To determine % K2O of organic manures, flame photometric technique is used whereas in inrganic fertiliser for K2O determination STPB method is used. I am trying to find out any method of determination of K2O in organic manure by STPB method.
As manures have much lower amounts of K than mineral fertilizers I recommend flame photometry. We solubilize organic matter by using wet digestion (HClO4 + HNO3) or combustion 400-550ºC followed by ash dissolution.