Hmmm I see that explains all the white lime particles on the soil surface, I tried to mix it into the top soil a bit.Lime is not water soluble.
Another thing I realised is that I have recently been watering with tapwater that ran through a screw-on filter on the tap - maybe that filter removed some calcium.
Anyway I just applied an epsom foilar spray at 1/4tsp per gallon. I also checked the run off again this morning, I added pH6.1 water and runoff was 6.7.