Correcting Water pH For Better Plant Health

When we drink water, our kidneys adjust the blood pH to 7.35-7.45. Plants also have the ability to change pH around their roots, up to 1.5 pH points. But in more extreme pH situations, plants suffer.

Water pH measures the concentration of hydrogen ions. If you have more hydrogen ions, you have acidic water. If you have more hydroxide ions, it’s alkaline. The pH scale runs from 0 to 14, with 0 being ridiculously acidic and 14 being obnoxiously alkaline. 7 is neutral.

For the best plant health—and to grow the broadest range of species—we should aim for the middle. 6-7 pH is usually ok, with 6.5 being about optimal.

The thing is, this isn’t a simple scale. A pH reading of 2 is not twice as alkaline as 1. It’s 10 times as alkaline. It’s logarithmic. This means a pH of 8 is 100 times as alkaline as 6.

That might seem like a scary range, but plants are pretty tough. Your pines and azaleas will prefer things acidic, junipers are famously adaptable, while crabapple and elm can perform well in alkaline situations. Many plants do OK if things are not ideal.

But when we’re well beyond a good pH range, mineral deficiencies and toxicities show up:

• In highly acidic situations, calcium, magnesium, and phosphorus become insoluble, so the plant can’t take them up
• Metals like aluminum and manganese become soluble in highly acidic conditions, which results in toxicities
• Highly acid water can damage root cells
• In highly alkaline situations, nutrient lockouts happen, seen in stunted top and root growth
• Leaf chlorosis, or iron deficiency, is seen with highly alkaline water

How then do we know the pH of our water?

There are several methods of testing water, the easiest being a hand-held, battery operated, digital tester, which gives readouts of both pH and dissolved salts. Alternatively, you can use drops from an aquarist’s store to test pH. Avoid litmus paper, which is a vague reading.

Above, a digital tester for both pH and TDS (dissolved salts). As it tests both key measures of water quality, this is the recommended way to test. Stick the end in water and hey presto, a digital readout.

For just testing pH, an aquarist’s drop test kit is good enough. Fill water to the white line, add three drops of test solution, cap it, and shake. This test came out about right for bonsai—about pH 6.2 or 6.3. A pH of 6.5 is ideal.

If you’ve soft water that is alkaline, you may be fine. Usually high pH comes with hard water, and that’s a real challenge for a plant. (See this post for more on hard water and how it affects plant health.) In general, changing your pH to be mildly acidic corrects moderate hard water issues.

OK then, enough of the warm up to the subject… How then do we adjust the pH coming out of our hoses to get the best bonsai health?

Most water problems will be too alkaline. In that case, the easiest fix is to use a siphon and inject acid. Fertilizer siphons come in many sizes and complications and expense levels, the simplest being a siphon with a hoselet that goes into an acid dilution. It sucks up a certain amount to your bib, which then mixes in the hose as you begin to water.

A simple brass siphon that attaches to your hose bib, with a black hoselet that snakes into a tank with the acid dilution. You’ll have to fiddle with the dilution to get the pH you want out of the hose. These are not foolproof, so keep checking your pH.

A system with more control is the Dosetron system. Above is the Dosetron D14MZ2, the injection system we use. It’s more expensive than the simple siphon, and has a tangle of plumbing to sort out at the bib, but it does have several controls which can make either acid injection or liquid fertilizing a top end experience.

Which acid?

• Vinegar is easiest for home applications. We used muriatic for a few years before discovering it can lead to chlorine burn on sensitive plants (Vine Maple, for one). Vinegar is also safe—no chance of burning yourself, unlike muriatic, which is dirty hydrochloric
• Be sure you’re using up the vinegar dilution in a few days, as over time the pH will rise. A more stable acid is phosphoric (most commonly used acid for hydroponics), but take care when using if concentrated
• Keep testing the pH through the watering hose as you get the dilution dialed in

If your water is too acidic, like a 3.5 pH, then adding baking soda will buffer it and bring the pH up. Again, test the water out of the hose to get the right tank mix dilution for whatever siphon you’re using. Whether using acid to bring pH down or baking soda to bring it up, out of the hose the water should be somewhere between 6 and 7 pH, with 6.5 about optimal.

And then…be patient!

• If water quality was the problem, better plant health is often seen within a few months to a year. Conifers may take two years before they look better
• Any kind of water source, from a well or a municipal source, should be checked occasionally for pH
• Some municipal water sources in particular can shift over the year, from a pH in the low 6’s in the winter to over 9 or in the summer, which they do to protect the pipes and prevent metal leach
• Stay consistent. Keep testing your water to be sure your system is working

Please see this related post on water hardness, which covers what to do in extreme hard water situations.