Water quality: soluble salts, hardness, and pH

Every watering and feeding leaves dissolved minerals behind when the water leaves. This is how the buildup shows up, and how to clear it.

Salt buildup follows directly from how you water, and it's worth understanding before you ever pick up a fertilizer bottle.

Soluble salts

Fertilizer and water leave dissolved minerals, called soluble salts, in the mix. They stay behind and become concentrated as water evaporates, which can make it harder for the plant to take up water; at extremely high levels they draw water out of the root tips and damage the roots directly (University of Maryland Extension, 2023). The signs include slower growth, brown leaf tips, lower leaves dropping, and a ring of salt crust at the soil line or around the pot (University of Maryland Extension, 2023). Growers measure this concentration as electrical conductivity (EC), since water with more salts dissolved in it conducts electricity better (Penn State Extension, n.d.), but you don't need a meter to manage it. The fix is leaching: every four to six months, pour at least twice the pot's volume of plain water through the mix and let it drain completely (University of Maryland Extension, 2023). Watering thoroughly with a little runoff each time, from Lesson 4, is the everyday version of the same housekeeping. If the salt level still seems high, repot into fresh substrate (University of Maryland Extension, 2023). Maryland's extension service calls salt buildup a problem of great importance for houseplants, and one that proper watering largely avoids.

Hardness, pH, and your options

Two more properties round this out, briefly. Hardness is a measure of the calcium and magnesium in your water, and both are nutrients plants need (UMass Extension Greenhouse Crops and Floriculture Program, n.d.). Softened water isn't the answer to hard water: it carries more dissolved minerals and should be avoided for houseplants (University of Maryland Extension, 2023). As for pH, what shifts the mix over time isn't your water's pH but its alkalinity: the carbonates and bicarbonates dissolved in it. Water with a high pH but low alkalinity has little effect on the mix (UMass Extension Greenhouse Crops and Floriculture Program, n.d.). For the small number of cases where tap water is a real problem, rainwater collected in clean buckets has no added chemicals (University of Maryland Extension, 2023). Distilled water is free of minerals, but it lacks dissolved nutrients, and using it for a long time may lead to deficiencies (Criscione & Holliday-Goulart, n.d.). Filters vary: an activated-carbon filter removes chloramine, and reverse osmosis doesn't (Lesson 6).

One last thing, which has nothing to do with chemistry. Extension guides advise watering with room-temperature water, between 62 and 72°F (about 17 to 22°C) (Shaughnessy & Pertuit, n.d.). One study tested cold water at the roots directly, and its scope is narrow. It used four commercial varieties of Phalaenopsis orchids grown in bark, and gave each plant either three ice cubes a week or the same volume of room-temperature water. The ice cubes didn't change how long the flowers lasted, how long each plant stayed in bloom, or the health of the roots at the end of the trial (South et al., 2017). That tells you about orchids in bark, not every houseplant, so we still use water at room temperature.

Leaves are a different story. In detached African violet leaves, a rapid drop in temperature injured the cells just under the upper surface, and the injury grew with the size of the drop; the same drop made slowly did no harm (Kadohama et al., 2013). The damage showed up about 10 minutes after the drop, and the leaves turned brown over the next day. Earlier work, reported by Kadohama et al. (2013), found that even 77°F (25°C) water injured African violet leaves kept above 95°F (35°C). So keep water off the leaves of plants like these, and water them from below, the method recommended for plants that can't tolerate wet leaves (Criscione & Holliday-Goulart, n.d.).

White and yellowish soluble-salt crust on the surface of potting substrate and around a terracotta pot rim.

Fig. 8. A white crust on the substrate and pot rim is soluble salts that have built up from fertilizer and water. Leach the pot with at least twice its volume of plain water every four to six months to flush them out.

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

A white crust has built up on the soil surface and around the pot rim, and the leaf tips are browning. What is it, and how do you fix it?

Most likely soluble salts from fertilizer and water, which stay behind and concentrate as the mix dries and can make it harder for the plant to take up water. Leach the pot: run at least twice its volume of plain water through and let it drain, every four to six months. If the salt level still seems high, repot into fresh substrate.

When is your tap water's pH worth worrying about, and where does water temperature matter?

Rarely, on its own. For the mix, the number that matters is the water's alkalinity, its carbonates and bicarbonates, rather than its pH: high-pH water with low alkalinity has little effect on the mix. On temperature, extension advice is to use room-temperature water. Leaves of plants like African violets can be injured when their temperature drops suddenly, so keep water off those leaves and water them from below.

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