The nutrient solution: pH, EC and what the plant actually takes up

Every grower who feeds from a bottle ends up with two small meters and two numbers to chase: pH, the acidity of the feed, and EC, electrical conductivity, a rough measure of how strong it is. Any grow shop will give you a confident pair of target figures, and the tutorial in this app does too. For plants in coco coir it asks for a feed pH of 5.8–6.0 for seedlings, 5.8–6.2 in vegetative growth (aiming for 5.9–6.1) and 6.0–6.2 in flower. Feed strength climbs from an EC of 0.4–0.6 mS/cm for seedlings to 1.2–1.6 in veg, 1.5–1.8 through the stretch and 1.8–2.2 at peak flowering. It also asks growers to measure the runoff, the water that drains out of the bottom of the pot, and to treat a runoff EC 0.6 higher than the feed as a sign that salts are building up.

These are practice values, worked out by growers. This article looks at what lies behind them: what pH and EC measure, why they matter to a root, how horticulturists sample the root zone, and what the few cannabis feeding trials found. It is educational; in the UK, growing cannabis, even with a prescription, requires a Home Office licence.

pH: the gatekeeper of availability

pH is a measure of how acidic or alkaline a solution is, on a scale where 7 is neutral, lower numbers are acidic and higher numbers alkaline. The scale is logarithmic, so a drop of one unit means ten times more hydrogen ions. Plants do not need a particular pH for its own sake. What matters is what pH does to the nutrients dissolved around the root.

Extension guides for greenhouse growers put it simply. In soilless substrates, "all nutrients are readily available" at a pH of 5.4 to 6.2, although each species has its own optimum, and pH matters especially for the micronutrients (Camberato et al., 2009). When the substrate pH drifts too high, iron becomes hard to take up and the plant shows interveinal chlorosis: yellow leaves with green veins. In hydrangeas this readily occurs above pH 6.3 to 6.5 (Whipker et al., 2018), and the Purdue guide illustrates the same iron deficiency, induced by high pH, in calibrachoa and maple (Camberato et al., 2009).

The chemistry behind this is precipitation. Sambo et al. (2019), reviewing nutrient solutions for soilless crops, explain that above pH 7 iron, zinc, copper, nickel and manganese can drop out of solution as insoluble hydroxides and oxides, and that iron in its oxidised form may precipitate even well below neutral. Phosphate becomes less available above pH 7, mostly because it precipitates with calcium, and in doing so it can drag calcium, magnesium, iron and manganese down with it. Organic binding agents known as chelates, such as EDTA, DTPA or EDDHA, help keep iron, copper and zinc in solution (Sambo et al., 2019).

Too low is a problem as well. In the introduction to their cannabis trial, Velechovský et al. (2024) summarise the guidance they draw on: hydroponic culture typically runs at pH 5.5–6.0 and soilless production at 5.5–6.5; below 5.5 there is a risk of manganese toxicity, and above 6.5 phosphorus, iron and manganese become less available. A long-standing guide for hydroponic tomatoes in Florida sets the final solution pH at 5.8 to 6.2 (Hochmuth and Hochmuth, 2018).

The sources draw the window slightly differently but agree on its shape: a band a little on the acidic side of neutral, roughly 5.5 to 6.2 for soilless media.

The root moves the pH

A grower sets the pH of the feed, but the root does not leave it there. Roots exchange charged particles with the solution around them, and the form of nitrogen they take up matters most. According to Sambo et al. (2019), feeding nitrogen only as nitrate tends to make the solution more alkaline, while ammonium makes it more acidic, which is why most nutrient solutions contain both. Pure ammonium nutrition can be toxic to many plants and can inhibit nitrate uptake; in tomato, root growth was best with nitrate and ammonium in a 3:1 ratio, and was strongly inhibited when ammonium was too high.

