I think the problem is that I damaged the roots while trying to refill the jar with water? Which I guess is why Kratky himself and his hardcore followers use containers big enough so that refilling isn’t necessary during the life of the plant.
It’s certainly not because the roots weren’t getting water:
Oh, well. The four spinach that have come up are still doing okay. I may end up growing lettuce in plastic cups with holes, sitting on a bed of rocks to make room for root growth, and just keep that bed watered.
We shall see…