., 2025). |
| Manganese (Mn) | 5.0–6.5 | Can become toxic below 5.0 if overfed; locked out above 7.0 | Behaves like iron. Mn toxicity looks like Mg deficiency. |
| Zinc (Zn) | 5.0–6.5 | Locked out above 7.0 | Too much P competes with Zn for uptake. Zn deficiency gives small, twisted new leaves. |
| Copper (Cu) | 5.0–6.5 | Locked out above 7.0 | Needed in very small amounts: 0.1–0.18 ppm. Dissolves well at low pH. |
| Boron (B) | 5.0–7.0 | Toxic above 1.0 ppm | Available over a fairly wide range. Recommended dose: 0.875–1.0 ppm. |
| Molybdenum (Mo) | 6.0–8.0 | Locked out below 5.5 | The exception: more available as pH rises. The plant needs it to convert nitrate. |
ppm means parts per million. In water, 1 ppm is about 1 mg per litre. The boron dose comes from a 2021 column by Veazie, Whipker and colleagues in Cannabis Business Times, a US trade magazine for cannabis growers.
The overlap zone
Macronutrients (N, P, K, Ca, Mg) are most available at pH 6.0–7.0. Micronutrients (Fe, Mn, Zn, Cu) are most available at pH 5.0–6.5. Molybdenum is the only exception. The two ranges overlap at about 5.8–6.5. That is why this band is the usual target for coco and hydroponics.
What to do: when a leaf symptom appears, use the deficiency diagnostic guide to find the likely nutrient. Then look up its pH range in the tables above. If your measured pH is outside that range, fix the pH first. Adding more of a locked-out nutrient will not help.
EC, TDS and ppm: measuring how strong your solution is
EC stands for electrical conductivity. It measures how well the solution carries an electric current. Dissolved fertiliser salts carry the current, so a higher EC means more dissolved minerals. The unit is millisiemens per centimetre (mS/cm). EC gives the total amount of salts but not which nutrients they are.
TDS stands for total dissolved solids, shown in ppm. A TDS meter does not measure solids directly. It measures EC and multiplies it by a conversion factor. Meter makers use two different factors:
| Scale | Formula | Meter brands using it | EC 2.0 reads as |
|---|---|---|---|
| 500 scale (also called NaCl) | ppm = EC × 500 | Hanna, Milwaukee | 1000 ppm |
| 700 scale (also called Truncheon) | ppm = EC × 700 | Bluelab, Eutech | 1400 ppm |
The name NaCl refers to table salt, which the 500 scale is based on. Truncheon is the name of a Bluelab EC meter that uses the 700 scale.
Because the factors differ, two meters can show the same solution 40% apart (700 ÷ 500 = 1.4). EC in mS/cm is the physical measurement itself. It reads the same on any calibrated meter. Use EC as your main unit. Convert only when a feeding chart is written in ppm:
- From 500-scale ppm to EC: divide by 500. From 700-scale ppm to EC: divide by 700.
- From 500-scale to 700-scale ppm: multiply by 1.4. For the reverse, divide by 1.4.
- Some charts use µS/cm (microsiemens): 1 mS/cm = 1000 µS/cm.
What to do: look up your meter’s ppm scale in its manual or settings. Write the scale on the meter with a marker. This stops you feeding 1.4 times too much or too little when a chart uses the other scale.
Why the plant’s water use limits the EC it can handle
Transpiration is the water a plant loses as vapour through its leaves. That loss pulls water up from the roots, and dissolved nutrients travel with it. So how much food the plant takes in depends on how fast it drinks.
In hot, dry air the plant drinks fast. Salts it does not use can build up in the medium, so growers often lower EC in these conditions. In very humid air the plant drinks slowly, so less food moves up with the water even at the same EC.
Translator’s note: Thailand’s hot season and rainy season change transpiration in different ways. The climate control guide explains how temperature and humidity together drive it, measured as vapour pressure deficit (VPD). When the season changes, do not keep the same EC out of habit. Watch leaf tips and runoff EC (Part 2) over the next few waterings, then adjust.
Target EC for each growth stage
| Stage | EC (mS/cm) | ppm, 500 scale | ppm, 700 scale |
|---|---|---|---|
| Seedling or clone | 0.2–0.8 | 100–400 | 140–560 |
| Early veg (weeks 1–3) | 0.8–1.2 | 400–600 | 560–840 |
| Late veg (week 4 on) | 1.2–1.6 | 600–800 | 840–1120 |
| Early flower (weeks 1–3) | 1.4–1.8 | 700–900 | 980–1260 |
| Mid flower (weeks 4–6) | 1.6–2.2 | 800–1100 | 1120–1540 |
| Late flower (week 7 on) | 1.2–1.6 | 600–800 | 840–1120 |
| Flush (last 1–2 weeks) | 0.0–0.4 | 0–200 | 0–280 |
The 700-scale column is calculated from the EC values (EC × 700).
