Soil pH and Living Soil

So I've been told there's no need to pH water going into soil nor the medium itself since the microbes provide a type of pH buffer which makes sense to me seeing as how naturally plants grow this way in the wild. One of my girls started showing signs of what I believe to be either a potassium or magnesium deficiency (I could be wrong, this is my first grow). Before I go supplementing either of these, I took a slurry test of the soil pH and it's at 5.8, just short of the recommended pH for cannabis, but since this is living Soil am I correct in assuming this doesn't matter, it's not locked out of the nutrients it needs in the soil and the soil itself is in fact in need of supplements?

Background: Dutch Passion CBD Compassion Lime 57 days old. Sohum living Soil, no added nutes, just top dress of Build-a-flower compost from Build-a-soil

Looks like phosphorus def to me, let’s see what everybody else says