pH, the gatekeeper

Module III, Lesson 7, talked about pH as a property of your water and said it matters less than people fear. That's true of the water on its own. The pH that matters is the pH of the root zone, the water sitting in the substrate inside the pot (Lopez et al., 2010), and it's set by several things together. This lesson is about that number: what it does, what moves it, and how to read it.

What pH is

pH measures how acidic or basic a solution is, on a scale from 0 to 14: 7 is neutral, below 7 is acidic, above 7 is basic (alkaline) (U.S. Geological Survey, n.d.). The scale is logarithmic, so each step is a tenfold change: a pH 5 solution is ten times more acidic than pH 6 (U.S. Geological Survey, n.d.), and a substrate at pH 5.0 has a hundred times the hydrogen ions of one at 7.0 (Pennisi & Thomas, 2023). Small-looking moves on this scale are big moves.

Why it's the gatekeeper

Substrate pH affects how available each nutrient is, and the micronutrients most of all (Camberato et al., 2009). When pH is high, iron, manganese, boron, zinc and copper are harder for the plant to get at; when it's low, they come loose too easily and can be taken up to toxic excess (Pennisi & Thomas, 2023). Iron is the one that usually goes first. In soilless substrates, what decides whether iron is available is mostly the pH of the root zone: higher pH turns iron into forms that won't dissolve, which is why iron deficiency often turns up in greenhouse flower crops that were fertilized correctly; the supply was there, the availability wasn't (Owen, 2026). It shows as yellowing between the veins, especially on new leaves (Lopez et al., 2010); iron doesn't move from old leaves to new ones, which is why the new growth shows it first (Lopez et al., 2010). Lesson 4 explains that mobility rule properly.

Fig. 4. (a) In general, when pH is high, iron, manganese, boron, zinc and copper are harder for the plant to get at; when it's low, they come loose too easily and can be taken up to toxic excess (Pennisi & Thomas, 2023). (b) The same University of Georgia bulletin's chart of nutrient availability by substrate pH, redrawn from its Figure 3, which the bulletin credits as adapted from Bailey (1996). Wider bands mean more available, and the bulletin marks 5.4 to 6.0 as the range that suits all nutrients (Pennisi & Thomas, 2023).

The low end has its own problems. When substrate pH runs low, geraniums, marigolds, lisianthus and pentas can suffer iron and manganese toxicity, because those nutrients become readily available at low pH (Lopez et al., 2010); for geraniums the North Carolina State guidance puts the toxicity risk below about pH 5.8 and the deficiency risk above about 6.8 (Whipker et al., 2000). One qualification before you blame pH for every yellow leaf: the University of Georgia bulletin shows an ivy geranium grown hot and bright, with normal pH and EC, that yellowed anyway, and notes that leaf pigment genetics can produce the look too (Pennisi & Thomas, 2023). pH is a prime suspect, not the only one.

So what's the right number?

It depends who you ask and what they grow, so here are the ranges with their owners. Purdue's pH and EC guide says all nutrients are readily available in soilless substrates at pH 5.4 to 6.2, but that each species has its own optimal range (Camberato et al., 2009); its alkalinity guide groups greenhouse crops into 5.4 to 6.2 for plants that struggle to take up iron, 5.8 to 6.4 for the general group, and 6.0 to 6.6 for plants that take up iron easily (Lopez et al., 2010). The University of Georgia gives 5.4 to 6.8 as the range for maximum growth in general (Pennisi & Thomas, 2023), and a Cornell extension newsletter gives 5.5 to 6.5 (Rusinek, 2016). For the plants most of us keep indoors, the University of Florida's commercial production guides recommend a medium pH of 6.0 to 6.5 for Philodendron, Anthurium and Aglaonema, and 6.5 to 7.0 for pothos (McConnell et al., 2003b; Chen et al., 2015b; Chen et al., 2015a; McConnell et al., 2003a). Those are greenhouse production figures, written for growers running crops on fertilizer programs. The same newsletter notes that the optimal range in soilless mixes sits about 1.0 to 1.5 units lower than in mineral soils, because nutrient availability works differently in them (Rusinek, 2016), which is why garden-soil pH advice doesn't transfer to pots.

Our target range is 5.8–6.3, which sits inside the Georgia and Cornell ranges, overlaps the Purdue ranges and Florida's 6.0 to 6.5 for Philodendron, Anthurium and Aglaonema, and sits just below Florida's 6.5 to 7.0 for pothos. It's a target, not a measurement of anything.

