Just for information purposes, I exist on city water here and it is fluoridated. Most municipal waters in the United States have small amounts of soluble fluoride. Large amounts can, indeed, be toxic. But the rules for fluoridation keep the level quite low and it should be safe (see my comment on salt buildup, later).=C2=A0As I mentioned previously, my experience here is that the leaf tip burn goes away with sufficient moisture and my practice to keep them moist uses only plain tap water. Most municipal waters in the U. S. also contain a small amount of chlorine as a disinfectant. The chlorine dissipates quickly upon sitting (overnight). I also have chlorine in my trap water.
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Recall also that all (almost all?) municipal water sources are also lightly alkaline, meaning a pH in the range of 8 or 9. The reason for that is to minimize water pipe corrosion. This also applies to my water supply, which=C2=A0originates from the Great Lakes.
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One problem that people sometimes have is keeping their plantings for extended periods without refreshing the soil (whatever the composition). If tap water (or well water)=C2=A0is added from time to time (except for deionized or distilled water, from which no salts will originate) salts gradually build and can lead to "burn" phenomena. The kind and extent of dissolved salts in your water can be seen by evaporating some of it in a clean glass vessel. Look for the milky white residuals. These are salts. Water used to keep plants wet will transpire through the leaves and evaporate from the soil, but any salts will be left behind. To some extent excess salts can be purged with leaching and subsequent disposal of the leachate. People often ask me about the "calculus" of whitish salt that appears on pot rims over time. This is mostly calcium sulfate (gypsum). It is diagnostic of an accumulation of salts in the potting medium, although the calc is not soluble in new added water. Fluoride used to treat tap water=C2=A0is a typical salt and is not volatile. It also gets converted in horticulture to a very insoluble form (as calcium fluoride), meaning that it will not be flushed with a leaching. That means fluoride will accumulate as a planting ages without periodic refreshing of the planting mix. (My guess is that since calcium fluoride is so insoluble it must be essentially inert once created.)
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Finally, it should go without saying that once tip burn occurs it will not go away. Brown tips are dead tissue. Success with any remedy will need to be noted on new leaves as they appear. If you are successful the new leaves will stay green while the affected leaves will stay with their brown tips. If you have a big display of Spaths, it might take a couple of years or more of better growing conditions to finally have a planting without brown tips, as old leaves die off and are replaced by nice green ones.
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Ted Held
Detroit.
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