But my real question is broader: In soil, does it matter if you use pure (RO/DI) or tap water? What are the (dis)advantages of using RO, tap or mixing in terms of improved plant productivity and quality (presuming Ca/Mg deficiencies not an issue, with suitable preventative Ca/Mg supplementation provided during the grow)?
To circle back to the original topic for posterity/other readers:
The main reason for using a neutral media (e.g., coco or peat) with pure water (RO/DI) in a mineral feed-based approach is so that you can calculate / know / control the cation balance (i.e., ratio of K:Ca:Mg) ahead of time and avoid Ca/Mg deficiencies from occurring in the first place.
That is, by the time any deficiencies are evident during the grow, then (technically) productivity and quality have already been impacted (to a degree). However, if you're used to adapting / compensating for that during the grow and correct it in time, then the overall impact on final harvest (i.e., final weights and quality) is typically negligible (or, at least within an acceptable and/or non-noticeable range for an individual grower).
The main reason I don't use tap water is that you don't really know how much Ca is in it. Municipal water reports are published after-the-fact, and you can't really measure Ca ions individually at home. Many municipalities switch water sources depending on season, local arrangements with neighboring counties, etc. My area sources from the Potomac too, but my county sometimes purchases excess supply from neighboring counties which pull from different locations in the Potomac - so, the Ca levels can change randomly without me knowing.
The second reason is chlorine. The 'just let tap water sit for 1-2 days' method is a bit antiquated and ineffective in a sense. Most municipalities use chloramine which is a more-stable combination of ammonia and chlorine (NH2Cl) that eventually breaks down into free chlorine (Cl-). It's the free chlorine that evaporates, but the remaining chloramine must be filtered out or left to decompose naturally over time. I've seen reports that this can up to weeks and/or months. Overall, I suspect that free chlorine is more of a concern for the soil biome and organic-based grows rather than for mineral feed-based grows and the plant itself. However, I have not tested that hypothesis yet.
Another (dis)advantage is micronutrients. Tap water does include some, which can be beneficial. For RO/DI, you need a feed component that adds these back in (for the OP, that's what one of the
trio bottles does for you). Micronutrient issues typically do not become apparent until around 8-10 weeks into the grow (at which point, it is too late to do anything about them for that grow). However, their impact is mainly cosmetic - i.e., discoloration and/or burnt tips starting in the fan leaves of the upper buds (i.e., new growth). As long as they don't spread to the buds / sugar leaves, they are not a huge impact - just possibly more trimming / manicuring.
Another potential concern is incidental / toxic elements in tap water, and I can't really say whether it should or should not be a concern in practice. I don't know how much bioaccumulation there actually is in cannabis, or how that's impacted by the final extraction / concentration / processing / ingestion methods (e.g., smoking vs. edibles vs. oil concentrates, etc) that we use. I will just say that pharmaceuticals are manufactured using USP-grade water (essentially RO/DI), and tobacco is grown using non-potable water from agricultural sources (which can be 'worse than tap water' in a sense...at least, I wouldn't drink it).
The last (dis)advantage between the two is just convenience / cost. RO/DI filters can be annoying because the final IX (ion exchange) cartridge has a high pressure loss and slows down the overall flow rate. A 1 gal bucket that can be filled in 30 secs from the tap can take 30 mins to fill from a consumer RO/DI filter. However, the majority of the Ca and Cl should be removed in the first stages (i.e., the upstream sediment / charcoal filter cartridges) and the IX cartridge is just removing the final remaining small elements / free ions. That is, I suspect just 'filtered water' (i.e., through a sediment and charcoal filter) would work just fine as opposed to 'pure' RO/DI water (i.e., sediment, charcoal, and IX cartridge) - but, I have not tried a grow like that yet.
