I would agree with flushing from the top. I have really old houseplants, some of which are bottom watered, which show salt build up on the top of the soil and even on the pot sides themselves.
This would suggest that the salts can indeed float to the surface with the moisture and therefore a thorough flushing would need to start from the top.
After flushing down into the rez, I think I would be tempted to suck or tilt all of that out of there and start fresh. A need to flush is going to be a PITA even in a sip.
Thank you Emilya and Azimuth for commenting. Unfortunately I wasn't able to flush a few days ago, so I'm doing it this morning. And yes, it's a huge pain in the ass. So much so that for the next cycle something is going to have to change. I've poured 3 gallons so far into Blueberry #2 from the top, and because of how small my overflow hole is, I'm going to have to wait quite a while before I can continue adding water.
The res was 1/3rd of the way full before I started which tells me they're not drinking nearly as much as they have been in the last 1.5 weeks. The deficiency hasn't gotten overly worse, but now that it's more developed it does look like a lock out. As you can see in the pic above, the media is so saturated I have to wait for water to drain out before continuing to force water in. This is going to take a monumental amount of time, considering I have 2 plants I have to do this to.
The overflow hole. Unless I stick a drill in there and make it bigger, risking losing material, I don't see any other way to accomplish this in any sane amount of time. The plant is in the bathtub, not under lights, and will have to stay there until I've given it water. Emilya mentioned 3x the container volume, that'd be 15 gallons. While that might be the right answer, I don't have the time to wait for the water to exit the overflow hole until I can add more.
So this presents a serious problem for using SIPs moving forward with the FF nute line. The easiest solution is to make the hole bigger and deal with the consequences. For the next cycle, since I'll still have to use the remainder of this nute line (it's expensive, and is working, so won't be thrown away) I believe I'll put in a drain hole with a valve or something to make flushing easier and faster.
As for getting liquid out, that won't be a big deal. I've already done this with a 1/4" air tubing siphon that worked really well. I'm not worried about getting it out. The problem right now is getting it in. I'm going to shoot for 10 gallons run through the container (15 is out of the question right now) but even then putting in 10 gallons is going to take a ton of time getting 10 gallons out.
Dunno what I'm going to do here. I'll keep putting in a gallon at a time until I have to move on to the next thing. At that point, it is what it is.
Edit to add: I stopped caring about the size of the overflow hole and just drilled it with a 3/8" bit. It pulled some material out, but because I added a perlite channel between the res and the overflow hole, I'm not losing any more material through the hole. It's spurting out with some force too, so I estimate in not-so-scientific-terms I've cut the time needed to drain down by a factor of 2 or 3. That sounds legit.
