I have been using tap water and saw a difference when I started aerating my water for a day before I water. Made a difference with my house plants as well. I dont have the means to test it but that supposedly removes the chlorine. I wonder about the fluoride still.
I wonder if slow-release fertiliser ties into this reasoning somehow, well at least the "right" brand.
I had a few orchids outdoors this year, same as every year. However I used dynamite fertiliser for the first time. Last year I used osmocote which doesn't last as long - I can tell because I can still see "whole" dynamite granules.
The results so far:-
- a cattleya with 4 new growths
- a dendrobium rescue from Lowes that is about to flower for the first time in 3 years
- zygo's with lots of new growth. 5 spikes on a single plant which is the first time too.
- noticeably different strong growth on other dens and catts
- catasetums that went "crazy"
The plants received rain and a watering when I remembered.
Perhaps slow-release is BETTER than other forms of fertilising because it is available when the plant really needs it and the plant is not dependent on a random fertilising when you remember to ?
I might continue this fertilising regime indoors too.
Since you are science inclined and apparently have the space (600 plants wow!).
Maybe you could set up another trial with control groups. One group with twice weekly watering, one with twice weekly plus fertilizer, one with daily water, and one with daily water plus fertilizer.
It would answer your question as to whether it is the water schedule or the fertilizer schedule.
Did you weigh the plants, estimate the size, or count growths/leaves