Level: Advanced
Most houseplants do not need routine pH or EC tests. Measure when new leaves yellow between green veins, a white crust forms with crisped leaf tips, or after you change the mix. pH affects nutrient uptake, especially micronutrients, and EC reflects dissolved salts. Use one method every time and watch the trend [1].
Key points
- pH affects nutrient access. For peat-based mixes Clemson lists 5.3 to 6.5, and Purdue says nutrients are readily available at 5.4 to 6.2 [1][2].
- EC must be read with the method: the same pot can be “normal” at 2.7 to 4.6 mS/cm by PourThru, but 0.76 to 1.25 by 1:2 dilution [1].
- Purdue advises against sticking a probe straight into the pot unless moisture and timing are consistent [1].
- Hong Kong tap water averages pH 8.3, alkalinity 29 mg/L and conductivity 141 µS/cm [5].
What do pH and EC actually measure?
pH tells you how acidic or alkaline the mix solution is. Clemson says it directly influences nutrient availability, and a value too high or too low can cause toxicity or deficiency [2]. Purdue adds that pH matters most for micronutrients and shows iron deficiency caused by high pH [1].
EC measures the total dissolved salts. It can show how much fertiliser is present or whether salts have built up [1][2]. High EC reduces growth, and low EC can mean too few nutrients [2].
Source setting matters. Purdue wrote its methods and ranges for greenhouses and nurseries, and Clemson’s ranges come from its laboratory reports [1][2]. We found no university table of pH or EC targets for individual houseplant species, so treat the numbers below as rough guides.
When is it worth testing?
If the plant looks normal, skip it. If one of these clues appears, rule out light, watering and roots first, then test.
On a phone, swipe sideways to see every column.
| Clue | Possible cause | Next step |
|---|---|---|
| New leaves yellow, veins still green | Possibly high pH limiting iron and other micronutrients [1], though other causes exist | Read yellow new leaves with green veins, then test pH |
| White crust on the surface or rim, brown tips | Possible salt build-up [3] | Flush first and read brown leaf tips; test EC afterwards for a baseline |
| Brown edges and wilting soon after feeding | Possibly fertiliser too strong [4] | Stop feeding, read fertiliser burn; confirm with EC |
| Slow, pale growth, no crust | Low EC may mean too few nutrients [2]; low light gives the same look | Check light numbers first |
| New mix or new water source | You want a starting point | Test once, record it, repeat with the same method |
How do you measure it so the number means something?
Purdue extracts the mix solution with distilled water, which has no minerals, then measures that extract [1]. It does not recommend pushing a probe into the pot unless moisture is consistent and you always test on the same day [1]. The two methods below are Purdue’s greenhouse procedures; scaling them down for one home pot is our adaptation.
Method A: PourThru (no digging)
- Water or feed as usual, then let the pot drain for 30 to 60 minutes with a saucer underneath [1].
- Slowly pour distilled water on the surface to collect about 50 ml of leachate. Purdue lists about 75 ml for a 4 to 6 inch pot, 100 ml for 6.5 inch and 150 ml for a 1 gallon pot [1].
- More than 70 ml of leachate dilutes the salts, and under 50 ml may not cover the probe [1].
- Measure pH and EC, and write down the date and water volume [1].
Method B: 1:2 dilution. Mix one part mix with two parts distilled water by volume, for example one cup to two cups. Leave it for 30 minutes, filter through a coffee filter, then measure [1]. The added water is fixed, so it depends less on personal judgement [1].
Calibrate the meter with standard solutions, keep the same person, method and timing, and plot the readings [1]. Purdue calls the trend the real value of testing [1].
How do you read the result? The same pot gives different numbers by method
Purdue lists EC ranges for the same nutrient status under three methods, in mS/cm [1]. Reading 0.9 by 1:2 dilution against the PourThru column would wrongly label it “very low” [1].
On a phone, swipe sideways to see every column.
| Purdue class | PourThru | 1:2 dilution | Saturated extract (SME) |
|---|---|---|---|
| Very low | 0 to 0.9 | 0 to 0.25 | 0 to 0.75 |
| Low (seedlings, salt-sensitive plants) | 1.0 to 2.6 | 0.26 to 0.75 | 0.76 to 2.0 |
| Normal (most established plants) | 2.7 to 4.6 | 0.76 to 1.25 | 2.1 to 3.5 |
| High (growth may suffer) | 4.7 to 6.5 | 1.26 to 1.75 | 3.6 to 5.0 |
| Very high (possible salt injury) | 6.6 to 7.8 | 1.76 to 2.25 | 5.1 to 6.0 |
Purdue says very high readings may cause leaf-edge burn and wilting [1]. Clemson lists 0.8 to 5.0 mmhos/cm for peat-based mixes and 0.7 to 3.0 for bark-based mixes [2]. Mmhos/cm and mS/cm are two names for the same unit (a unit conversion, not a source figure). Clemson does not name its extraction method, so we cannot line its range up with the table (our inference) [2].
Advanced. The Water Supplies Department reports average tap-water conductivity of 141 µS/cm [5], which is 0.141 mS/cm. Against the table, tap water adds little EC itself; high pot EC more likely comes from fertiliser and salts left behind as water evaporates (our inference).
How does Hong Kong water and indoor life change the picture?
