I gave up chemistry and physics before O levels, and only got a C in biology, so all this technical stuff is way beyond me!!!
Busy Bee 2, put it this way, in its best form fire ash would contain less than one percent of Nitrogen, Phosphate, Potash. where as Growmore would have 7 percent of each. With commercial fertiliser you know exactly what you have, with ash it is guess work
My question is it worth risking valuable plants just to use something which even back in my childhood Dad said were old wives tales. He did use ash from the fires and soot the sweep left from the chimneys but it was put into a brick box and left for a year then never put directly on plants, or else into the midden with the fresh manure for a year.
As a check Potash K or Kalium is certainly not Phosphate and most certainly not Phosphorous, we played silly tricks with that in the school lab not realising just how dangerous it was, no H&S back then.
My last word honest, Frank.