Why ammonium nitrogen fertilizer should not be mixed with plant ash is because ammonium bicarbonate is unstable, and it can decompose at room temperature when it is wet. The higher the temperature, the faster the decomposition. NH4CO3 = NH3 ↑+H2O+CO2 ↑, which makes ammonia escape and reduces fertilizer efficiency. When alkaline plant ash is mixed with ammonium nitrogen fertilizer, it will produce ammonia, which is volatile and will also reduce fertilizer efficiency.