When plant based solid fuels, such as firewood, coal etc. are burned, their potassium and phosphorus content stay in the solid ashes. Quick search on the net shows ashes contain about 4% potassium and 1-2% phosphorus.
What chemical methods exist to separate potassium-something, something-phosphate out of ashes?
It seems mixing ashes with water will dissolve potassium salts, but still unsure what can we do the get the phosphorus out.
(I find the question interesting because currently both potash and phosphate are mined, and thus non-renewable. So if we want to be sustainable, we will need a way to recover these nutrients from organic waste and ashes.)