rainwater ph adjustment question
 

I currently use a rainwater cistern - via gutters collecting water from my roof - always assumed ph of it to be on the acid side - tested yesterday and it was 6.75 , does this first need to be adjusted?, what to; and with what? use on about 230 orchids , mixed types, in heated greenhouse..thanks so much.

Not by my standards. It's not the water that is the potential problem, but the way fertilizer acts with this water. I started using MSU mix from Ray's (a Orchid Board memeber and vendor) on his recommendation. I am sure he will jump in here with more info than I can muster. I use rainwater during spring-summer-fall months and RO water during winter months with MSU mix blended for pure water (like rainwater) and fertilize at every watering.

Hi Kathleen,

Orchids are happiest with a Ph of between 6 and 7 so you're good :clap:

thank you
 

I appreciate your help on this -

Not sure I agree. My pH for RO runs approx 6.2 (slightly acid). Rainwater here runs lower pH than that (don't have any right now or I would test it.) Solution after adding MSU mix for 125ppm runs approx the same (surprize!)

Thank you for your response - Think now I will adjust it ,as I said previously was shocked ph was so high - assumed all rain water was on the acid side- especially after it sits for a bit in large container - our tap water here is RO but still has too much salt in it and lots of chlorine.

That's curious! RO has lots of salt and chlorine? Doesn't sound right to me.

PURE water has a pH of 7.000000000. Because it has no dissolved mineral ions in it to buffer the pH, the addition of the tiniest amount of almost anything will cause it to change - including carbon dioxide absorbed from the air to form H2CO3 (carbonic acid, which lowers the pH). Adding fertilizer to pure water is a much stronger influence, so you definitely need to check, and possibly adjust the pH after its addition.

A couple of more comments:

1) 1kathleen, rsfrid is right. If you have RO tapwater, it should be essentially free of dissolved solids. I think it is doubtful that it is RO, as it would be dissolving your copper pipes! If anything, it could be RO (from seawater, for example) to which minerals have been reintroduced.

2) It is my understanding that most orchids do best with a pH in the 5.8-6.2 range, but except for a few really picky ones, can easily handle a wider range.

Thanks Ray - I need to lower the ph for spiking phals per Bob Gordon's book - I live on an island and the RO water is seawater - however the additives are heavy on the chlorine - also not all salt removed - it does however have a ph of 6...oh well - I will be ordering the cal-mag supplement shortly from you. Thanks so much for your help...