A hydroponic study on flowering Chinese cabbage shows how large the effect can be. Zhu et al. (2021) grew the plants in a greenhouse in aerated buckets of nutrient solution that was topped up with water but never replaced, using four ratios of ammonium to nitrate. By harvest, the all-nitrate control and the 10:90 mix had drifted to about pH 8.0; the 25:75 mix sat at 5.8; and the 50:50 mix had fallen to 3.6. The 25:75 mix gave 1.54 times the yield of the all-nitrate control, while the 50:50 mix reduced yield. This is a leafy vegetable, not cannabis, and the solution was never changed, so the drift is larger than a grower who feeds fresh solution would see. But it shows that pH drift is information: it reflects what the roots are taking up.

The same thing happened in a cannabis trial. Kpai et al. (2024) made their solutions from nitrate salts and set them to pH 5.8; in deep-water culture, both the pH and the EC of the solutions rose between the start and end measurements. The authors suggest that nitrate uptake by the roots pushed the pH up, and that the EC rose because, as the plants took up water and nutrients, what remained in the tank became more concentrated. They also note that a high concentration of hydrogen ions, that is a low pH, inhibits the uptake of cations such as magnesium, calcium, zinc and manganese.

EC: how strong, not what's in it

Electrical conductivity measures how easily a current passes through a solution. Pure water conducts poorly; dissolved salts, which split into charged ions, make it conduct. EC is therefore "a measure of the dissolved salt concentration" (Camberato et al., 2009), and for a nutrient solution it is a quick gauge of strength. Growers usually read it in millisiemens per centimetre (mS/cm); scientific papers often use decisiemens per metre (dS/m), which is the same number. Sambo et al. (2019) give the typical range for soilless nutrient solutions as an EC of about 0.8 to 4.0 dS/m.

What EC cannot tell you is the recipe. A reading of 1.8 could be a balanced feed, a feed heavy in potassium and light in calcium, or tap water with a lot of sodium chloride in it. Whipker et al. (2018) make the point plainly: an in-house EC test will confirm a low fertiliser level, but it will not identify which elements are short. For that, growers in commercial horticulture send samples to a lab. Sambo et al. (2019) add that as plants take up some nutrients faster than others and water evaporates, the composition of a solution changes and unwanted ions such as sodium and chloride can accumulate, all of which shifts the EC without saying which ion moved. They also note a subtler point: in concentrated solutions ions interfere with one another, so the "active" concentration a root experiences is lower than the amount dissolved, by around 15% at typical nutrient-solution strengths.

The balance between nutrients matters as much as the total. The Florida tomato guide warns that high potassium can interfere with the uptake of calcium and magnesium, leading to magnesium deficiency on lower leaves and blossom-end rot in fruit (Hochmuth and Hochmuth, 2018). In cannabis, Kpai et al. (2024) found that raising phosphorus and potassium in the solution lowered the magnesium concentration in the leaves. A strong EC with the wrong balance can still leave a plant short.

Signs at either end

Extension guides describe the symptoms at both ends. Low substrate EC typically shows as yellowing of the lower leaves (Whipker et al., 2018). High EC brings reduced vigour, especially in hot weather, and at very high levels salt injury through reduced water uptake, with "marginal leaf burn and wilting" (Camberato et al., 2009). These descriptions are for ornamental pot plants, but the mechanism, salty water being harder for roots to take up, is general.

Feed strength in professional programmes rises with the crop. In the Florida tomato programme, nitrogen in the delivered solution goes from 70 parts per million at transplant to 150 late in the season, and potassium from 120 to 200, while phosphorus stays at 50 (Hochmuth and Hochmuth, 2018). The tutorial's rising EC follows the same logic: small plants need little, and demand grows with the canopy and the flowers.

Three ways to ask the root zone

The feed is what you put in; what the roots live in is the solution held in the substrate, which can be stronger or weaker and more or less acidic. Commercial horticulture uses three accepted methods to sample it, described in the Purdue guide (Camberato et al., 2009) and a Greenhouse Grower series by Whipker et al. (2018).

