Hey Pete, I'm happy to share my experiences with you as I grow in similar setup as you in terms of lights and RO water usage (also watering 2-3 times a week). I have learnt a few things along the way that I can relate back to you. I have made quite a few mistakes over the last 15 years!!
1. Yes I keep track of the pH and TDS with my digital meters. I calibrate with the standard solutions of pH 4 and pH 7 every time I start making the fertilzers to make sure the pH meter is +/- 0.1 parameter. Then I test the RO water once with both meters, which is usually 10 ppm TDS and pH 6.8. This ensures that your filters and RO membrane are functioning properly.
Then I mix my fertilzers and make sure the TDS is not more than 333 pm (easy to remember), although 200 ppm is ideal. The pH that I aim for is 6.2 but a range of 5.8 to 6.6 is acceptable. Many experts say that mineral bioavailabilty is best between pH 5.2 to 6.2. I prefer to keep closer to 6.5 because I know its usually acidic in the media due to breakdown etc. So when it enters it will go down to those levels. I use a pH down and pH up solutions from the hydroponics store to up or down regulate the pH, using drops (1-2 drops in a 2L watering can can drop it by 0.4 pH).
2. I usually fertilize every third watering with 1/5 to 1/4 recommended strength of MSU (for RO) and 5 drops of sea kelp (the thick dark ones) in 2L watering can. I check the pH and TDS every single mixture (5 times if I use five 2L can to water the entire collection). The reason is that pH is affected with even a little extra powder. Note that it is important to use the MSU for RO water. It regulates pH better.
3. I never use CaMg with MSU as MSU has it already. Instead I use the CaMg solution separately on the 6th watering mix with micro-elements solution (with manganese, iron etc). I use 1/4 to 1/2 tablespoon each in 2L RO water and adjust the pH to 5.8-6.5 and TDS 333 or less. This is a summer schedule. Winter I fertilize maybe once a month, maybe twice. Orchids are good at storing so I fertilze when I feel like they need food lol.
Sometimes I use foliar fertilizer (once a month) as a backup esp since many aerial roots don't get the pot fertilizers. I use the one by Norman's Orchids.
Sometimes I also use a variety of bloom booster solutions from hydroponic stores but those are rare (like humic acids, mycorrhizae, hygrozyme, etc). Mostly just to see LOL but they are not the main feeds.
Again I test pH and TDS every time.
Prior to implementation of testing, I was naively watering fertilizers with a pH of 4.0 and TDS of 600-800 because I was thinking RO is so pure that I can put more nutrients without harming. Boy was I wrong. RO water has unique water chemistry that fluctuates the pH and TDS in crazy ways. I was slowly killing roots and then my plants wasted away. Once I cleaned up my misconception and started using meters, plants started to grow better.
I'm also testing some plants on semi-water culture and they seem to be responding by growing roots into the mix. the mix is 1/3 LECA clay pellets on bottom, then 2/3 large orchiata/perlite/charcoal and top with thin layer of NZ moss. Each pot sits in clean water plastic trays. These are usually watered less, 1-2 times a week but the moss is sprayed wet daily. Roots grow towards the water which seep up the clay pellets but stop at the bark mix. The moss draws roots into mix (without the moss, the roots tend to wander outside the pot). I most likely will switch most if not all cattleyas to this method.
Hope I answered your questions.
Thanks for taking the time and the detailed reply, your insight is very helpful. I follow many of the same steps and feel like a chemistry class every time I start to water, glad others are in the same boat and I'm not 'a crazy'. I measure pH and TDS before every watering (spraying roots/ top of pots to damp down) with my hand pump (1 gallon). But I don't calibrate every application, only every once in a while with the standard solution of 7. I'll have to read up on calibrating, I want to understand the benefits to calibrating to both @ 7 and 4. I'm going to take steps to do this, also calibrate at every watering as I'm getting the sense from you that this might be very important. I also use pH UP, I have not found the need to pull down the pH, usually if I am adding supplements to the RO water it only ever moves the pH down the scale. And I only ever have to slowly adjust it up.
I aim for a pH of 6.6 - 6.8. - I understand the pH windows where nutrient uptakes are maximized. However, I seem to have strong root growth performance when keeping the pH in this range. I fear the log scale, but will relook at this and may set aside a few weeks in the coming growing season and winter season, target a lower pH and see if a better calibrated pH meter and lower overall pH will provide different results.
Watering from the hand pump is almost daily on the roots( light spray), rotating between fert (MSU, green jungle, Maxsea 16-16-16, Flower Fuel 1-34-32, kelp max, Inocucor Garden Solution, GREAT WHITE Mycorrhizae with Beneficial Bacteria & Trichoderma, CalMag). After which I flush the dailys (vandas, phrags, Aerangis, other mounted) with a hose of RO. Every 3-7 days depending on how dry the conditions are I deeply 'water' / flush the entire collection with the RO hose.
I aim for a TDS of 50 ppm, never over 80. - I believe orchid roots are incredibly efficient at taking up nutrients and I have many sensitive plants, including spanish moss and some carnivorous trays to knock down gnats and other flies, which I don't like to risk accidentally ODing. I have opted to keep a low but consistent application of supplements. Switching to the lower nitrogen ferts over the winter periods... with a once in a while dose of a balanced fert. I do plan to try foliar (mega thrive) feeding this spring / summer once the vegetative growth is in high gear.
I agree, RO water is a strange medium. It's got no buffer and you really have to be careful with it across the board as it produces a range of results.
Good luck with the semi hydro tests. Let us know when you get across a year or two and how the media / roots respond. I am starting to experiment with rockwool, net baskets, and styrofoam peanuts. I have a problem with roots growing all over the place out of the kiwi bark mix in clay pots, and out of wood baskets. I am trying to find a way for the roots to come out of the basket then grow back in. I'm seeing some Cattleya roots seek and dive into neighboring rockwool pots that I have my phrags and other wet footed friends in. I am also observing aggressive Cattleyas potted in large plastic net pots have roots that grow out then dive straight down along the net pot . I think there might be a way to open the media up, layer it properly and encourage root tip growth back into the pot with a thin layer of rockwool somewhere. TBD.
Thanks again, glad we have all this posted on the board. Looking forward to seeing your future blooms.
-Pete
