Rain water does contain N in the form of Nitric acid. And sulfur in the form of sulfuric acid. And carbon, in guess what form?
If you want to use rain water, you have to be prepared to pH test and adjust it every time.

'Praying for magnesium' is a myth. Mg def shows as very regular pale stripes between the veins.

You scorched the plants with Epsom salts.