• PourThru. Water or feed the pots as usual, let them drain for 30 to 60 minutes, set a saucer underneath and pour a measured small amount of distilled water on top, enough to push out about 50 ml of leachate. That liquid is the root-zone solution displaced from the pot. It is non-destructive, samples the whole root zone and works in coconut coir, but results vary: dry pots give higher EC, and too much water dilutes the sample.
• Saturated media extract. Take a sample of substrate from the middle of the pot, wet it with distilled water just to saturation, let it stand for 30 minutes and filter out the liquid. It is accurate and is the standard method of commercial labs, but it means digging into the pot and disturbing roots.
• 1:2 dilution. Mix one part substrate with two parts distilled water by volume, let it stand, and filter. It is quick and the amount of water is fixed, but it is also destructive, and because of the extra water its EC readings are much lower.

Because each method dilutes the root-zone solution differently, the same pot gives different EC numbers, and each method has its own interpretation table. Here is a slice of the Purdue table, which was drawn up for greenhouse ornamentals (Camberato et al., 2009):

pH, by contrast, comes out similar whichever method is used (Whipker et al., 2018).

The "normal" PourThru band looks high next to the tutorial's feed EC of 1.8–2.2, but the two are not comparable: the table describes the solution inside the pot of top-watered ornamentals, not the feed, and the guides say to adjust ranges by crop and watering method.

What home growers usually measure is a fourth thing again: the runoff collected while watering. It sits somewhere between the feed and the root-zone solution, depending on how much feed passes through and how long it spends in the pot. No source in this library checks simple runoff readings against the PourThru method, so runoff numbers should be read as a home-made index, consistent with itself, rather than compared with published tables.

Calibration and trends

A pH pen or EC meter is only as good as its last calibration. The guides recommend calibrating with standard solutions: pH 4, 7 and 10 buffers, and an EC standard of 1.41 mS/cm for many meters (Whipker et al., 2018), with the calibration matched to the pH range being tested (Camberato et al., 2009). Meters and solutions should be stored at room temperature and low humidity, not in the growing area. Purdue advises against probes pushed straight into the substrate unless substrate moisture is very consistent and readings are taken on the same day of the week as part of a fixed routine (Camberato et al., 2009). The tutorial's set-up step, which calibrates with pH 4.01 and 7.01 buffers and an EC standard of 1.413, matches this advice.

The most useful point in the extension literature is about trends. Purdue recommends taking several samples and working from the average, testing the same way and ideally by the same person each time, plotting pH and EC on a graph, and adjusting fertilisation in good time based on the trend: "This is the real value of testing" (Camberato et al., 2009). One reading can be thrown off by a dry pot, a dirty probe or a heavy watering. A line that creeps steadily upward over two weeks is much harder to argue with.

What the cannabis trials found

Research on cannabis nutrition is still thin, largely because of past legal restrictions, as Kpai et al. (2024) note. Two recent trials in this library are worth looking at closely, partly for what they found and partly for how different their conditions are from a home tent.

A two-week trial in a tent

Kpai et al. (2024), at McGill University in Canada, grew two-week-old clones of a medical cannabis accession called 'The New' in deep-water culture: roots hanging in 15-litre tanks of aerated solution, replaced weekly. The plants grew in a grow tent under an 18-hour day at 360–400 µmol/m²/s, at 25 °C, 65% relative humidity and 470–520 ppm CO₂, for just two weeks of vegetative growth. Fifteen combinations of nitrogen, phosphorus and potassium were tested, with five replicates each, and solutions were set to pH 5.8.

From the growth responses and the efficiency with which nutrients were used, the authors recommend 160–200 mg/L nitrogen, 30 mg/L phosphorus and 60 mg/L potassium for the vegetative stage. Raising phosphorus from 30 to 100 mg/L made little difference to growth. As described above, pH and EC rose in the tanks. It is a careful trial, but short, with one cultivar, young clones and water culture; it says nothing about flowering and gives nutrients in mg/L, not a target EC for a coco pot.

