Target pH and EC numbers only help if your readings are correct and you can fix them safely. This part covers pH and EC meters, calibration and the products that raise or lower pH. It explains how to test the water that drains from the pot and lists the ten most common mistakes. It ends with a six-step check to run before you add more fertiliser.
Here is a short recap so this part stands on its own. pH shows how acidic or alkaline the solution is, on a scale from 0 to 14. EC (electrical conductivity, in mS/cm) shows how much fertiliser salt is dissolved. Target pH is 6.0–7.0 for soil, 5.8–6.3 for coco coir and 5.5–6.5 for hydroponics. When pH is wrong, nutrients become locked out. They are in the pot, but roots cannot absorb them.
This is Part 2. Part 1: pH & EC: The Nutritional Foundation Every Grower Needs to Know
pH and EC meters
The models below come from the source articles. Listing them is not a recommendation, and the forum does not favour any brand.
Some of these meters have automatic temperature compensation (ATC). ATC means the meter corrects its reading for water temperature. Without it, you have to make the correction yourself.
pH pens:
- Apera PH20: has ATC.
- Bluelab pH Pen: described as durable. The probe can be replaced.
- Hanna HI98107: a basic pocket tester. It needs calibration more often.
EC and TDS meters:
- Bluelab Truncheon: shows EC, 500-scale ppm and 700-scale ppm. Described as durable and waterproof. Like any EC meter, check it against a 1.413 mS/cm standard solution from time to time.
- Hanna HI98312: an EC/TDS meter with ATC.
- TDS-3: a basic tester sold under many brand names. Before you rely on it, check whether it uses the 500 or the 700 scale.
Translator’s note: pH and EC readings both shift with water temperature. Nutrient water in Thai grow rooms is often 30 °C or warmer, so ATC saves you a correction step. Before you buy, ask the official distributor in Thailand whether calibration and probe storage solutions are sold locally. If you cannot calibrate a meter, its readings soon go wrong. For hands-on experience with specific meters, ask in the forum’s General Nutrients Q&A section.
Calibrating your meters
Calibration means checking the meter in a liquid with an exactly known value, then correcting the meter. These liquids are called standard or buffer solutions.
- pH meter: calibrate with pH 4.0 and pH 7.0 standard solutions. Adding pH 10.0 is optional. Do it every week.
- EC meter: calibrate with a 1.413 mS/cm standard solution. Most EC pens need this only once a month.
- pH probe storage: always keep the probe moist in storage solution made of potassium chloride (KCl). Never let it dry out, because drying damages it for good. Do not store it in distilled water.
Translator’s note: keep solution bottles closed and out of direct sun. For each calibration, pour a small amount into a clean cup and throw it away afterwards. Never dip the probe into the bottle, because that contaminates the rest. Check the expiry date when you buy.
What to do: set phone reminders: pH calibration every week and EC once a month. Before each calibration, note what the meter reads in the standard solution. This shows how far the meter drifts. If the drift grows week after week, the probe is wearing out.
Adjusting pH safely
Products that lower pH (pH Down)
| Product | Active ingredient | Advantages | Drawbacks |
|---|---|---|---|
| General Hydroponics pH Down, Advanced Nutrients pH Down | Phosphoric acid (H₃PO₄) | Industry standard. Holds pH steady. Adds P, which is useful in flower. | Too much P can lock out Zn and Fe. Corrosive. |
| Citric acid | Citric acid (C₆H₈O₇) | Safe, cheap, food grade. Suits organic soil. | pH does not hold: microbes break down the citrate and pH climbs back. Not for hydroponics. |
| Nitric acid | HNO₃ | Adds nitrate (NO₃⁻), useful in veg. Used by professionals. | Very dangerous. Commercial use only. |
| Sulfuric acid | H₂SO₄ | Cheapest. Sulfate does the plant no harm. | The most dangerous. Use food grade only. Not recommended for hobby growers. |
| Vinegar or lemon | Acetic or citric acid | Natural, safe, easy to find. | Short effect: pH climbs back within hours. Organic soil only. |
Products that raise pH (pH Up)
| Product | Active ingredient | Advantages | Drawbacks |
|---|---|---|---|
| General Hydroponics pH Up, Advanced Nutrients pH Up | Potassium hydroxide (KOH) | Fast, strong, precise. Industry standard. | Corrosive. Easy to overshoot. Wear gloves and eye protection. |
| Potassium silicate | K₂SiO₃ | Raises pH and supplies silicon, said to strengthen stems and pest resistance. | Can settle out with calcium in concentrated solutions. Dilute it first. |
| Dolomite lime | CaMg(CO₃)₂ | Natural buffer that holds pH near 7.0. Adds Ca and Mg. Dose: 1 tablespoon per gallon of soil, about 1 tablespoon per 4 litres. | Slow: takes days. Only for mixing into soil beforehand. |
| Oyster shell | CaCO₃ | Slow release and stable over the long term. Very good for living soil. | Too slow for hydroponics. Works only in soil or organic media. |
| Wood ash | K₂CO₃, CaCO₃ | Very alkaline. Adds K, Ca and trace elements. | Easy to overdose. Imprecise. Use very sparingly. |
The forum’s silicon topic reviews which of the silicon claims the evidence supports.
