"George Patterson" wrote in message news:6JPIf.37126$Eq.22423@trnddc02...
He said he's using RO generated from creek water. The creek water at least will
be different from day to day and maybe from hour to hour. If somebody fertilized
upstream on that day, the RO filter will have to work real hard to get all the
phosphates out.

That is true.

Anytime you make a claim about phosphates or nitrates, you HAVE to test the
source water before mixing in the salt. If you don't, your tests are inconclusive.

Or, just use the same bucket of water and split it between two salt mixes.