nice troubleshooting! Saying first that i do not grow in soil but in hydro so i have only basic knowledge about the science behing growing in living soil, the problem is not the soil itself (i've never used"commercial" soil without adding a ton of perlite to it, i hate the way soil growing sometimes leeds to problem with root oxigenation), but the micro organisms that should brake down your raw organic nutrients into plant's assimilable food, are not able to live in an environment with such ph. Soil usually buffers Ph, is not difficult to maintain the right values if you bring your tap/nutrient solution to around 6.5 and try to stay consistent with it when you water..
Foliar feeding is surely the fastest way to make plants uptake what they need despite the root situation, so if you have to flush or think something wrong is happening, you can flush the plant and still feed her. I honestly don't know how to solve a nutrient buildup in a soil grow, without having to water just with low ppms and no nutrients for entire weeks somethimes, but i hope it won't be your case.
So, most likely it was an uptaking issue then caused the deficiency when the nutrient were already in the substrate, so maybe you won't need to add more.
Just take it super easy with nutes, i'll just adjust ph and environmental condition and continue to foliar spray.
Good grow, keep us updated!
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