Translator’s note: for home growers in Thailand, ready-made pH Up and pH Down from an official distributor is the practical choice. Nitric and sulfuric acid are too dangerous for home growers. Always add the acid or alkali to the water, never the other way round. Wear gloves and eye protection. To check a product’s Thai registration, see how to verify that a fertiliser is legal in Thailand.
Step by step
- Mix the nutrients first, because fertilisers change the pH of the water.
- Measure the pH of the mixed solution.
- Add pH Up or pH Down. Start with 1–2 mL per US gallon (3.8 L), about 0.25–0.5 mL per litre.
- Stir well, wait 10 minutes and measure again.
- Repeat until you reach the target pH.
- Move pH by no more than 0.3 per adjustment. Small steps prevent overshooting and then correcting back the other way, which stresses roots.
What to do: note how many mL of pH Down or pH Up your usual tank volume needs. Next time, start from that dose instead of guessing.
Runoff testing: reading the root zone
Runoff is the water that drains out of the bottom of the pot after watering. It carries salts out of the medium. Its pH and EC are the only way to know what is really happening in the root zone. The root zone, or rhizosphere, is the thin layer of medium around the roots.
Run-to-waste and PourThru
Run-to-waste is a watering method in which you give enough solution for some to drain out. You then throw that drainage away instead of reusing it. Coco growers often work this way. Each watering rinses part of the old solution out, so the drainage shows what was left in the medium.
PourThru is a related sampling method used in commercial container growing. After a normal watering, you pour a small, fixed amount of distilled water onto the medium. You then test the liquid it pushes out of the drainage holes. A University of Florida extension guide says to wait about 30 minutes for small pots and up to 2 hours for large containers, then add enough water to collect about 50 mL. Its estimates are about 150 mL of water for a 1-gallon (3.8 L) pot and 350 mL for a 3-gallon (11 L) pot. You can only compare readings if you use the same timing and volume every time.
How to collect a runoff sample
- Water the plant as usual with pH-adjusted nutrient solution.
- Put a tray under the pot to catch the drainage.
- Let the first few seconds drain away, because that is water left from the previous watering. Then collect the runoff.
- Measure the pH and EC of the runoff straight away.
- Write the result in a log so you can follow the trend.
Reading the results: compare what goes in with what comes out
| What you see | What it means | What to do |
|---|---|---|
| Runoff EC lower than input EC | The plant is absorbing nutrients well. This is normal and ideal. | Keep your feeding schedule. |
| Runoff EC about the same as input EC | The plant is not absorbing. Possible causes are pH lockout or root damage. | Check runoff pH and root health. Consider lowering EC. |
| Runoff EC higher than input by 0.5 mS/cm or more | Salt is building up in the root zone. This can damage roots. | Flush with plain pH-adjusted water until runoff EC reaches target. Then feed at 50% strength. |
| Runoff pH more than 0.5 below input pH | The medium is turning acidic. Roots may be stressed. | Raise input pH by 0.2–0.3. Do a light flush. Check for purple or brown roots. |
To flush means to water with plenty of plain water to wash salts out of the medium.
How often: in coco and hydroponics, measure runoff at every watering. In soil, measure at least every 2–3 waterings. Look at the trend over 3–5 readings in a row before you make a big change.
Translator’s note: power cuts stop pumps, air pumps and water chillers. Hydroponics has no medium to buffer the solution, so pH and EC can drift while the water warms up. After a power cut, measure reservoir pH, EC and temperature before the next feed.
What to do: keep a log with five columns: date, input pH, input EC, runoff pH and runoff EC. A trend in this log shows a problem before the leaves do.
