by Steven Hendricks

This article and the following article on page 6 were taken from "BONSAI NEWS" A monthly publication of the Lake Charles Bonsai Society (July 2009). It was originally published in the Fort Worth Bonsai Society Newsletter, Jan 1994 pp4-6. Make sure you read both articles before you draw any conclusions - ED.

Did the leaves of your maples or azaleas burn around the edges this summer? A Japanese maple and Satsuki azalea of mine did burn and badly enough that I fear for their survival. I asked around and heard various theories as to why, but people seem fairly evenly divided between "moisture stress" and "salt build-up in the leaves". Since I can't do anything about the summer heat, I decided to do what I could for the salt build-up. At a recent workshop Warren Hill provided an in-depth look at this phenomenon and the most complete explanation I have ever heard. Warren stated that the leaf burn is a sign of sodium build-up in the soil and the leaves of our plants. It is caused by the retention of sodium by our soils which act like an ion exchange medium.

What is an ion exchange medium? Think of the water softener unit that many people install in their homes. It usually consists of a big tank that contains a granular material (medium) onto which some sort of chemical group is bonded. Generally this group is the sodium salt of sulfonic acid. The sodium is attached to this group only by a weak electrostatic attraction. The sulfonic acid group is negatively charged and the sodium ion is positively charged. When water containing hard water components such as calcium (+2 charge) and magnesium (+2 charge) pass through this material, the weakly charged sodium ions (+1 charge) are kicked off by the more highly charged elements. The hard water components are retained in the exchange medium and soft (read "sodium laden") water is released. This is the same kind of process that is happening in our soils.

Many of us use Turface, a baked clay product, in our bonsai potting soil mix. According to Warren, the clays in Turface are no more desirable in our potting mix than in the clay soils with which most of the Southwest is blessed. The clays in the Turface possess a net negative charge and act as an ion exchange medium, just like a water softener.

The positively charged ions (like sodium) in our tap water are attracted to and attach themselves to the clay particles and any other negatively charged soil constituents. They displace hydrogen ions that possess an even weaker charge. Because our water contains high levels of sodium, the soil quickly becomes saturated with retained sodium. As transpiration takes place, water, containing dissolved nutrients and sodium, is drawn up from the soil into the leaves. Since the leaf tips are the areas of greatest transpiration, the sodium concentrates preferentially in these areas. When the sodium concentration reaches too high a level, tissue damage occurs.

Though the leaves are the most visible manifestation, sodium buildup causes other problems in the soil. The sodium that binds to the clays and other negatively charged soil particles forms a sticky, gelatinous mass. This blocks the pores, impeding the circulation of air through the soil. As oxygen is depleted, undesirable fungi and anaerobic bacteria begin to grow and root rot sets in. This is what causes the peat moss in our azaleas to decompose so quickly to a black muck.

And sodium retention does not only affect maples and azaleas. Warren emphasized that it affects all of our plants, even though they may not show it as dramatically. I was happy to hear that should I successfully address the sodium problem, I should be able to keep my moss alive. This knowledge also helped to explain other questions. Like many others, I have heard the adage that azaleas prefer deeper pots, yet I had never understood why. Azaleas are well known to have shallow roots. Warren conducted his own experiments to determine how depth of pot affects soil pH. He found that the deeper the soil, the lower, or more acidic, the pH.

Why? Because the soil saturates with sodium the same way an ion exchange column does, from the top down. As you water, the top layers of soil are the first to be exposed to the high levels of sodium. As the water percolates down through the soil, the sodium is adsorbed onto the clays and othernegatively charged particles in the soil. The further down the water flows, the fewer sodium ions are left, so that by the time the water reaches the bottom third of the pot it has been filtered of most of its detrimental contaminants and is a neutral pH. This is the same principle of filtration that gives us pure well water and aquifers. If you plant an azalea or a maple in a deep pot, some of their roots will reach that healthier layer of soil. Plant them in a pot that is only one third as deep, and the roots never find an area of soil that has a pH and a sodium level to their liking. According to Warren, in Japan they regularly grow maples in pots no more than one inch in depth, yet they have no problems with leaf burning.

