Reasonable TDS for orchids

[estación seca]

What is a reasonable TDS target in municipal tap water to be used for orchids? In other words, above Xppm don't use this for orchids? Not for me; for friends. Their TDS in municipal tap water are 132ppm, pH 7.7. (Mine is 388-1,390ppm, which is why I collect rain.)

[Orchid Whisperer]

138 ppm should be OK for most commonly grown orchids (Phals, etc. routinely available). The tap water pH of 7.7 is usually adjusted and set by the water utility (prevents pipe corrosion, therefore reduces the heavy metals dissolving out of your pipes and getting into your drinking water).

At one time, the Houston Orchid Society posted on their website an information sheet that recommended no more than 200 ppm TDS; unfortunately, that link is no longer active. At 138 ppm I would want to flush pretty thoroughly at each watering. I assume your friend is in the AZ area too, If they can also collect some rain occasionally, that would be good too.

[estación seca]

Thanks. They're in Alabama, so they should be able to get plenty of rain, but sometimes there are dry spells.

[naoki]

Estación, I came across this:
http://www.ecaa.ntu.edu.tw/weifang/l...-72orchids.pdf
I hope that it is viewable by everyone. Well-known orchid nutrition researchers are included as the authors. So the recommendation is based on some science (unlike opinions by "experts"). p.72 contains the salinity discussion for Phalaenopsis. Here is the relevant quote:

"A desirable electroconductivity (EC) of the water before adding fertilizer is 0.5 dS/m or less. We recommend that growers maintain a medium leachate of 1.5 dS/m or lower when using the pour-through technique."

They use dS/m for EC, but you may be more familiar with microS/cm. 0.5 dS/m = 500 microS/cm, which is approximately TDS of 250 ppm (with US style TDS meter). If you use European or Australian TDS meters, here is the conversion:
EC to ppm Conversion Chart - Manic Botanix
(in this page, it uses mS/cm, which is same as dS/m).

Obviously, different species have quite bit different tolerance to salt, and Phalaenopsis seems to be able to handle fairly high salt level. Also, potting media, drying rate, watering method should all influence the decision somewhat.

[estación seca]

Thank you; I will look at that.