I've watched your grows so I know you know this, but sometimes reading it all at once helps, so here we go. This is your SIP fix. It's about calcium, so grab a coffee.
Your issue is usually more of a SIP mechanical problem, not an ammendment issue. The soil mechanics occurring in a SIP pot, at least from my experience, are not conducive to proper calcium cycling.
With LOS and SIPs you need to keep the calcium rotataing. In top watering, the calcium in your EWC/topdressings waters in and moves down, creating the perfect environment for feeder roots in the top of the pot. The entire pot for that matter.
In most SIPs that fail, feeder roots disappear and the top half of the pot goes hard, dry, and crusty.
That crustiness is magnesium, in the absence of proper calcium (the cal to mag ratio is out). The excess magnesium crusts things up. It also locks up nitrogen on a 1 to 1 basis for every molecule of excess magnesium. So a dose of CalMag will fix the crusties, but it will also unleash all that nitrogen at once. In veg a few curled leaves aren't a big deal. In flower you do not want nitrogen toxicity.
Fix this before flip, and don't flip until it's fixed.
That crustiness chokes off oxygen, and every speck of plant food has to be attached to an oxygen molecule for the plant to recognize it as food, so low calcium is just the 1st domino in a death spiral.
In the absence of calcium, it's magnesium's job to kill the plant before it can produce weak genetics. Bugs are on their way. Brix levels are crashing.
Calcium works on contact, it's an electrical thing. Calcium is a nutrient, but it's also a soil conditioner, and it's heavy, so it moves downwards when it comes in contact with water.
It's double positive charge neutralizes magnesium's charge when in proper ratio, and Mag releases nitro, causing the soil to go from crusty to fluffy. Now your leaves won't die and fall off.
This is how CalMag fixes a nitrogen deficiency. It's chemistry, not biology, and it confuses people.
Occaisonal top watering until the res is full with EWC topdressing will eliminate this problem.
The Rev's system is a great example. He uses SIPs pots, but fills the reservoirs via top watering. At least that was his system in the 1st two books. I haven't read his 3rd book yet.
Your refractometer will tell you the state of the calcium content in the plant. If you are studious here, you can stay ahead and healthy and learn the plants required rhythm of needed top watering/EWC.
Once you have a top watered calcium source, or a way to cycle calcium, figured out for your style, this problem will disappear.
The health (healthy being without disease or impairment) of your plant is directly tied to the state of soil calcium. If soil calcium is off, you are either running slow on low voltage, or too fast and frying from too much voltage. Either way, you aren't processing nutrients or photosynthesizing properly.
Too slow or too fast are both no good, but you only need to be in the ball park to hit a home run, you don't have to get it perfect. Just close. The rhizosphere will dial it to perfection.
Refractometer.... thats your calcium monitoring device. The split second the line is less than very fuzzy, a calcium deficiency is arising. Stay ahead.
Somehow in the SIP world it became taboo to top water. I'm not sure what that's about, but I suspect it's related to a synthetic mentality.
LOS works differently in SIPs. Calcium must be constantly cycled back to the top, or constantly added up top. EWC is the best readily available calcium rich top dressing. It needs to be watered in.
If you topdress EWC from the very beginning, and water it in, theres a really good chance you will be successful.
Follow the refractometer. It's your best friend here. Analog refractometers are far superior to digital ones. Digital ones won't tell you about calcium levels.
