by Mike Hansen

Taken from LCBS May 2010 Newsletter

(Original article written in 1987, updated in 1988 and in 1999)

It is with some reluctance that I write this article because so much has already been said and written about pH. In some respects it's a little like the old cliché, "Everybody talks about the weather, but nobody does anything about it." I have been talking about soil pH since I moved to Austin, TX, and for the past thirteen years, I have been trying to do something about it. Now, at the urging of a friend, I am writing about it.

To understand the significance of pH and its relationship to plants, it helps to understand what pH is. pH is a unit of measure that indicates the relative acidity or alkalinity of a substance. pH is measured on a scale of 0 to 14 with 0 being very acid, 14 being very alkaline and 7 as neutral (distilled water). The measurement along the pH scale is logarithmic which means that the each value along the scale is 10 times more concentrated than the previous value. For example, if we are comparing three samples of water one with a pH of 6, another with a pH of 5 and the third with a pH of 4, the sample with a pH of 5 is 10 times more concentrated than the sample with a pH of 6 and the sample with a pH of 4 is 10 times stronger than the pH 5 sample and 100 times stronger than the pH 6 sample. The same relationship holds true on the alkaline side of the scale.

Many years of agricultural research have established that most plants will absorb nutrients from the soil within a fairly narrow band of between pH 6.0 and pH 7.0 with pH 6.5 about optimum. Some plants like junipers prefer pH 7.0 to pH 7.5 and some plants like pines and azaleas prefer pH 6.0. Therefore, we try to tailor our bonsai soil to be somewhere between 6.0 and 7.0 Why, after raising bonsai for many years without great concern about soil pH, am I spending so much time and effort trying to understand pH and its effects on plants, bonsai in particular? The answer is simple: within a year after moving to Austin, TX, from St. Paul, MN, some of my previously very healthy and most valuable plants were stressed and were showing signs of nutrient deficiencies. In short, they were dying.
When I asked other bonsaists in the area if they have had similar experiences, the most common answer was, "You can't raise those varieties here, it's too hot."

While that advice was true about some alpine species, one thing I have learned over the years, is that plants are more adaptable to climate than we give them credit for. On the other hand, plants, especially bonsai, are a great deal more fussy about the soil environment they are grown in. Further observation revealed that the plants showing the most stress were acid loving plants, pines in particular. I dug out the do-it-yourself soil test kit and quickly discovered that my previously mildly acid soil was testing between pH 7.5 and pH 8.0. (Slightly alkaline). A little more checking with extension agents, a plant pathologist and a chemist and the cause became clear. It's in the water.

The culprit is calcium carbonate (limestone). Little wonder since all of Austin sits on the biggest chunk of limestone I've ever seen and our water comes from caverns deep within that same limestone. Normally calcium carbonate is not considered to be soluble in water but more than likely the water we use has been in the ground a very long time and over time relatively small amounts of calcium carbonate have dissolved into the water.

This property of insolubility means that after you water your bonsai, the water that stays in the pot either is absorbed by the plant or evaporates leaving the calcium carbonate behind changing the effective pH of the soil. With repeated watering the calcium carbonate builds up not only in the soil but on your pot as well. Witness the hard white deposits that begin to appear after a few weeks of watering.
What to do? Every person I spoke with had a different recommendation and most of them would have worked. When each recommendation was considered, they could be summarized under one of three categories:

• Do nothing and specialize in native and other alkaline tolerant plants.
• Treat the soil to compensate for the alkalinity introduced by the water.
• Treat the water to neutralize the alkalinity of the calcium carbonate.
  

The first recommendation was not acceptable because I have many acid loving plants in my collection and I wasn't about to give them up without a fight.

The second recommendation was a little more acceptable but I was reluctant to change my soil formulation which until now had been so successful. I did change it some by substituting acid producing materials. Later research from papers published by Dr. Carl Whitcomb and conversations with Dr. Thomas Perry lends strong support the proposition that due to the leaching effect of repeated watering, the pH of the soil in container grown plants the pH of the soil will approach the pH of the irrigation water within a few weeks after the irrigation begins. Soil amendments have little long term affect.