Fig. 5. pH runs from 0 to 14: 7 is neutral, below 7 is acidic and above 7 is basic, and each step is a tenfold change (U.S. Geological Survey, n.d.). Enlarged below are the ranges from this lesson with their owners: Purdue 5.4 to 6.2 (Camberato et al., 2009), the University of Georgia 5.4 to 6.8 (Pennisi & Thomas, 2023), a Cornell extension newsletter 5.5 to 6.5 (Rusinek, 2016), and the University of Florida's 6.0 to 6.5 for Philodendron, Anthurium and Aglaonema (McConnell et al., 2003b; Chen et al., 2015a, 2015b) and 6.5 to 7.0 for pothos (McConnell et al., 2003a). Our 5.8 to 6.3 is a target, not a measurement.

What sets it, and what moves it

Start with the ingredients. Peat is very acidic, with sources putting unamended sphagnum peat around pH 3 to 4.5, so limestone is routinely added to peat mixes to bring the starting pH up (Pennisi & Thomas, 2023; Evans, n.d.); commercial greenhouse blends typically arrive with limestone already incorporated, a nutrient charge added, and pH adjusted to about 6.0 (UMass Extension, n.d.). Coir starts higher than peat and needs less lime (Owen & Lopez, 2015). Then the pot starts drifting, and three things push it.

The first is your water, and it's the one Module III set up. What raises substrate pH over time is the water's alkalinity, its dissolved bicarbonate and carbonate, not the water's pH. Watering with high-alkalinity water works on the pot the way adding lime does, while the water's own pH has minimal effect (Lopez et al., 2010). The Georgia bulletin's example makes the point: pH 9.0 water with low alkalinity barely moves a substrate, while pH 7.0 water with high alkalinity moves it a lot (Pennisi & Thomas, 2023), and UMass notes that high-pH water with low alkalinity usually causes no problems at all (Cox, 1995). Small pots have less buffering (less resistance to a change in pH), so high-alkalinity water changes their pH quickly; Purdue's rule of thumb is that alkalinity around 120 ppm as calcium carbonate is likely to cause high-pH trouble over time (Lopez et al., 2010). We look for it on the water supplier's annual quality report, where it's usually listed as alkalinity.

Fig. 6. The Georgia bulletin's example: pH 9.0 water with low alkalinity barely moves a substrate, while pH 7.0 water with high alkalinity moves it a lot (Pennisi & Thomas, 2023). Watering with high-alkalinity water works on the pot the way adding lime does (Lopez et al., 2010).

The second is fertilizer, which Module VI covers in full. In short: an ammonium-based fertilizer acidifies the substrate, and a nitrate-based one raises pH as the plant takes it up (Pennisi & Thomas, 2023); with very low-alkalinity water and an acidic ammonium fertilizer, substrate pH can fall (Lopez et al., 2010). Greenhouse guidance treats nitrogen form as having a much greater effect on substrate pH than the pH of the solution going in (Argo & Fisher, n.d.). The third is time: as peat and bark break down, their ability to impart acidity can decrease (Pennisi & Thomas, 2023). Put together, substrate pH is the outcome of water alkalinity, fertilizer, the lime in the mix, the substrate's components and the plant itself, all interacting (Lopez et al., 2010). None of them is "the" cause.

Reading it at home

The standard way to read a pot's pH without disturbing the roots is the pour-through. Water or feed as usual, let the pot drain for 30 to 60 minutes, set a saucer under it, then pour enough distilled water over the surface to collect about 50 mL (a little under a quarter cup) of drainage; for a 6.5-inch (16.5 cm) pot that takes about 100 mL (a little under half a cup) of distilled water (Camberato et al., 2009). Test what comes out. An inexpensive pH/EC pen does the job. Two cautions. Readings depend on the method, and that matters far more for EC (electrical conductivity, the reading that tracks dissolved salts) than for pH: pour-through, saturated-media and 1:2 extractions give quite different EC numbers while their pH results differ much less (Cox, 2019). And a dry pot reads higher in EC while too much water dilutes it, which is why the pour-through needs a consistent routine (Camberato et al., 2009). Purdue also advises against pushing a probe directly into the mix unless its moisture is very consistent (Camberato et al., 2009). If you prefer a simpler approach, the 1:2 extraction method of one part air-dried substrate to two parts water provides a reasonable occasional check (Cox, 2019).

One last thing on fixing it. Chelated iron will green a plant up at a higher pH, but it doesn't lower the substrate pH or neutralize the water's alkalinity, so the deficiency comes back, and it only supplies iron (Lopez et al., 2010; Owen, 2026). The durable fix is to the water and the mix. How, exactly, is Module VI.