Ten common pH and EC mistakes
| # | Mistake | What happens | Fix |
|---|---|---|---|
| 1 | Not measuring pH | Hidden lockout. It looks like deficiency, but more fertiliser does not help. | Get a pH pen and measure at every watering. |
| 2 | Not calibrating meters | Wrong readings lead to wrong adjustments, which damage plants. | Calibrate the pH pen weekly with 4.0 and 7.0 solutions. Calibrate the EC pen monthly. |
| 3 | Chasing pH (over-adjusting) | pH Up overshoots, then pH Down overshoots the other way. Roots get stressed. | Add 1–2 mL per gallon (about 0.25–0.5 mL per litre) at a time. Wait 10 minutes and re-measure. Move pH no more than 0.3 per adjustment. |
| 4 | Ignoring runoff | Salt builds up unseen until it poisons the roots. | Measure runoff pH and EC regularly and compare them with the input. |
| 5 | Wrong pH for the medium | Soil watered at pH 5.5 locks out Ca and Mg. Hydroponics at pH 6.8 locks out Fe, Mn and Zn. | Soil 6.0–7.0. Coco 5.8–6.3. Hydroponics 5.5–6.5. |
| 6 | Adjusting pH before mixing nutrients | Fertilisers shift pH a lot. You end up adjusting twice and wasting product. | Always mix the nutrients first, then adjust pH. |
| 7 | EC too high for seedlings | Young roots burn. Seedlings grow slowly and leaf tips turn yellow. | Seedlings: EC 0.2–0.8. Raise by 0.2 per week. |
| 8 | Not buffering new coco | Fresh coco holds Na⁺ and K⁺. This causes severe Ca and Mg deficiency in the first week. | Before use, soak coco overnight in CalMag solution at EC 1.5–2.0. |
| 9 | Letting the pH probe dry out | A dry probe is ruined for good and must be replaced. | Always store it in KCl storage solution, never in distilled water. |
| 10 | Mixing up ppm scales | A 700-scale chart read with a 500-scale meter means feeding 1.4 times too much, which burns plants. The reverse underfeeds by the same factor. | Know your meter’s scale. Better still, work in EC (mS/cm). |
Diagnosing lockout before you add fertiliser
Your plant may show deficiency signs such as yellow leaves, brown spots or slow growth. Do not add more fertiliser straight away. Work through these six steps in order. Each step rules out one cause.
Step 1 – Check your meters. Are they calibrated? If not, calibrate first and measure again. This rules out a false reading.
Step 2 – Measure runoff. Water with plain pH-adjusted water. Collect the runoff and measure its pH and EC.
Step 3 – Read the runoff pH. Is it in the right range for your medium?
- No: this is pH lockout. Flush with correctly adjusted water until runoff pH reaches the target.
- Yes: go to step 4.
Step 4 – Read the runoff EC. Is it more than 0.5 mS/cm above your input EC?
- Yes: salt has built up. Flush until runoff EC reaches the target. Then resume feeding at 50% of your usual EC.
- No: go to step 5.
Step 5 – Check the roots. White or light cream roots are healthy. Brown, slimy roots mean root rot.
- Brown roots: the treatment given in the sources is 3% hydrogen peroxide (H₂O₂) at 3 mL per litre. The other option is Hydroguard, a commercial root-zone product. Also check the water temperature, which should stay below 22 °C.
- White roots: go to step 6.
Step 6 – Confirm a real deficiency. Only now should you consider adding the specific nutrient that is short. Match the leaf symptoms against a deficiency chart.
Grower communities often say that most apparent “deficiency” cases are really pH lockout or salt buildup, and only a few are a real shortage. No published study gives a reliable breakdown, and none of the studies in the reference list supports one. Its practical message matches the six steps: rule out pH and salt first.
Translator’s note: keeping nutrient water below 22 °C is hard in Thailand, where rooms often run at 30–35 °C. Without a water chiller, the reservoir usually stays well above that. The topic on running hydroponics at 31 °C compares three options: a chiller, beneficial microbes or a different system. For root treatments, use only products registered with the Department of Agriculture (กรมวิชาการเกษตร), and follow the label exactly. See what you may legally spray under Thai GACP.
What to do: print the six steps and keep them next to your nutrient tank. That way, you rule out pH and salt before you spend money on more fertiliser.
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 meters, pH adjustment or runoff readings? Post it below this topic. Include your medium, your input pH and EC, and your runoff pH and EC, so other growers can help you read them.