What happens at each stage:
- Seedling or clone: roots are weak and burn easily. The young plant mainly needs moisture and light. For the first week, plain water adjusted to the right pH is enough.
- Early veg (vegetative growth): raise EC step by step. Give priority to nitrogen, which builds leaves and stems.
- Late veg: the plant is now strong. If it responds well and the leaf tips are not burnt, go to full strength.
- Early flower: switch to bloom nutrients, with more P and K and less N. This is the stretch, when plants shoot up in height after the switch to flowering.
- Mid flower: nutrient demand peaks because the buds (flower clusters) grow fastest now. Watch for tip burn.
- Late flower: lower EC gradually. The plant is ripening and needs less. With less nitrogen, the leaves change colour naturally.
- Flush: give plain pH-adjusted water or a very low EC. The aim is to push excess salts out of the medium. Many growers also flush for flavour, but controlled trials have not shown a flavour benefit. The forum’s flushing topic reviews the evidence for the flavour claim.
Light feeders and heavy feeders
Strains differ in how much salt they can handle. Light feeders include autoflowers and landrace sativas (traditional local varieties). At peak they can only handle EC 1.0–1.4. Heavy feeders such as OG Kush or Gorilla Glue can handle EC 2.5–3.0.
What to do: start low and raise EC by 0.2 at a time. Watch the plant for 2–3 days before the next increase. Log the date, the EC and the state of the leaf tips. This shows you your strain’s real limit in your own room.
References
- Veazie P et al. (2025). Impact of substrate pH and micronutrient fertility rates on Cannabis sativa. Agrosystems, Geosciences & Environment, 8(1), e70044. doi:10.1002/agg2.70044.
- Veazie P, Whipker BE et al. (2021). When Micronutrients Become Macro Problems. Cannabis Business Times (Cultivation Matters column).
- Rx Green Technologies. Identifying pH Levels in Cannabis (pH technical paper). Identifying pH Levels in Cannabis | Rx Green Technologies
- Advanced Nutrients (2017). Nutrient Lockout: What It Is and How To Prevent It! Nutrient Lockout: How to Prevent It - Advanced Nutrients
- Quesada Moya F / Pevgrow (2024). Marijuana also has its PH — Best PH for Cannabis (Soil & Hydro) Chart. Best PH for Cannabis [Soil & Hydro] Chart | Pevgrow Blog
- CocoForCannabis. How To Set EC Targets; Growing Cannabis in Coco Coir with High Frequency Fertigation. https://www.cocoforcannabis.com/ectargets/
- Science in Hydroponics (2020). A Guide to Different pH Down Options in Hydroponics.
- BVB Substrates (2021). Cannabis substrate and pH buffer. Cannabis substrate and pH buffer - BVB Substrates
- HortGrow Solutions. Coco Coir as a Medium-CEC Substrate: Balancing Precision and Buffering. Coco Coir as a Medium-CEC Substrate: Balancing Precision and Buffering | HortGrow Blog
- Advanced Nutrients (2025). How to Optimize Your Commercial Grow With Rockwool for Cannabis; Peat Moss for Cannabis: The Complete Guide to Maximizing Your Grow Medium. advancednutrients.com
- Modern Farms (2026). DWC Cannabis Setup Guide.
- The Cannigma (2026). Living Soil Cannabis: Why Top Cultivators Are Ditching Synthetic Nutrients. Living Soil Cannabis: Why Top Cultivators Are Ditching Synthetic Nutrients | The Cannigma
- Malík M, Tlustoš P (2025). Soilless Growing Media for Cannabis Cultivation. Agriculture (MDPI), 15(18), 1955. doi:10.3390/agriculture15181955
- Blimburn Seeds (2024). Best EC and pH for Cannabis Nutrient Uptake.
- GrowDiaries (2024). EC and PPM in Cannabis + Common Nutrient Deficiencies from Incorrect pH.
Legal note: this article is for education. In Thailand, cannabis flower is a controlled herb. This is set by the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Flower may be sold only with a PT 33 (ภ.ท.33) prescription covering at most 30 days. Advertising and online sale of flower are prohibited. Two ministerial regulations of B.E. 2569 followed. The regulation on cannabis and hemp extracts took effect on 26 April 2026. Controlled Herbs (No. 2), which tightens licence criteria for flower, took effect on 30 April 2026. Whether you may grow cannabis, and whether you need a licence, depends on the law where you live. This forum does not encourage recreational use.
Have a question about your own readings? Post it below this topic. Include your medium, the pH and EC you feed, and your runoff readings if you have them. Other growers can then compare them with their own numbers.
Continued in Part 2: pH and EC for Cannabis: Meters, Calibration, Adjusting pH, Runoff Testing and Lockout Diagnosis (Part 2 of 2)