The WSD 2024 to 2025 report gives tap-water pH of 7.3 to 9.2 (average 8.3), and total alkalinity as calcium carbonate of 8 to 56 mg/L (average 29) [5]. Purdue says high water alkalinity can raise substrate pH over time [1].
We found no university source stating what alkalinity in mg/L counts as high, so we cannot say whether Hong Kong water will push your pH up. Measuring your own pot over several months is the safer route (our inference). Clemson also notes that nutrient uptake and heavy irrigation can raise pH over time [2].
Illinois Extension warns that plants grow slowly in dim light and are more easily damaged by too much fertiliser [4]. In a dim, air-conditioned corner, feed conservatively. See using numbers to judge light and Hong Kong tap water for plants.
What should you do if the reading is high or low?
High EC or a white crust. Colorado State suggests scraping off the crust and about half an inch to an inch of surface mix, then filling the pot to the top with tepid water a couple of times, letting it drain between rinses [3]. Clemson says container mixes can be flushed with at least twice the pot volume of water [2]. The amounts differ; scale to the root ball and keep drainage holes clear [2][3].
Low EC. Clemson suggests adding fertiliser or organic matter [2]. Illinois Extension says most houseplants need feeding only every 1 to 3 months between March and September, and little or none in winter [4]. Follow the product label for the dose.
pH out of range. Clemson says acidifying agents lower pH and lime raises it [2], but we found no university guidance on safe home doses for a small pot, so we do not give a recipe. Overshooting is easy in a small volume of mix (our inference). Fresh mix suited to the plant is simpler; see potting mix science.
Flushing will not turn already scorched leaf tips green. Judge success by the new leaves.
What should you not do?
- Do not judge from one reading. Purdue stresses tracking values over time with the same method [1].
- Do not mix PourThru, 1:2 and SME numbers, as the table above shows [1].
- Do not use tap water instead of distilled water for the extract; Purdue uses water without minerals [1].
- Do not add a large dose of fertiliser because of one reading. Illinois Extension says too much fertiliser can burn roots [4].
- Do not trust online tips to adjust pH with vinegar, baking soda or coffee grounds. We found no university or botanic garden support for them in home pots.
- Do not push a probe into a very dry or freshly watered pot and trust the number. Purdue warns that results depend on consistent moisture [1].
How do you prevent problems, and when should you ask for help or let go?
Prevention: feed to the label, let water drain from the base and discard what collects in the saucer [3], leach the pot every 4 to 6 months as Illinois Extension advises [4], and log date, volume and method each time [1].
When to ask for help: if the same symptom returns on new leaves after flushing or a fresh mix, take your trend chart to a nursery, plant doctor or lab. Purdue says such records are the key evidence when problems are only indirectly related to pH or EC [1]. If you suspect roots, read root rot first.
When to give up: repeated high EC alongside black, soft roots usually means root health is the real issue, not the reading (our inference). If most roots are gone, recovery is unlikely.
Frequently asked questions
Can I push a pen-style pH or EC probe straight into the pot?
Purdue does not recommend it unless moisture is very consistent and you always test on the same day with the same routine [1]. Measuring the extract with distilled water is more reliable.
Why distilled water?
Purdue’s methods extract the mix solution with water that has no minerals and then measure the extract [1]. Tap water carries its own calcium, magnesium and salts, which would blur the result (our inference).
If my water pH is high, will my mix pH be high?
Not necessarily. Purdue points to alkalinity, which can raise substrate pH over time [1]. The WSD report gives local pH and alkalinity [5], but we found no university statement of what is high, so measure your own pot over time.
Can I compare my reading with Clemson’s EC range?
Be careful. Clemson’s range comes from laboratory reports and the page does not name the method [2], while Purdue shows the same status gives very different numbers by method [1].
How often should I flush the pot?
Illinois Extension suggests every 4 to 6 months [4], while Colorado State describes flushing once symptoms such as a white crust appear [3]. You can combine both: flush on a schedule and act sooner if symptoms show.
Sources
- Purdue University Extension: “HO-237: pH and Electrical Conductivity Measurements in Soilless Substrates” — Definitions, PourThru and 1:2 dilution steps, EC table by method, iron deficiency from high pH, probe-in-pot caveat (greenhouse and nursery setting)
- Clemson Cooperative Extension (HGIC): “How To Interpret Soilless Mix Test Results for Healthy Plant Growth” — pH and EC ranges for peat-based and bark-based mixes, responses to high or low EC, flush volume for containers
- Colorado State University Extension (PlantTalk Colorado): “1339 – Leaching Salts From Potting Mixes” — Salt build-up signs (white crust, brown tips) and the home leaching steps
- University of Illinois Extension: “Houseplants: Care” — Fertiliser frequency, fertiliser damage in dim light, leaching every 4-6 months
- Water Supplies Department, Hong Kong: “Drinking Water Quality for the Period of April 2024 – March 2025” — Minimum, maximum and average pH, alkalinity, conductivity and hardness of Hong Kong drinking water
Further reading
- Back to Plant Doctor
- Yellow new leaves with green veins
- Fertiliser burn after feeding
- Hong Kong tap water for plants
- Tap water or filtered water for houseplants
- White marks on houseplant potting mix
Sources checked: 31 October 2026. This is general guidance, not an on-site diagnosis. Always follow the product label.