A full crop in a commercial-style room

Velechovský et al. (2024), at the Czech University of Life Sciences in Prague, grew a full crop of a THC-dominant cultivar, 'McLove'. The 220 plants grew in 3.45-litre pots of expanded clay on fertigated tables in a 15 m² room, at 27.5 plants per square metre, under six 1000 W HPS lamps delivering about 1,029 µmol/m²/s, with CO₂ raised to 550 ppm by a methane burner. Air was 25 °C, with 60% humidity in the one-week vegetative phase and 40% in flower. The feed was set to pH 5.9.

The control feed's EC (the paper's tables give no unit) rose from 0.97 in week one to 1.74 in weeks six to nine. An enhanced feed with more phosphorus, potassium and iron reached 2.05 in week five and 2.34 from week six. The trial also compared a recirculating system with drain-to-waste. Neither the richer feed nor the type of system had a statistically conclusive effect on final flower mass, and the richer feed did not significantly change cannabinoid concentrations. The authors do report a higher calculated THC yield per square metre for the enriched feed, up to 50.7%, and they conclude, in line with earlier work, that raising potassium beyond about 60 mg/L brings no benefit. With three plants sampled per treatment each week, the headline result is best read cautiously: pushing the feed from about 1.7 to 2.3 did not measurably increase the final flower harvest. From week ten, the plants received only demineralised water, a final clear-water phase built into the protocol rather than tested as a variable.

What transfers

Neither trial tests what a coco grower actually faces: which root-zone pH and EC give the best result in a fabric pot under LED. What does transfer is the chemistry. Solution pH drifts with nitrogen uptake, EC climbs as plants drink, and a stronger feed is not automatically a bigger harvest.

The tutorial's numbers, read against the research

The pH column sits comfortably inside the window the extension literature describes, 5.4 to 6.2 (Camberato et al., 2009), and matches the 5.8 to 6.2 used for hydroponic tomatoes (Hochmuth and Hochmuth, 2018). The slight rise towards 6.0–6.2 in flower is a practice choice; no source here tests it.

The EC column rises with the crop, as professional feeding programmes do. Its peak of 1.8–2.2 lies between the two feeds in the Velechovský et al. (2024) trial. Since the stronger of those did not give a measurably larger flower harvest, the research gives no reason to go higher, which agrees with the tutorial's own warning against pushing past EC 2.4 in the belief that "more feed = bigger yield".

Two parts of the tutorial are pure craft. One is the runoff rule: a runoff EC about 0.6 above the feed signals salt build-up, and in late flower a feed of 1.9 coming out at 2.6 calls for a lighter feed. It is a reasonable reading of a trend, in the spirit of the extension advice, but the threshold itself has not been tested. The other is the advice to raise EC until the leaf tips show a slight burn. Extension guides list marginal leaf burn as a symptom of salt injury (Camberato et al., 2009), so using it as a target means steering at the edge of excess. The tutorial pairs it with a safety valve, cutting the feed by 15% if tips burn back more than a millimetre, and a cautious grower may prefer to stop short of any burn at all.

The tutorial's final flush with plain water is a separate topic; the site's view is that a brief, moderate flush remains a recommended small-grow practice.

What this means for a 1×1 tent

• Keep feed pH in roughly 5.8–6.2 for coco. The tutorial's values fit the research window. Calibrate the pH pen with 4 and 7 buffers regularly, and the EC meter with its standard, and check it again whenever a reading looks odd.
• Treat EC as strength, not recipe. A good number on the meter does not guarantee a balanced feed. Mix the same way each time, use a complete nutrient line, and remember that very high potassium can crowd out magnesium and calcium.
• Measure the same way every time. Collect runoff from the same pot, at the same point in the watering, after the same amount of feed. Write down feed and runoff pH and EC at least weekly, and look at the trend rather than the single number.
• Read the drift. Runoff EC climbing steadily above the feed suggests salts are building up: water through with more runoff, or ease the feed. Runoff EC well below the feed, with pale lower leaves, suggests the plant is hungry. A steady pH drift tells you something about what the roots are taking up.
• Don't chase the high end. In the only full-crop cannabis trial here, a feed of about 2.3 did not give a measurably heavier harvest than one of about 1.7. Start lower and let the plant show you it wants more.
• Do not compare runoff with lab tables. Published PourThru and extract ranges were made for other crops and methods.