Where does sodium come from? It is ubiquitous in nature and generally present in the form of common salt or sodium chloride (NaCl). Municipal water supplies, however, contain unnaturally high levels due to the addition of sodium hydroxide (NaOH) by the water treatment facility. This is done to make the pH of the water alkaline in order to prevent our metal pipes from dissolving, something that happens quite rapidly if the water is acidic.

Can you eliminate sodium from your tap water? Sure, if you want to buy, install, and maintain water treatment equipment. You can install a water still that boils the water and recondenses it, or a reverse osmosis unit that uses a semi-permeable membrane that will allow water across but excludes other dissolved particles. But all of this equipment is expensive, comes with regular maintenance requirements, and most people find them more trouble than they are worth.

Other possibilities are collecting rainwater or buying distilled water in bottles. The first is unreliable in arid Texas summers [and tends to propagate mosquitoes in humid Louisiana], and the second is somewhat expensive.

You can help alleviate the sodium problem to some extent by altering the pH of your tap water. Acidic water contains an abundance of hydrogen ions which, by their sheer numbers, will displace some of the sodium in the soil. Some of the more enthusiastic bonsaiists and commercial growers have set up systems whereby sulfuric acid is metered into the water while a pH meter continuously monitors the pH and adjusts the acid additions as needed. A simpler approach (as suggested by Vito Megna) for the hobbyist is to add a teaspoonful of white vinegar (acetic acid) to one gallon of water and use this to water your alkaline sensitive plants.

Warren stressed, however, that while lowering the pH of your water is helpful, it is not a complete solution to your sodium problem. The sodium must be flushed from the soil and prevented from reaccumulating. For this he suggests gypsum (calcium sulfate). Gypsum is a naturally occurring mineral that is sold by most garden centers for soil amendment. I bought a twenty pound bag at Home Depot for $3.49. When dissolved in water, the gypsum separates into calcium ions (Ca with a charge of +2) and sulfate ions (SO4 with a charge of -2). Since the calcium has more positive charges (+2) than sodium (+1), it will have a greater affinity for the clay particles and displace the sodium ions allowing them to be washed out with the water. The sulfate ions will combine with water to form sulfuric acid, serving, thereby, to help acidify the water.

The gypsum can be mixed into the soil before potting or sprinkled on top of the soil for plants already potted. If your plants are severely sick, as mine are, Warren suggests placing the plant, pot and all, into a tub of water containing a handful of dissolved gypsum and allowing the water to rise through the pot to the top. Better yet, if the plant is root bound, to remove it from the pot and place the root ball in the tub to provide even better exposure to the dissolved gypsum.

While this is good for emergency treatment, Warren insisted that next spring each ailing tree should be bare rooted using a water spray and the soil replaced. For maples Warren uses a potting mix consisting of five parts crushed granite, two parts screened planters mix, and one part perlite. Planters mix is a product sold at most garden centers BONSAI NEWS July 2009 that contains a number of soil amending ingredients such as a humus and peat moss. The mix he uses in California also happens to include gypsum. If your local planters mix does not contain gypsum, then add some to your potting mix. Warren suggested that only by your own experimenting will you be able to tell the proper amount of gypsum to use, but a good place to start would be a handful per gallon of potting mix. George Gray says he mixes a teaspoonful into the potting soil of each plant he pots. Based on the healthy appearance of his maples, it appears to work for him.

In summary, there are various theories as to why our maples burn so badly around here. Since there is very little I can do about the heat and moisture stress, I am giving gypsum a try. I have given my ailing trees a gypsum bath. I may also try Vito’s vinegar water as well. It’s too late in the season to expect a visible difference in my plants this year, but if they were not too badly damaged to make it through the winter, I hope to see a notable difference next year.