The third seemed to have the most promise; however, water treatment is a tricky business. Every time you add something to water to get a desired effect, you will also have side effects which can cause a bigger problem than the one you are trying to solve.

WARNING! THE METHOD OF WATER TREATMENT DESCRIBED BELOW WORKS FOR THE WATER DELIVERED TO MY HOME BY WILLIAMSON COUNTY MUNICIPAL UTILITY DISTRICT #2. THE MATERIALS USED ARE VERY HAZARDOUS TO HUMANS AS WELL AS PLANTS IF THEY NOT HANDLED CORRECTLY. I DO NOT RECOMMEND ANYONE USE THIS METHOD UNLESS THEY HAVE THOROUGHLY ANALYZED THEIR OWN WATER AND ARE COMPLETELY COMFORTABLE WITH HANDLING SULPHURIC ACID.

After considerable thought and conversations with local wholesale nurserymen, I chose sulphuric acid as the neutralizing agent. It is inexpensive and it is readily available from local chemical supply companies. I originally used hydrochloric acid because it was also readily available but when we used enough acid to change the pH of the irrigation water, it had an undesirable side effect, chlorine gas.
The sulfuric acid keeps the calcium carbonate in suspension and thereby eliminates the nasty buildup in the soil. While this sounds good and simple enough, it is anything but simple. Too little acid and it doesn't do any good, too much acid and you kill the plants. I found that I had to mix about 7/8 of a gallon of sulfuric acid to 2500 gallons of water with pH 7.3 before I noticed any change in pH. That's because the acid must react with all of the calcium carbonate, which is a powerful buffer, in the water before the acid could begin to change the pH.

The second revelation was that it took only one cup of additional acid to change the pH of that same 2500 gallons of water to pH 6.1.

Third, since I didn't have space for a 2500 gallon water tank in my back yard, I purchased a commercial metering injection pump to add the concentrated acid to my irrigation water in just the right amount. I quickly found out that concentrated acid will destroy stainless steel and other materials in some metering pumps. I had to replace my original pump to one made of all plastic materials made by DOSMATIC Inc. We recommend DOSMATIC Inc. metering pumps. They are high quality and well suited for this purpose. Also, the DOSMATIC Company offers excellent service when service is needed. Call DOSMATIC at 1-800-344-6767 (www.dosmatic.com ), and they can recommend a pump made of materials that will work for you.

Fourth, I did not trust the litmus paper and similar acid/alkaline indicators since the pH window I was trying to meet was very narrow so I purchased a *digital pH meter from the Cole-Parmer Instrument company.

Note: After accidentally destroying three different models we recommend their LX-59000-05 Waterproof hand held pH tester. So far it has stood up to our handling and continues to work well.
For what it's worth, the system works. The acid loving plants are green and healthy again and so are the junipers and other native species. All seem to be much more heat tolerant and less susceptible to iron chlorosis.

Is water treatment on the scale I have described it for everyone? Definitely not! For most people with modest collections if water treatment is needed, watering by hand with an appropriate watering can, purchasing an inexpensive pH meter and batch treating your bonsai water is the best overall answer. There are mild, effective water naturalizing products such as DynaGro’s ”pH Down,” and “pH Up” available that are safe, economical and easy to use.

If you choose to amend your bonsai soil, be cautious of soil additives that acidify the soil because they do just that. An overdose of those materials in a bonsai pot can be just as fatal as putting too much acid in the water.

There are no easy answers to complicated problems, just learning experiences. Others in our area have copied what works for us and it also seems to work well for them. Before you add acid of any kind to your water, be sure that pH is the problem before you treat your water supply. There are other common components in drinking water that cause serious problems such as sodium chloride (salt) that cannot be successfully treated with acid or alkaline additives.

Note, small changes in pH over time are more acceptable to plants, especially plants that are already stressed than making a big change at one time.

Good luck!