pH and TDS

[Gin]

Today was Fert. day I test the water before using it , had some rain water in an 18 gal. container tested the tds. it was way high before adding the Fert . approx 235 .. could not use that so tested another 18 gal. container of rain it was approx . 11 all I could figure out it was the first rain water after a dry period . It is collected from the greenhouse roof no shingle residue .
Ph. runs 6.5 to 7.
Awhile back I tested runoff from various mixes , bark , Spag. , Pro Mix all were diffrent , I wrote it down somewhere if I find it will post the results . gin

[Tmai]

Great info, thanks, Julie. Looking forward to your mix runoff results too, Gin.
question: Is there a correlation between pH and TDS? I mean, could high pH be indicative of higher TDS, or of some components of TDS such as sodium? I see how dissolved nutrients would affect pH, but would TDS?

Tmai

[Piper]

Hi Tami,

Without digging around in some chemistry books, I'm going to hazard an answer...

pH is influenced by the hydrogen ion activity relative to hydroxide ion activity they're oppositely charged. When it's hydrogen ion activity is high, a solution is acidic (low pH). When hydroxide ion activity is high, a solution is basic or alkaloid (high pH).

I don't think those ions are large enough to be measurable as solids in a TDS reading. Other charged particles (anions and cations), though, are large enough and can influence TDS. But pH can effect when certain chemicals dissolve or precipitate out of a liquid, so there is an effect.

Whether or not larger charged particles can influence pH, I'm not sure, but I don't think so.

Julie