What we know, and what we don't

The chemistry is well established and transfers directly to a tent. pH controls the solubility of nutrients, with iron, manganese, zinc, copper and phosphate becoming harder to reach on the alkaline side (Sambo et al., 2019), and greenhouse guides consistently place the comfortable window for soilless media at about 5.4–6.2 (Camberato et al., 2009). EC measures total dissolved salts and says nothing about the balance between them. Nitrogen form shifts solution pH (Sambo et al., 2019; Zhu et al., 2021), and in cannabis, as in other crops, solution pH and EC rise as plants feed (Kpai et al., 2024). The methods of sampling the root zone, and the value of calibration and trend-plotting, are standard extension practice.

Less certain is how this translates into numbers for cannabis. The extension EC bands were drawn up for ornamentals, the hydroponic guide for tomatoes, and the two cannabis trials used water culture with young clones (Kpai et al., 2024) and a dense commercial-style room with HPS and added CO₂ (Velechovský et al., 2024); they report feed concentrations, not root-zone targets.

What we do not know, from this library, is the best root-zone EC for cannabis in coco at different stages, whether simple runoff readings track the established PourThru method, how cultivars differ in their appetite, or whether the slight pH rise many growers use in flower makes any difference. Until such work exists, the tutorial's numbers are best read as they are meant: sound practice that sits inside the research window, to be adjusted by watching the plant and the trend.

Sources

Camberato, D.M., Lopez, R.G. and Mickelbart, M.V. (2009) pH and Electrical Conductivity Measurements in Soilless Substrates (HO-237-W). West Lafayette, IN: Purdue Extension.

Hochmuth, G.J. and Hochmuth, R.C. (2018) Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida (HS796). Gainesville, FL: UF/IFAS Extension (EDIS). doi:10.32473/edis-cv216-1990.

Kpai, P.Y., Adaramola, O., Addo, P.W., MacPherson, S. and Lefsrud, M. (2024) 'Mineral nutrition for Cannabis sativa in the vegetative stage using response surface analysis', Frontiers in Plant Science, 15, 1501484. doi:10.3389/fpls.2024.1501484.

Sambo, P., Nicoletto, C., Giro, A., Pii, Y., Valentinuzzi, F., Mimmo, T., Lugli, P., Orzes, G., Mazzetto, F., Astolfi, S., Terzano, R. and Cesco, S. (2019) 'Hydroponic solutions for soilless production systems: issues and opportunities in a smart agriculture perspective', Frontiers in Plant Science, 10, 923. doi:10.3389/fpls.2019.00923.

Velechovský, J., Malík, M., Šenkyřík, J.B. and Tlustoš, P. (2024) 'Effect of augmented nutrient composition and fertigation system on biomass yield and cannabinoid content of medicinal cannabis (Cannabis sativa L.) cultivation', Frontiers in Plant Science, 15, 1322824. doi:10.3389/fpls.2024.1322824.

Whipker, B., Henry, J., Owen, W.G., Currey, C., Krug, B. and Lopez, R. (2018) 'Greenhouse Toolkit Series: keeping in front of the curve: pH and EC monitoring', Greenhouse Grower, January, pp. 62–70.

Zhu, Y., Qi, B., Hao, Y., Liu, H., Sun, G., Chen, R. and Song, S. (2021) 'Appropriate NH4+/NO3– ratio triggers plant growth and nutrient uptake of flowering Chinese cabbage by optimizing the pH value of nutrient solution', Frontiers in Plant Science, 12, 656144. doi:10.3389/fpls.2021.656144.